Jim Mohr's SCO Companion

Index

Copyright 1996-1998 by James Mohr. All rights reserved. Used by permission of the author.

Be sure to visit Jim's great Linux Tutorial web site at http://www.linux-tutorial.info/


Talking to Other Machines

Long ago (at least in terms of the history of electronic data processing) having two computers at the same time was something you read about in science fiction novels. As systems became more common, the time did come when a company or university would have two computers. The need then arose that data be exchanged between the two machines. This was the beginning of SNEAKER-Net (Slow, Non-Electrical Activity, Keeping Everyone Running), which was developed in the 1950s. With SNEAKER-Net, the technician would copy data onto a tape or other media and using his sneakers, would run the tape over to the other machine to be loaded. In many organizations, SNEAKER-Net is still employed today as this is often the only type of network they think they can afford.

In 1976, researchers at AT&T Bell Laboratories came to the rescue. This was the development of a serial line protocol to exchange data between UNIX machines, which came to be known as UUCP, for Unix-to-Unix Copy. Over the years there were several changes, upgrades revisions, etc. In 1983, AT&T released a new version that came to be known as Honeydanber UUCP, as it was had been developed by Peter Honeyman, David A. Nowitz and Brian E. Redman. This is the version currently employed on SCO systems.

Although, UUCP was a good thing, it was limited by the speed of the serial line connecting the two computers. Since the system could only be as fast as it's slowest component, there needed to be a way to speed up that slowest component. Well, serial line speeds increased, but that still was not enough. In the 1970s, Xerox came out with Ethernet, which made high speed communication between computers possible. It was now possible for users to access remote computers and expect response times comparable to being logged in locally, rather than experiencing delays as was common with the serial line communication of the day. (We'll get into more details on Ethernet later.)

Today, both are still wide spread. Although prices have dropped to the point that Ethernet networks are common place (I even have one in my house), UUCP is still used regularly when distances prevent other types of connection, or when the connection is going to be quick or short term and the administrator doesn't want the added hassles of first installing the Ethernet cards.

We are going to talk about both in this chapter. First, we'll talk about UUCP as this is a set of programs that everyone has on their SCO system, even if they don't have ODT or OpenServer. Later, we'll talk about networking principles, especially TCP/IP and the treasure chest (or Pandora's box) of tools and programs that go with it.

UUCP

Perhaps the oldest form of UNIX computer networking is UUCP. UUCP stands for UNIX-to-UNIX-copy and stems from the time when even the ability to copy files directly between computers was a big deal. Today, UUCP is still widely used in systems where more complex networking is either not necessary or not possible. UUCP also provides the ability to remotely execute jobs as well as interactively log in to remote sites.

UUCP and most of it's associated programs are batch oriented. This means that transfers and remote execute requests are "spooled" to be carried out later. The user submits a request and waits for the request to be carried out. There is often little control over when the transfer actually takes place. This is essentially the same way the print spooler works. Jobs are done when the system gets around to it. In most cases, communications between remote systems is carried out according to specific schedules, usually when the phone rates are cheaper.

Because of it's complex nature and the large number of support programs, UUCP could also be considered a system service. UUCP is not only the name of the package or service within SCO, but is also the name of one of the programs within the package. The uucp command is perhaps the most commonly used of the commands, with the interactive login program (cu) a close second. The remote execution command, uux, also places in the top three.

Build into UUCP is several levels of security. The first level is normal file permissions that everyone must adhere to, whether local or remote. The next is the system level security that allows or denies access to your system or to remote system. There is also the ability within UUCP to restrict access to specific directories and commands. This means that although the permissions on a file or directory might give you access to it and your have a valid UUCP account on that system, you still cannot access it. With UUCP you can not only limit access to certain directories, but also to certain commands that can be executed. (Remember UUCP also allows remote execution.)

The SCO UUCP package is essentially the same between ODT and OpenServer. The differences really show themselves when you are trying to configure and administer UUCP, so we will wait until we talk about configuration to talk about these differences. The SCO UUCP package provides two programs for copying files between systems: uucp and uuto. The uucp command is the more common of the two and provides more flexibility. On the other hand, uuto is a little easier to use. Personally, I have never used uuto other than to test a user's command to see if they were doing it right.

For the purpose of this discussion, we're going to assume that there is a valid uucp connection between the two machines. In the second half of the book, when we'll talk about configuring and troubleshooting uucp, we talk about problems ands what to do. We'll also assume that we have three machines, all of which use UUCP. The machine local machine is scoburg. It connects via modem to the machine siemau. Siemau is connected via a direct serial line to the machine vesta.

The reason we are including this third machine is that is possible to transfer file to and from machines via UUCP to which there is no connection. So, if you have connection to one machine (in our case scoburg can talk to siemau) and the second machine can talk to he third (siemau and vesta) you can use the second machine as a bridge between the other two.

Before we start, we need a crash course in the basics. There are three primary files, all of which are located in /usr/lib/uucp . First the Systems file contains a list of which sites (machines) are known to you. The device that UUCP uses to connect to the remote machine is defined in the Devices file. What each machine can do in terms of copying or remotely executing commands is defined in the local Permissions file. For example, what siemau is authorized to do on scoburg is defined in scoburg's Permissions file regardless of who called whom.

Let's assume that we were on scoburg and we wanted to transfer a file to siemau. We start up UUCP which looks in the Systems file to see if it could find siemau. When it does, it then checks to see if this is a proper time to call (also defined in Systems). It then looks through the Devices file looking for an appropriate device (serial port). Once it finds the device UUCP can then beginning calling siemau.

In order for scoburg to be able to log into siemau, an account has to be established. For UUCP to login, it goes through the same procedures as "real" users: it needs both a valid logname (an account) and a password. When UUCP on scoburg gives the correct logname and password, UUCP is started up on siemau and the two UUCPs begin to talk to each other. When they start talking, UUCP on scoburg tells UUCP on siemau that it wants to send a file and where it wants to put that file. UUCP on siemau checks its Permissions file to see if scoburg has authorization to send files in the first place and if so, can UUCP on siemau send the file to the location specified. If it can, then the file is copied. (actually the work is done by the program uucico.)

By default, ODT 3.0 and OpenServer provides a UUCP login account: nuucp. The default Permissions file also allows every uucp login to be able to write to the /usr/spool/uucppublic directory. This directory is available on many different versions of UNIX as a common, generally accessible directory for transferring files. This allows you to easily transfer files with new sites, without having to establish special accounts for them. On the other hand. If every site shares the same UUCP account, they all have access to the same files. Therefore, if security is a concern, then you should consider individual accounts.

Copying Files

So let's get to it. Just keep in mind something I have been saying all along. This book is not a tutorial. I want to show you relationships and how things fit together. In order to do that I need to establish some base level understanding, which is what I am going to do with UUCP We're going to quickly go through some of basics UUCP commands to get a feel for how they work. Then we are going to go into details of what happens behind the scenes.

The primary (read: "most used") command in UUCP is uucp. Surprised? This command allows you to copy files from one UNIX machine to another. UUCP has spread itself so widely throughout the computer world that many vendors supply UUCP for non-UNIX machines, such as what is provided in the MKS Toolkit from Mortice-Kern Systems for DOS.

The syntax of the uucp is essentially the same as the "normal" UNIX copy command: command source destination. The only difference is that normally you want to specify a machine name in the source or destination name (or both). However, this is not a requirement. If you want, you could leave the machine name out completely and use UUCP to copy files on your local machine.

Let's say I wanted to copy a letter I have from my home directory on scoburg and put in the uucppublic directory on siemau. The syntax would be:

uucp /usr/jimmo/letter siemau!/usr/spool/uucppublic/letter

The first thing to notice is the machine name (siemau) is followed by an exclamation mark. This is a flag to UUCP to signify that this is a machine name and not part of the file name. (If you are using the C-Shell, you should switch to the ksh.) If you really don't want to (or can't) then you need to be careful because (remember the chapter on shell basics?) the exclamation mark means something to the C-Shell. Therefore you need to 'escape' it. The syntax for the C-Shell would be;

uucp /usr/jimmo/letter siemau\!/usr/spool/uucppublic/letter

As with other sections, I am not going to give you both csh and (k)sh syntax. Instead, I will just say that when you use csh you need to be careful about the exclamation mark.

Just like the normal cp command, the tilde (~) also has special significant for UUCP. By itself, it refers to the /usr/spool/uucppublic directory. Therefore, the above command could be written as:

uucp /usr/jimmo/letter siemau!~/letter

Not only does this save time in typing, you don't have to worry (too much) about misspelling anything. When I don't do use the tilde, I often have only one 'p' in the middle of uucppublic and my UUCP request will fail.

You can also use the tilde with a user name so it expands to mean their home directory. Therefore, I could have written the command above as:

uucp ~jimmo/letter siemau!~/letter

Which saves me a whole four characters. However, some systems I have seen with hundreds of users, the home directories are scattered across several filesystems. Using this mechanism, you can don't have to know what filesystem a user is on in order to get the file to or from their home directory.

Keep in mind that although you can specify a pathname with this short-cut, you do not automatically have access to that directory. Even if it is your own account. The problem is that UUCP does not know that you are you. To UUCP these are two different accounts, even if they have the same UID and GID. Also keep in mind that you as the submitter of this job must have access to the source file. However, it is UUCP that needs to have access to the destination.

Another way of specifying the file is with relative paths, or no paths, provided I am specifying a file in that directory. If I were already in /usr/jimmo, the above command could look like:

uucp letter siemau!~/letter

or:

uucp ./letter siemau!~/letter

This also works for relative paths, such as:

uucp ../jimmo/letter siemau!~/letter

When the job is submitted to UUCP it will create the appropriate full path name for that file. Also, if you specify the home directory for a user on a remote machine, that user need not exist locally for the expansion to take place. The reason is that the path name is not expanded locally. Rather, the system sends the path name you specify to the remote machine. It is there that the expansion takes place. Because there is no rule as to when and where the expansion takes place, it would be best not to use relative paths when specifying files on a remote machine.

Just as you can copy files from one directory to another, although neither is your current directory, you can copy files from one machine to another, although neither is you local machine. For example, assume our local machine is scoburg, which knows about both the source machine, siemau, and the destination machine, vesta. We can copy files from siemau to vesta with the following command:

uucp siemau!/usr/spool/uucppublic/file vesta!/usr/spool/uucppublic/file

We see from this example that the source machine is remote (remember that we are assuming that the local machine is scoburg). We are taking a file from siemau and copying it to vesta. What happens if we left out the reference to a remote machine when we specified the destination for the file? We would be taking a file from the remote machine and copying it to our local machine. That is, we would be copying from siemau to scoburg.

We can take this one step further and specify multiple machines in our destination. For example:

uucp /etc/motd siemau!vesta!~/filename

Since the local machine (scoburg) does not know about vesta, this is actually the way we would have to run the command. If we tried the first example on scoburg, uucp would have complained immediately that it did not know about vesta.

There are a couple of commonly used options to uucp. For example, you can use the '-j' option to print out the job-ID. If you later decide that you really don't want to send the file (or remotely execute the command), you can use this job-ID to cancel the request.

The '-m' option can be used to send mail to the requester that the file has been transferred or the job executed. Or if you want to notify someone on the remote machine, you can use the '-n' option followed by that user's name.

Depending on how your system is configured, the actual transfer of the file might not take place immediately. As I mentioned before, the Systems file contains information on what is an appropriate time to call. Often calls are only permitted during certain hours to either reduce the traffic on the line or to limit calls to time when the phone rates are less expensive. It may also be the case that your system is configured to never call the other system. Transfer is dependent on the other system calling you. Therefore, as a user, if your request does not go through as quickly as you want, talk to your administrator to find out how UUCP has been configured. If your are an administrator and the request doesn't go through as quickly as you expect, then you probably configured something incorrectly.

What happens to requests is that they are "spooled". This is essentially the same as for the print spooler. There is a special holding area for spooled uucp jobs. At what time UUCP decides to send the message is dependent on your system configuration and we'll get to that a little later.

As with spooled print jobs, there will probably be a time when you would like to find out the status of your spooled UUCP requests. To check the status in the print spooler, you use the lpstat command. To check the status in the UUCP spooler, you use the uustat command.

If I would run uustat after submitting a request:

uucp /etc/motd siemau!~/filename

the output of uustat would look like this:

siemauN522d 05/28-14:45 S siemau jimmo 90 /etc/motd

In this example, there is only one output line. Like lpstat when several print jobs are queued, uustat will also show information on multiple UUCP jobs that are queued. The entries in each line are:

job-ID date/time type_of_job remote machine requester size_of_file file_name

If we take a more complicated example, then the output of uustat get more complicated as well. For example, let's say we want to copy a file from siemau to vesta. Assume there is nothing in the queue and we issue the following uucp command:

uucp siemau!/etc/motd vesta!~/filename

When we run uustat, we now get:


Nothing. Hmmm. What happened? Well, it has to do with the default behavior of uustat. Just as the default behavior of lpstat is to only show us our pending jobs, uustat only shows us the pending uucp jobs. As a result we don't see anything. Because we are actually creating a local uux job that sets up a uucp job on vesta.

Instead we need to use the '-a' option to uustat, which says to show us all queues, not just uucp. This time we get:

vestaN6451 05/28-14:56 S vesta uucp uucp -C siemau!/etc/motd !/tmp

The biggest change is that instead of a size and file name at the end of the line, we have what appears to be a uucp command. This is exactly what it is. What we said was that wanted to copy a file from siemau to vesta. What happens is that UUCP actually creates a job for remote execution on vesta which says to go get the file from siemau. This is why we don't get any output from uustat. It is no longer considered a uucp request, but rather a uux request. If we look in /usr/spool/uucp/vesta we will see both a control file (C.) and a data file (D.)

Remote Execution

Just as uucp allows you to copy to and from different sets of machines, uux allows you to execute commands across sets of machines. For example, I want to print a file on a printer attached to vesta, but the file I want to print is on siemau. I could do this will the command:

uux vesta!lp siemau!/etc/motd

This would actually create two UUCP jobs. This first would transfer the file /etc/motd from siemau to vesta. The second then prints out the file on vesta. In principle we can use uux to execute any command on a remote system. There are two limitations. First, uux is an SUID program, so it is run as the user uucp. Therefore, the user uucp needs to have permissions to run the command. Second, the commands that can be remotely executed may be limited by the Permissions file on the remote machine.

Interactive UUCP Connections

There is one interactive program included in the UUCP package: cu (call UNIX). It's purpose is to access the modem and to dial the number specified, either through the Systems file or directly on the command line. Once the remote modem has answered, cu's work is over. Granted cu does serve as a kind of terminal emulator as it displays the output from the remote system on your screen, however it responds to your commands and does nothing on it's own. Because cu is simply echoing the output of the remote system, you don't even have to be calling another UNIX system!

If the system you called was like UNIX, where you eventually got to a command prompt, then the commands you input would actually be executed on the remote system. Build into cu is also the ability to execute commands on your local system, even though you are attached to (logged into) the remote system.

The format of cu is:

cu [options] destination

As I mentioned above this destination can either be a phone number or the name of a system from the Systems file. If you use a telephone number you can use special character to represent secondary dial-tones(=), or pauses (-).

Let's assume the phone number for siemau was 555-6501. To use cu to connect to it I could do in two ways:

cu siemau

or:

cu 5556501

You can also tell cu to access a particular device using the -l option. rather than trying to dial a phone. For example:

cu -l tty2a dir

This would connect me to the modem attached to /dev/tty2a (assuming there is a modem there)

Build into cu are several commands that are accessed by preceding the command with a tilde (~). Normally cu passes everything to the remote machine. However, if the first character on a line is the tilde, the line is interpreted as a command for cu.

A tilde followed by an exclamation mark (~!) tells cu that you want to execute a shell on the local machine. Although you are still connected to the remote machine, everything you now input is executed on the local machine. To return to the cu session, simply type exit. You can include a command name with the ~!. This will execute the command locally and immediately return you to the cu sessions.

Two other important tilde commands are ~%take and ~%put. These allow you to take a file from the remote machine and copy it to a local file, or put a file onto the remote system from the local system. The syntax is:

~%command source destination

So, if I wanted to copy the remote file file_name and copy it to new_file on my local machine, the command would be:

~%take file_name new_file

Keep in mind that this really only works for ASCII (text) files. The reason for this is that cu command was designed for 7-bit communication. Cu treats every 7-bits as a character. However, the bytes the compose a binary file are 8-bits long. You end up having everything garbled. However, there is a nifty trick that you can use with one of the other UUCP commands: uuencode.

The primary function of uuencode is to convert a set of 8-bit characters to 7-bit and then back again. Well, to the uninitiated, the syntax for uuencode is a bit confusing. The first thing to note is that by default, uuencode sends the results of it's work to stdout. You do give it two names as arguments, but one is the destination name and not the place that the output goes. Confused? Well, lets take an example.

Assume I have a program that I want to transfer named newprg. To uuencode it I might be tempted to input:

uuencode newprg newprg.uue

I am using the convention of .uue as an extension to indicate that this is a uuencoded file. Well, if I were use this command, I would get screen after screen of characters scrolling by. As I said the output of uuecode is stdout, not the filename we give as an argument. To understand this, let's look at the first line of the output:

begin 711 newprg.uue

The first part of the line is the word 'begin'. When we finally get around to decoding the file, this is a flag to tell uudecode where to start. This allows us to include uuencode'd messages as part of email.

Next we have 711. This is the permissions the resulting file will have. At the end of the line is newprg.uue. This is what we input, expecting to be the output file name. This is actually the name of the resulting file when we decode the file. In order to get this into a file so we can transfer it, the command would actually be:

uuencode newprg newprg > temp.uue

Now we have a file (temp.uue) that contains the output of the previous command. If we look at the first line, we see that it is the same as the previous one except that the resulting file name is now newprg and not newprg.uue. We now have a 7-bit file that we can transfer using either ~%put or ~%take.

When we are done, we can terminate the cu session by inputting tilde-dot (~.).

The best way to learn about something is to do tech support for it. That's how I learned about UUCP. One of the first calls I had with UUCP was, fortunately for me, a very basic call. The user was simply having trouble communicating with another machine.

We checked all the configuration files on the local machine and everything made sense. That is, all the files had the right values in the right place. One of the first things to try is cu, which provides an interactive login. If cu works, then you know that the physical connection between the two sites and the software configuration is correct. That is, the modems are set up correctly, you are going through the right device and you have the correct system name and phone number.

We tried to call, but we just couldn't communicate. The biggest problem was that the user didn't have access to the other machine. He was relying on the fact that the administrator from the remote side said the everything was configured correctly. However, we were still not communicating.

After about 20 minutes of checking our configuration and trying to call site with which we could communicate, he both agreed that it had to be something on the remote side, despite what the other administrator said. So, the user called him up on another phone and had him watch and listen as we tried to communicate with him.

Well, we heard the phone ring on the remote side. It rang, and rang and rang. Finally, UUCP gave us an error saying it couldn't connect. So we had the other administrator turn the modem on and off to make sure that it had not gotten into some "weird" state. We tried it again. The phone rang and rang and rang.

Fortunately, there was a set of lights on the font of the modem. One of them was the AA light, for Auto-Answer. The other administrator failed to notice, or failed to tell us, that this light was off. This meant that no matter how hard we tried, the modem wouldn't answer. We were trying to communicate with another site that just wasn't listening. After setting the modem to Auto-Answer, we could communicate with no problems.

Oh, the organization with the communications problems where one side just was listening? The United Nations.

The Flow of Things

Most UUCP files can be found in one of two directories. The /usr/lib/uucp directory contains the UUCP configuration files and many of the support programs. The /usr/spool/uucp directory is the working directory for UUCP. There are several sub-directories here with various functions, which we'll get to in a moment.

There are three key programs that manage and well as supervise UUCP communication. The first, uucico, is perhaps the workhorse of the UUCP package. It is the responsibility of uucico to determine what device should be used to make the connection, starts up the modem dialer programs (more of them in a minute), logs into the remote site and performs the actual transfer of the data.


Figure 0-1 The flow of UUCP

Part of the configuration process of UUCP is to establish UUCP accounts. With these accounts, remote sites can access through the uucico program (among others). When uucico calls a remote site, it logs as a predefined user. This user should have been established as a UUCP user on the remote site. If so, that user does not have a log in shell as we are used to it, but rather has uucico. When your uucico logs into the remote site, the remote uucico starts up and the two start to talk to each other.

To be able to communication with each other, the two uucico programs need to determine whether they both speak the same language. The language they speak is called a protocol and the process they go through is called negotiating the protocol. The most common one used in SCO UNIX systems is the 'g' protocol. This comes in two flavors: G and g.

The entire time it is attempting to log in and afterwards as it is processing and carrying out requests, uucico is maintaining its supervisory roll. It is the responsibility of uucico to ensure that files and commands are only accessed if the system allows it.

To execute commands on a remote machine, you use the uux command. It searches for requests for remote execution, but is also concerned with security and can only execute them is the remote site allows it.

If the connection cannot be made (we'll get into that later), the job remains in the spool directory. Like the print spooler the files in the UUCP spool directory are the only record the system has of pending jobs. If they get removed, then the job must be re-submitted. This obviously not a good thing if the files are removed by accident, however it does allow you to remove files that may be causing problems.

If the remote computer or the device selected to make the connection to the remote computer is unavailable, the request remains queued in the spool directory. This is also what happens when you either tell uucp to specifically hold onto the job or the time permitted to call hasn't bee reached.

By default, there are several entries in the UUCP user's crontab that are related to job scheduling. At 9 and 39 minutes after each hour, cron will start the /usr/lib/uucp/uudemon.hour program. Why it is called uudemon.hour when it runs every half hour is beyond me. Maybe uudemon.half-hour was a bit too much to write. Actually, the reason is the number of characters. UNIX file names are restricted to 14 characters on the traditional UNIX filesystems. Therefore, they couldn't have a file called uudemon.half-hour.

The uudemon.hour program is actually a shell script that contains two commands:

/usr/lib/uucp/uusched &

/usr/lib/uucp/uuxqt &

The uusched program (often called a daemon) checks for pending jobs and schedules them for execution by uucico. The uuxqt program is the remote end of uux, in that it is what executes the jobs that have been submitted for remote execution. It is the uusched program that eventually starts up uucico on the local machine.

To get a better understanding of what happens, lets go through an example. Let's assume your local machine is scoburg and you wish to send a file to the remote machine siemau. The command you would enter would be

uucp file_name siemau!/usr/spool/uucppublic

Remember from our discussion on the various shells that the exclamation marks means something to the C-shell. Therefore you have to escape it by placing a backslash in from of it, as in:

uucp file_name siemau\!/usr/spool/uucppublic

Work files are created in the /usr/spool/uucp/siemau directory. Each system will have its own sub-directory under /usr/spool/uucp, but these are not created until the first request is submitted. There is a control file, whose name has the format C.siemau#xxxx, where # is the grade, or processing priority of the request and xxxx is the job number of that particular request. The grade is a single alphabet character (letter) and grades are processed in order from A-Z and then from a-z. The default under SCO OpenServer is 'N'.

After submitting the job above we might have a file in /usr/spool/uucp/siemau that looked like this:

-rw-r--r-- 1 uucp sys 41 May 27 10:49 C.siemauN5214

Since this file is readable by everyone, we could look at it no matter who submitted the job. It contents look like this:

S /use/jimmo/file_name /tmp jimmo -dc D.0 644 jimmo

the format of this file is as follows:

Type of request. Here we have an 'S' for Send a file to the remote machine. This could also be an 'R' to request a file. If we want to remotely execute a job, this would be an 'S' as we are sending a request for remote execution.

Source path name of the file. If we specified a relative path (or no path) uucp will determine the full-path and use the full-path here. If we had specified a path relative to the home directory of a particular user (with the ~username), then the entry would also appear here in that form. In this case we are sending the file /usr/jimmo/file_name. If we are requesting remote executing the source file is the data file (D.).

Destination path. Here too, if we specified a relative path (or no path) uucp will determine the full-path and use the full-path here. If we had specified a path relative to the home directory of a particular user, then the entry would also appear here in that form. Here we are simply sending it to the /tmp directory. UUCP will ensure that it gets the right name, in this case motd. This is the same behavior as the cp command. If we are requesting remote execution, the destination name is the same as the control file, but will have an X instead of a C.

The login name of the sender.

List of command options specified to either uucp or uux. Here we have -dc. These are the defaults. The -d says to create directories necessary to copy the file. The -c says not to copy the local file into the spool directory prior to transferring. Instead the file will be copied directly from the source, in this case /usr/jimmo/file_name. (Copy the file if you don't want to change permissions)

Name of the data file. Here is the default of D.0. This means that UUCP will copy the file directly from the source. If I had chosen to copy the file into the spool directory this would point to that file.

Mode of the source file in octal. This is only used when sending files. Check the chmod(C) man-page for details on the octal format of file permissions.

Who to notify. The default is the sender. However, if you used either the -n or -m options.

In this example, I am copying the file directly from the source, in this case /usr/jimmo/file_name. If I had specified the -C option, uucp would have first copied the file into the spool directory. Check the permission here as well. You see that they are 644. That means that everyone, including UUCP has the right to read the file. If UUCP did not have the right to read the file, it would matter whether I specified a copy or not. UUCP would automatically make a copy of that file in the spool directory for me.

The file created by UUCPis referred to as the data file. Its name begins with a D and has the format:

D.siemauXXXXzzz

where XXXX is the sequence/ID number of that job and is the same as the job number on the control file, and zzz is a number used to identify any additional data files associated with that job. This is used when there are several data (D.) files created for a single control file. The format of the files is a single character to identify the type of data followed by the data itself. The meaning of each line is as follows:

U

User's name and system.

F

File to transmit. This normally only appears when sending mail since the D. file is the file being transferred in other cases.

I

Identifies standard input.

0

Identifies standard output.

N

If present, prevents mailing acknowledgment of completion of remote execution.

Z

If present indicates message should only be send if the job failed.

R

Return address of the requester. This is used by mailers that can forward to more than one hop. This is the user mmdf (assuming you are using MMDF).

#

Comments

After completion of the work files, UUCP can now send the job. If UUCP had been told to simply queue up the job (with the -r) option, the job will sit and wait until contact is initiated by either side. It is also possible that the system administrator had specified particular times to call that site and this is not one of them. So, even if you don't specify the -r option, the job will wait in the queue.

First, uucico checks the file /usr/lib/uucp/Systems. If the system you are trying to reachis not listed, or you are not allowed to call at this time, you get an error message.

An entry in the Systems file would look like this:

siemau Any ACU 300-2400 5551212 -\r\d-ogin:-\K\d-ogin:-\K\d-ogin: uusiemau asswd: NoSecret

The entries are:

system time-to-call device-type modem-speed phone-number chat-script

The time-to-call field tells uucico what times you can call the remote site. Here we have the entry Any which means calls are permitted at any time. We could specify specif times of the day or everm days of the week. We'll get into details on this in Chapter 14.

The third entry in the Systems file is the device type that should be used to call out. Using this device type, it searches through the file /usr/lib/uucpDevices for a matching entry. For example, I have an entry in my Devices file that looks like this:

ACU tty1A - 9600-19200 /usr/lib/uucpatdialHA96

The entries are

device-type port dialer-line baud-rate dialer

Here, uucico is looking for an ACU device type. This is an Automatic Call Unit or modem. Since this is the same device as specified in the System file, this line would match. The entry in the Devices file also tells what physical device to use and what modem dialer to use (A modem dialer is a program that is used to access the modem and dial the appropriate number). Using the telephone number from the Systems file, along with the device and dialer from the Devices file, uucico can now call out.

In order to prevent other UUCP processes from accessing the same device and to prevent them from trying to call the same system, uucico creates two lock files in /usr/spool/uucp. The port is locked with the file LCK..ttyxx, where xx is the non-modem control device. This is the convention used, although it is usually the modem control device that you are calling out on. In our example, we are calling out on device tty1A. therefore the lock file would be LCK..tty1a. Check the serial(HW) man-page or the section on device nodes for mode details on this.

The system lock is an similar form: LCK..siemau. In both cases, the LCK file is ASCII and contains the process ID of the process that created the lock. This is useful when trying to figure out who's hogging the modem by downloading dozens of image files when you are trying to transfer your end of month reports.

Now uucico can beginning calling the remote site. At this point we will assume that the connection can be made and the remote side answers. Uucico will attempt to login using the values defined in the chat script portion of the Systems file. Essentially, the chat script tells the calling system what to expect the remote system to send it and what it should send in return. We'll get into more details about this in the second part of the book.

Assuming that the logname and password were correct, scoburg has now successfully logged into siemau. One of the first things that siemau needs to do is to create it's own lock files. It would not be a good thing if one process starting to dial out on the port you were calling in on. These have the same format on siemau as on scoburg, however the system lock (obviously) has the form LCK..scoburg.

As I mentioned before, the shell that uucp users have is uucico. It starts up on siemau and the two uucico's start to talk to each other. One of the first things they do it to negotiate the protocol. Keep in mind that there is a lot more to the actual login process than that. However, this covers the main steps and provides a good background for understanding the flow of things.

The uucico on siemau needs to ensure that the uucp user calling from scoburg is allowed access to where it wants to go. It does so by checking the file /usr/lib/uucp/Permissions. Once siemau has determined that the UUCP user calling from scoburg has access it begins transmitting the file in small "packets". These packets are of a specific size. Changing them in SCO ODT 3.0 and earlier was not as straightforward as the /usr/lib/uucp/Configuration file in SCO OpenServer. Within each packet is a checksum, If siemau 0 determines that a packet was garbled during transmission, it will ignore (drop) the packet. Since it acknowledges each packet it receives successfully, scoburg will see that this packet was not received and will resend it.

To ensure integrity of the destination file, the uucico on siemau writes the file into a temporary file as it is receiving it. This is in the /usr/spool/uucp/scoburg directory, with the name TM.xxxx. When the transfer is complete, the file is moved to the specified destination, in this case /tmp/file_name. (We hadn't specified the destination file name, so uucico assumes that it the same name)

As this whole thing is going on, both sides are logging the information. On scoburg there will be a file /tmp/siemau. You can watch the progress of the transfer by using:

tail -f /tmp/siemau

The -f option tells tail to keep reading from the file as new lines are appended to the end. If you are siemau, there is a file /tmp/scoburg which you can watch with tail as well. The information in these files is for a particular session. Information is logged on a semi-permanent basis in /usr/spool/uucp/.Log/uucp/<system_name>.

If scoburg has more files to send to siemau, the process continues until there are none left. Scoburg tells siemau that it wants to hang up. If siemau doesn't have any files to send to scoburg, then they shut down communications. If siemau does have files, the roles are reversed and now siemau is the master and scoburg is the slave. The process continues until siemau no longer has files to transfer. Siemau requests to hang up. It's possible that during the time siemau was the master, a new job was submitted on scoburg. It then becomes the master once again. In theory, this go could back and forth indefinitely although I have never seen more than two role reversals.

After the connection is stopped, the uucico on both sides can remove the lock files for both the port and the system.

Look back at the beginning of the chapter when we covered the basics of UUCP command syntax. There was an example, where we copied a file from a remote machine to another remote machine:

uucp /etc/motd siemau!vesta!~/filename

If we change it slightly so that the source is also on a remote machine, we get:

uucp siemau!/etc/motd siemau!vesta!~/filename

UUCP recognizes the fact that both the source and the destination are on a remote machine. It therefore creates a remote execution command and not a simple file transfer. In other words, the local uucp command creates the necessary files so that another uucp command is executed on the remote machine (siemau). The contents of this command is to copy the local file /etc/motd to the destination "vesta!!/filename". In this case it happens that there is another remote machine involved (vesta). As a result, the file gets copied from siemau to vesta. (Assuming all the Permissions, etc are set right on siemau and vesta)

Where Things Live

There are three directories that UUCP uses. The first one, we've talked about in great detail already. This is /usr/spool/uucp. Like the /usr/spool/lp directory is for the print spooler, this directory is the working directory for UUCP. Here you'll find all the work files, locks, log files, etc. There are several sub-directories here as well. These are:

  • .Admin

Administrative files. Contains information on transfer statistics and accounting information.

  • .Corrupt

Corrupt work and execute files that could not be processed

  • .Log

Log files. One sub-directory for each of uucp, uucico, uux, uuxqt.

  • .Old

Old log files

  • .Sequence

System sequence numbers

  • .Status

Status files. One file per system.

  • .Workspace

UUCP temporary working area

  • .Xqtdir

Temporary workspace for remote executions

Plus there is one sub-directory for each system.

Most of the UUCPconfiguration programs and files are store in /usr/lib/uucp. Many of them we have already mentioned when going through the example transfer, but I would like to cover them briefly again.

The Systems file contains a list of the remote systems that are know to you. This file determine what kind of device can be used to contact a particular site (Modem, direct-line or TCP), the times that are authorized to call that site, the phone number to call, as well as chat-script, which is sued when logging into the remote system.

The Devices file contains information used to access the physical device, usually a serial (tty) device. This file contains such information as the type of device, speed, and the dialer program to use.

The Permissions file contains a list of permissions/authorizations when transferring files or requesting remote execution. The entries in this file determine what programs you can run on the remote side, which directories you can access and whether or not the remote site can request or send files to your site.

If you are running SCO OpenServer then there is a new file: Configuration. This contains information that determines and configures the protocol used when transferring data. This includes such thing as the packet size (how large each data packet is) and the window size (how many packets can be send before an acknowledgment is required).


TCP/IP

Before we talk about the details of networking, we should first talk about the process of network communication. Let's take a network program such as telnet. The telnet program allows you to login to a remote system. You end up with a shell just as if you had logged in locally. Although you are input commands on your local keyboard and the output is appearing on your local screen, all other activity is happening on the remote machine.

For simplicities sake, we can say that there is a telnet program running on each computer. When you input something on local keyboard, the local copy of telnet is accepting input. It passes the information through the network to the telnet on the remote machine. The command is executed and the output is handed to the remote telnet. That information is passed back through the network to the local telnet, which then displays the information on your screen.

Although it may appear as there is a constant flow of information between your local machine and the remote one, this is not what is happening. At any given time there may be dozens, if not hundreds of programs using the network. Since only one can use the network at a time there needs to be some mechanism to allow each program to have its turn.

Think back on our discussion on the kernel. When we need something from the hard disk, the system does not read everything at once. If it did, one process could hog the computer if it needed to read in a large file. Instead, disk requests are send in smaller chunks and the program only thinks that it gets everything it wants. Something similar is done with network connections.

Computers are like humans beings in that they need to speak the same language in order to communicate. Regardless of how they are connected, be it serial or Ethernet, the computers must know how to talk to each other. The communication is carried out in a pre-defined manner, called a protocol. Like the protocols diplomats and politicians go through, computer protocols determine how each side behaves and how it should react to behavior by its counterpart. Roughly speaking even the interaction between the computer and the hardware, such as the hard disk, can be considered a protocol.

The most common protocol used by SCO is TCP/IP. However, it is more accurate to call TCP/IP a protocol suite, or protocol family. This is because TCP/IP actually consists of several different protocols. Even the name consists of two different protocols as TCP/IP stands for Transmission Control Protocol/Internet Protocol.

TCP/IP is often referred to as protocol suite as it contains many different protocols and therefore many different ways for computers to talk to each other. However, TCP/IP is not the only protocol suite. There are dozens, if not hundreds of different ones, although only a small portion have gained wide acceptance. SCO only uses a few itself, although the TCP/IP family is what is delivered by default and most commonly used.

Although the name refers to two specific protocols, TCP/IP actually means an entire suite of protocols and programs. The result of many years of planning and discussion, the TCP/IP suite includes a set of standards which specify how computers ought to communicate. By following these standards, computers "speak" the same language and can therefore communicate. In addition to the actual means of communication that the TCP/IP suite defines conventions for connecting different networks and routing traffic through routers, bridges and other types of connections.

The TCP/IP suite is result of a Defense Advanced Research Projects Agency (DARPA) research project on network connectivity. However, its availability has made it the most commonly installed network software. Many versions provide source-code which reside in the public domain allowing users to adapt it to many new systems. Most vendors of network hardware (e.g. bridges, routers) support the TCP/IP suite.

Whereas the data being transferred to and from the hard disk is talked about in terms of blocks, the unit of information transfer across a network connection is referred to as a packet. Depending on the program you are using, this packet can be a different size. In any event they are small enough to send across the network fast enough, so that no one process hog the network. In addition, the packets go across the network so fast that you don't notice that your data is broken in to packets. This is similar to the way the CPU manages processes. Each one gets a very small turn on the processor. Because it switches so fast between processes it only seems like you have the processor to your self.

If we take a step back and look at the process of network communication more abstractly, we see each portion supported by and supporting another. We can say that each portion sits on top of another. Or in other words the protocols are stacked on top of each other. Therefore, TCP/IP is often referred to as a protocol stack.

Each portion of the stack is referred to as a layer. At the bottom of the stack is the layer that is responsible for the physical connected between the two computers. This is the physical layer. Sitting on top of the physical layer is layer that is responsible for the network portion of the stack. That is, it ensures that packets either stay on the network or get to the right network and at the same time ensures that packets get to the right network address. this is the network layer.

On top of the network layer is the layer that ensures that the packets have been transmitted correctly. That is, there are no errors and all packets have been received. This is the transport layer. Finally, at the top of all of this is the layer that the user sees. Since the programs that we use are often called applications, this upper layer is called the application layer.


Figure 0-2 Network Layers

Conceptually, each layers is talking to its counter part on the other system. That is, telnet on the local machine is passing data to telnet on the remote machine. TCP on the remote machine sends an acknowledgment to TCP on the local machine when it receives a packet. IP on the local machine gets information from IP on the remote machine that tells it that this packet is destined for the local machine. Then there are the network interface cards that communicate with each other using their specific language.

This communication between corresponding layers is all conceptual. The actual communication takes place between the different layers on each machine not the corresponding layers on both machines.

When the application layer has data to send. It prepends an application header onto the data it needs to send. This header contains information necessary for the application to get the data to the right part of the application on the receiving side. The application then calls up TCP to send the information along. TCP wraps that data into a TCP packet, which contains a TCP header followed by the application data (including header). TCP then hands the packet (also called a TCP segment) to IP. Like the layers before it, IP wraps the packet up and prepends an IP header, to create an IP datagram. Finally, IP hands it off to the hardware driver. If Ethernet, this includes both an Ethernet header and Ethernet trailer. This creates an Ethernet frame.

As we see, it is the TCP layer that the application talks to. TCP sticks the data from the application into a kind of envelope (the process is called encapsulation) and passes it to the IP layer. Just as the operating system has a mechanism to keep track of which area of memory belongs to what processes, the network has a means of keeping track of what data belongs to what process. This is the job of TCP. It is the also the responsibility of TCP to ensure that the packets are delivered with the correct contents and then to put them in the right order.

Error detection is the job of the TCP "envelope" which contains a checksum of the data contained within the packet. This checksum information sits in the packet header and is checked on all packets. If the checksum doesn't match the contents of the packet or the packet doesn't arrive at all, it is the job of TCP to ensure that packet is resent. On the sending end, TCP waits for an acknowledgment that each packet has been received. If it hasn't received one within a specific period it will resend that packet. Because of this checksum and the resending of packets, TCP is considered a reliable connection.


Figure 0-3 Encapsulation

Another protocol that is often used is the User Datagram Protocol (UDP). Like TCP, UDP sits on top of IP. However, UDP provides a connectionless transport between applications. Services, such as the Network File Service (NFS), that utilize UDP, must provide their own mechanism to ensure delivery and correct sequencing of packets. Since it can be either broadcast or multicast, UDP also offers one-to-many services. Because there is no checking by UDP it is also considered unreliable.

Closest to the hardware level, IP is a protocol that provides the delivery mechanism for the protocols. The IP layer serves the same function as your house addresses, telling the upper layers how to get to where they need to. In fact, the information used by IP to get the pieces of information to their destination are called IP addresses. However, IP does not guarantee that the packets arrive in the right order or that they arrive at all. Just like a letter to your house requires it to be registered in order to ensure that it gets delivered with the content in-tact, IP depends on the upper layers to ensure the integrity and sequencing of the packets. Therefore, IP is considered unreliable.

Since the hardware, that is the network cards do the actual, physical transfer of the packets, it is important that they can be addressed somehow. Each card has its own, unique identifier. This is the Media Access Control, or MAC, address. The MAC address is a 48 bit number that is usually represented by 6 pairs of hexadecimal numbers, separated by (usually) dashes or colons. Each manufacturer of network card is assigned a specific range of addresses which usually are specified by the first three pairs of numbers. Each card has its own, individual address: the MAC address.

When sending a packet, the IP layer has to figure out how to send the packet. If the destination is on a different physical network, then IP needs to send it to the appropriate gateway. However, if the destination machine is on the local network, the IP layers uses the Address Resolution Protocol (ARP) to determine what the MAC address of the Ethernet card is with that IP address.

To figure this out, ARP will broadcast an ARP packet across the entire network asking which MAC address belongs to a particular IP address. Although every machines gets this broadcast, only the one out there that matches will respond. This is then stored by the IP layer in its internal ARP table. You can look at the ARP table at any time by running the command:

arp -a

This would give you a response similar to:

siemau 194.113.47.147 at 0:0:2:c:8c:d2

This has the general format:

<machine name> (IP address) at <MAC address>

Since the ARP table is cached, IP does not have to send out an ARP request every time it needs to make a connection. Instead, it can quickly look in the ARP table to make the IP-MAC translation. Then, the packet is sent to the appropriate machine. This is accomplished by the requesting machine sending out a special message (a broadcast) to all machines in the network asking for a response from whoever "own" a particular IP address. The machine with that address will then respond with the MAC address of that card. This mapping of IP address to MAC address is then stored within the requesting machines ARP cache and is then available for all subsequent transfers.

Status and error information is exchanged between machines through the Internet Control Message Protocol (ICMP). This information can be used by other protocols to recover from transmission problems or by system administrators to detect problems in the network. One of the most commonly used diagnostic tools, "ping", makes use of ICMP.

At the bottom of the pile is the hardware or link layer. As I mentioned before, this can be represented by many different kinds of physical connections: Ethernet, token-ring, fiber-optics, ISDN, RS-232 to name a few.

This four layer model is common when referring to computer networks. This is the model that SCO uses and the one that I will use through the book. There is another model that consists of seven layers. This is referred to as the OSI model, but we won't be using it here.

Network Services

In the discussion above, I used the telnet command as an example of one of the programs that uses TCP/IP. However, there are many others which provide addition services such as transferring files, electronic mail, networking printing, and access to remote filesystems. Products such as SCO OpenServer can expand upon these basics to provide additional services.

One kind of service that SCO OpenServer provides is remote administration. In contrast to previous versions of SCO products which required you to administer all machines local, the SCOAdmin suite of programs allows you to administer machines anywhere in your network. Other products, such as database applications may have one central machine containing all the data and access is gained from the other machines via TCP/IP. Often this access is invisible to the user who just sees the "front end" of the database.

This configuration, where one machine contains the data or resource that an other machine uses is very common in computer networking. The machine with the resource that it is providing to other machines is referred to as the server, because it is serving the resource to the other machine. The machine that is using the resource is called the client. This model, where one machine is the server and the other is the client is referred to as a client-server model. The client-server model is the primary model used in SCO networks.

Another common network model is the peer-to-peer model. In this model, there is no one central machine that has all the resources. Instead, all machines are on equal status. Often times, these two models sort of blend together. In SCO UNIX networks, it is possible to have multiple servers, each providing many of the same resources. In can also happen that multiple machines all have resources that the others need so everyone is acting as both a client and a server, similar to peer-to-peer.

On SCO systems, there are dozens of resources available. Many of which are well-known such as telnet, others, such as ntp are more obscure. Like calling into a large office building with a central switchboard, our server needs to know what numbers are associated with which programs in order to make the proper connection. In the same regard, you need to know what office you want to reach before you call. In some cases you can call and say you want a particular extension. In other cases, you say you want a particular office. In a office building there is a list of available "services", called a phone book. On a SCO UNIX system the phone book is the file /etc/services.

The /etc/services file contains a list of what services a particular machine may have to offer. The concept of a service is slightly different than the concept of a resource. A machine may provide many resources in the form of login shells that it provides to remote users, however all of them are accessing the machine through the one service: telnet.

In addition to what service the machine provides, /etc/services also lists the port. To understand the idea of a port, think about this as being the telephone number. When I call in to a machine (say using telnet), I am connected to the telnet program on the other side through a particular port. This is as if I were calling a large office building with a single switchboard. When I reach that switchboard, I tell the operator which office or person I want to talk to. In the ancient history of telephones, that operator had to make the connection between the incoming line and the office herself.

A port can be thought of as the sockets that the operator plugs the phone lines into. Like in that office building, there may be a set of these sockets, or ports, that are directly connected to a specific person (i.e. service). These are well-known ports. There may be offices with their own operator (maybe just a receptionist) who passes the incoming phone call to the right person or may even pick someone themselves to take the call (such as when you call a government agency with a generic question and there is no one person responsible for that area).

On an SCO UNIX system using TCP/IP, the principle is the same. There are dozens of services that one can connect to, but only one way into the system, that's through your network interface card. In order for you to be able to connect to the right service, there has do be something like an operator to make the connection for you. This is the program /etc/inetd. This is the "Internet Daemon" and often referred to as a "super server" since it is inetd's responsibility to wait for requests to access the other servers on your system and pass you along.

Like in our office building, you may know what number you want, that is, which port. When you make the connection to inetd, your process tells it what port you want to connect to and inetd makes the connection. On the other hand, you may have a program that does not have a well-known port. Therefore a new port needs to get created.

The inetd daemon "listens" for the connections. You can say that it is listening on multiple ports in the sense that it manages all the ports. However, it is inetd that makes the connection between the incoming connection and the local port, and therefore to the local server. This mechanism saves memory since you don't need to start up the servers you aren't going to use. This is similar to having a central switchboard and not requiring every office to have their own.



Normally, inetd is started during system start up from a script in /etc/rc2.d. (normally /etc/rc2.d/S85tcp) When it starts, inetd reads its configuration file (/etc/inetd.conf) to obtain the necessary information to start the various servers. It then builds the logical connection between the server and its respective port. Kind of like laying the cable from the central switchboard to the various offices. Technically it creates a socket, which is bound to the port for that server.

When inetd gets a connection request (the phone rings) for a connection-based port, it "accepts" the incoming call which creates a new socket. That is, there is a logical connection between the incoming request and the server. Inetd can now continue to listen on the original port for addition incoming calls.

If the port is connection-less (UDP), the behavior is dependent on entries in the /etc/inetd.conf file. If inetd is told to wait (there is a wait in the fourth column), then the server that was called must process the incoming message before inetd can go on. If told not to wait (there is a nowait in the fourth column), inetd will continue to process incoming requests on that port. If you look in /etc/inetd.conf you see that almost exclusively TCP ports are no wait and UDP ports are wait.

Network Standards

Before we do that we need to side step a little. We need to first talk about what goes into making a standard. Without standards, it makes communication between computers of different type very difficult. Just like you have bus standards like ISA and PCI so hardware can communicate with the CPU, you need some kind of standard.

In the Internet community, standards are both suggested and established through Request for Comments or RFCs. To some extent this are the law. If one product claims to comply with a particular RFC, you know that any other applications that does so should be able to communicate with it. However, RFCs include other things such as lists of previous RFCs and basic introductions to things like TCP.

Becoming a standard is a three step process. Usually, the first few paragraphs of an RFC will tell you to what stage it applies. Assuming of course, that the RFC is part of a standards proposal. At the first stage, the standard is proposed. Organinzing then decide to implement the proposed standard. It requires three separate implementations before the proposal becomes a standard. (Finally, it becomes a standard. This is an oversimplication of the process, since there were also be a lot of discussion about the proposed standard.)

If you need information about a specific network standard, the first place to look is the most current RFC index, which is also published as an RFC. Not only does this list all the RFCs, but will also tell you if one RFC has been replaced and by which one.

Originally I had planned to include a list of the more commonly used and significant RFCs. I eventually realized that this was an unending task. When I started this book, there were just over 1700 RFCs. The last time I checked before wrapping up this book, there were over 1800. Instead I will simply tell you where to get them.

The first place is from the "central repository." These are obtainable using ftp from ftp.ds.internic.net. There is an rfc directory, which contains the RFCs in ASCII as well as many in postscript format. If you know what RFC you want, this can be obtained by sending an email message to [email protected] List each RFC you want in the format:

document-by-name rfcXXXX

where XXXX is the number of the RFC. You can obtain the index by including the entry:

document-by-name rfc-index

In addition, the RFCS are available from archives all over the Internet. However, rather than tying up the Internet bandwidth with a lot of copy of files you may not need. Check out the Web page www.cdrom.com or mail [email protected] This is for Walnut Creek CD-ROM and the sell a CD packed with thousands of documents related to the Internet, including the RFCs. Another site with pretty much the same kind of offering is InfoMagic. They can be reached at www.infomagic.com or [email protected]

For SCO UNIX systems running TCP/IP one of the most important standards deals with Ethernet. The encapsulation (packaging) of IP datagrams is defined for Ethernet in RFC 894. Developed in 1982 by Digital Equipment Corporation (DEC), Intel and Xerox Ethernet (spelled with a capital) is a standard, rather than a physical entity. Several years later, the 802 Committee of the Institute of Electrical and Electronic Engineers (IEEE or I-triple E), published standards of its own that differed in many ways from the original Ethernet standard. Collectively, these are referred to as the 802 IEEE standards. The 802.3 standard covers networks similar to Ethernet. The IEEE 802 encapsulation was defined in RFC 1042. Both of these use an access method called Carrier Sense, Multiple Sense with Collision Detection or CSMA/CD.

Both of these framing types (RFC 894 and RFC 1042) using a 48-bit addressing scheme. These are generally referred to as the MAC or hardware address. The six bytes of both the destination and source machine are included in the header of both framing types. however, the remainder of the frame is different. As we talked about earlier, This layer is responsible for sending and receiving the IP datagrams. It is also responsible for sending and receiving other kinds of packets as well. These are packets from the Address Resolution Protocol (ARP) and the Reverse Address Resolution Protocol (RARP). We'll talk about both later on.

IP Addressing

In today's world of inter-connected computers, you may have a connection to hundred of thousands of other machines. Granted there is no single cable connecting all of these computers, however there is a logical connection in that you can use the telnet program from your PC in California and connect to a machine in Germany. The problem is, how do the packets get from one end to another. Added to that, how do you keep your local network in California from getting overloaded with packets that are being sent between machines in Germany and at the same time making sure that those telnet packets do get through? The answer is provided by the Internet Protocol (IP).

Just as a street address is not always sufficient to get your letter delivery, so is the IP not always sufficient to get the packet delivered. If I sent you a letter, it could be sent to a single, central post office, whose job it was to distribute mail throughout the entire US. Because of the incredibly large number of pieces of mail, this is impracticle. Instead, there are thousands of offices, all over the country, whose job it is to route the mail for us.

If we lived in a small town, the local post office could catch a letter destined for a local addresss before it goes further. Mail with addresses outside could be sent to other post offices to be processed.

A similar situation applies to IP addresses. In local, self-contained networks, the IP address alone is sufficient. However, when multiple networks are combined, machines spend more time trying to figure out if the packet belongs to them than actually processing information. The solution is a Network Mask. Just as a zip code tells a postal worker whether to process a particular piece of mail locally or not, the Network Mask (or netmask) tells machines whether or not they can simply ignore a packet or need to process it further. How this works, we'll get to in a moment.

Every machine on the network, needs to have its own, unique IP address. Just like every house has a unique mail address. If that network is connected to the rest of the world, that address must not only be unique within the local network, but unique within the rest of the world, as well. As of this writing, IP addresses are 32-bit values. They are usually represented by four sets of numbers, ranging from 0-255 separated by dots (.). This is referred to as dotted-decimal notation. In dotted-decimal notation, an address might look like this:

147.132.42.18

Since each of these numbers range between 0-255, they can be represented by eight bits and are therefore referred to as an octet. This IP address is often thought of as being composed of a network portion (at the beginning) and a node (or machine) portion at the end. This would be comparable to writing a street address as:

95061.Main_Street.42

Where 95061 is the zip code and Main Street is the street and 42 is the address on that street. The reason we write the street address in this fashion, is that it's common to think of the IP address as moving from the general to the more specific.

Currently, there are three classes of networks, which are broken down by both the range used in the first octet and the number of octets used to identify the network. Class A networks are the largest and use the first octet as the network address. Networks in the class will have the first octet in the range 1-126. Class B networks used the first two octets, with the first being in the range 128-192. The smallest networks, class C use the first three octets in the network address and with the first in the range 192-223. How IP addresses are broken down by the different network classes is shown in Table 0.1.


Class

Range within 1st octet

Network ID

Host ID

Possible networks

Possible hosts per network

A

1-126

a

b.c.d.

126

16,777,214

B

128-191

a.b

c.d

16,384

65,534

C

192-223

a.b.c

d

2,097,151

254

Table 0.1 IP Address Breakdown by Network

There are a couple of things I would like to point out about this table. First, the network address 127 represents the local computer, regardless of what network it is really on. This is helpful for testing as well as many internal operations. Network addresses 224 and above are reserved for special purposes such as multicast addresses. The terms "possible networks" and "possible hosts per network" are those that are calculated mathematically. In some cases, 0 and 255 are not acceptable values for either the network address or the host address.

Keep in mind that a Class A address does not necessarily mean that having 16 million hosts on a single network is impossible to administrate and would over burden most network technologies. What normally happens is that a single entitity, such as Hewlett-Packard is given a Class A address. They will then break it down futher into smaller sub-nets. We'll get into more details about this shortly.

A network host uses the network ID and host ID to determine which packets it should receive or ignore and to determine the scope of its transmissions (only nodes with the same network ID accept each other's IP-level broadcasts). Because the sender's IP address is included in every outgoing IP packet, it is useful for the receiving computer system to derive the originating network ID and host ID from the IP address field. This is done by using subnet masks, as described in the following section.

Subnet Masks

Subnet masks are 32-bit values that allow the recipient of IP packets to distinguish the network ID portion of the IP address from the host ID. Like an IP address, the value of a subnet mask is frequently represented in dotted decimal notation. Subnet masks are determined by assigning 1's to bits that belong to the network ID and 0's to the bits that belong to the host ID. Once the bits are in place, the 32-bit value is converted to dotted decimal notation, as shown in Table 0.2.


Address class

Bits for subnet mask

Subnet mask

Class A

1111111 00000000 00000000 00000000

255.0.0.0

Class B

1111111 11111111 00000000 00000000

255.255.0.0

Class C

11111111 11111111 11111111 00000000

255.255.255.0

Table 0.2 Default Subnet Masks for Standard IP Address Classes

The result allows TCP/IP to determine the host and network IDs of the local computer. For example, when the IP address is 102.54.94.97 and the subnet mask is 255.255.0.0, the network ID is 102.54 and the host ID is 94.97.

Keep in mind that all of this with the subnet masks is the principle and not necessarily the practice. If you (meaning your company) has been assigned a Class B address, then the the first two octents are assigned to you. You could then breakdown the class B net into Class C nets. If we take a look at Table 0.1, we see that there are 65,534 possible nodes in that network. That is really too many to manage.

However, if we considered each of the third octets to represent a sub-net of our class B network, they would all have 254 possible nodes per sub-net. This is basically what a class C net is anyway. We can then assign each sub-net to a department or building and then assign one person to manage each of the class C sub-nets, which is a little easier to do.

To keep the different class C subnet from interfering with each other, we give each sub-net a Class C subnet-mask, although the first octet is in the range for a Class B network. That way machines on this subnet are only concerned with packets for the subnet. We can also break down the sub-nets physically so that there is a gateway or router between the subnets. That way the physical network is not overburdened with traffic from 65,534 machines.

Let's look at a concrete example, like SCO, which has been assigned the Class B address 132.147.0.0. The different departments within SCO have been assigned a class C address that might look like this: 132.147.144.0. Although the first octet (132) says that this is a class B address, it is really the subnet-mask that makes that determination. In this case, our subnet mask would be: 255.255.255.0. Therefore, any packet that is destined for an address other than one starting 132.147.144.0 is not on this network.

It is the responsibility of IP to ensure that each packet ends up going to the right machine. This is accomplished, in part, by assigned a unique address to each machine. This address is refereed to as the Internet address or IP address. Each network gets a set of these IP addresses that are within a specific range. In general, packets that are destined for an IP address within that range will stay within the local network. Only when a packet is destined for somewhere outside of the local network is it "allowed" to pass.

In other words, IP is responsible for the delivery of the packet. It functions similar to the Post Office. Whereby you have both a sending and receiving address. Often times you have many more letters than a single mail bag can handle. The mail carrier (or someone else at the post office) will break down the number of letters into sets small enough to fit in a bag. This is what IP does.

Since there are many people using the line a once, IP will break down the TCP packets into units of a specific size. Although often refereed to also a packets, the more correct terminology is to refereed to IP packets as datagrams. Just like bags of mail need to go from one post office to the next to reach their final destination, IP datagrams must often go through different machine to reach their final destination.

Saying that IP routing can be accomplished completely in software isn't entirely accurate. Although, no physical router is needed, IP can't send a packet to someplace where there is no physical connection. This is normally accomplished by an additional network card. With two (or more) network cards a single machine can be connected to multiple networks. The IP layer on that one machine can then be used to route IP packets between the two networks.

Once configured (how that's done, we'll talk about in the second part of the book), IP maintains a table of routing information, called (logically) a routing table. Every time the IP layer receives a packet, it checks the destination address

Routing and IP Gateways

I mentioned a moment ago that IP is an unreliable, connectionless protocol. That is it contains no provision to ensure that the packet arrives correctly at the destination, nor is there anything the guarantees that when packets do arrive they arrive in the correct order. Although IP is responsible to ensure that the packets get to the right machine, it has essentially no understanding of the physical connection between the two machines. IP will happily run on machines that are connected with something as simple as a telephone wire, to something as complex as satellites. IP depends on some other means to "physically" carry it across the network.

What this means is that the system administrator (or network administrator) is responsible for laying the "map" that is used to define which network address go with what sets of machine and what IP addresses are assigned to individual machines.

One important job that IP does is routing. That is, getting the packet to the right machine. If the source and destination machines are directly connected, that is on the same network, then routing is easy. Essentially there isn't any. IP sees this fact and simply hands the packets off to the data link layer. Otherwise, IP has to figure out how and where to send it.

Usually the 'how' is over a router. A router is some piece of hardware that acts like an air traffic controller send one packet off one way and other off a different way. Often routers are separate pieces of equipment that can be configured in very detailed ways. The disadvantage to this is that with power comes price. The ability to configure a router in many different ways usually means a high price tag. Fortunately, many operating systems, including SCO UNIX allow IP to serve as router-software. Thereby avoiding the cost of router hardware.

In comparison to the router is the concept of a gateway. Like a router, a gateway has knowledge of other networks and how to reach them. In general, we can think of a router as a special piece of hardware that does the work for us. In fact, there are companies that sell equipment called routers. A gateway is more of a concept, in that is the means by which you go from one network to another. Today, the distinction between a router and a gateway is blurred. Routers can server as gateways, gateways can serve as routers.

The path the packet takes from one machine to the next is called a route. Although each machine can maintain static routes for specific destinations, the default gateway is usually used to find remote destinations. (The default gateway is needed only for computers that are part of an internetwork.) If you have a gateway connect to several other networks, there will (probably) be route definitions for each of those other networks.

Let's look at this process as if we were sending a letter, as we did a little while ago. Each letter we send has an envelope which contains a message. On the envelope we write the source and destination addresses. When we mail the letter it gets to the post office and the person sorting the mail checks the destination zip code. If it's the same as the local zip code, the envelope is sent to one of the carriers for deliver. If the zip code is different, then it is sent to some other location. Perhaps all non-local mail is sent to the same place.

If you live across the country from me, the letter probably doesn't go directly from my local post office to yours. (assuming I don't live in San Fransisco and you don't live in New York) The same applies to IP packets. My letter first goes to my local post office, if it is destined for a local address it is processed there. If not, it is sent along to a larger post office. If I sent a letter from Santa Cruz, destined for Annsville, Pennsylvania, it will probably go first to San Fransisco and then to New York (or Philadelphia) before it gets sent to Annsville.

Again, the same applies to IP packets. If I were communicating with a network on the other side of the country, my machine needs to know how to get to the other one. This is the concept of a "gateway". A gateway is the first step in the path, or "route" to the remote machine. Just as there are a couple of post office between Santa Cruz and Annsville, there can be multiple gateways between computers.

Since San Fransisco is the closest "major" city to Santa Cruz, it is possible that all mail bound for points beyond must first go through there. What if I lived in Fresno, which is about halfway between San Fransisco and Los Angeles? If I sent a letter to Annsville, it could go through Los Angeles or it could go through San Fransisco. To make things easy, it might always get sent through San Fransisco it not destined for a local address. What if the letter is bound for Los Angeles? It seems silly to go through San Fransisco first when it is bound for LA. At the post office in Fresno, they might have a special procedure that says all remote mail goes through San Fransisco, except for those with a zip code in a special range.

Here, too, the same applies to IP addresses. One machine my be defined as the "default" gateway, but if an IP packet was bound for a particular network it could be told to use a completely different gateway. Which gateway to use to get to a particular machine or network is the concept of "routes." If I want all remotely-bound packets to use a particular route, I add that route as a default to my machine. If packets bound for a particular network are to go via a different route, I can add that route as well.

Figure 0-4 Network gateway

When IP prepares to send a "message", it inserts the local (source) and destination address IP addresses in the IP header. It then checks whether the network ID of the destination and source match (the zip codes). If so, the packet is sent directly to the destination, since it is on the local network. If the network IDs don't match, the routing table is examined for static routes. If none are found, the packet is forwarded to the default gateway.

The default gateway is a computer connected to the local subnet and other networks that has knowledge of the network IDs for other networks and how to reach them. Because the default gateway knows how to reach the other networks, it can forwards the packet, either to other gateways or directly to that machine if the gateway is on the same network as the destination. This process is known as routing.

Obviously(?), if you only have a single network, there is no reason to have a gateway. As each machine is directly connected to every other. It's possible that you only want certain machines within your network to go beyond the local net to the outside. In this case, these machine can have a default (or static) route default, while the others have none. However, users can add routes themselves, using the route command.

The telnetd daemon is a server which supports the telnet program. Makes sense huh? Telnet is a terminal program that allows you to work interactively with remote machines, just as if you would with the local machine. When inetd receives a incoming telnet request, it invokes telnetd.

What you then see is no different that if you had logged in locally to that machine (probably). You are presented with a login: prompt, you enter you logname and password. If these are correct, you then are given a shell that you can enter commands starts applications, etc.

The way telnetd works is that it allocates a pseudo-terminal device for you. This pseudo-terminal has the same behavior as a "normal" terminal in that you input commands and see the results on your screen. Internal the pseudo-terminal is broken down into two parts. The master portion is the side that you see. Since your side is the one that is controlling things, your side is the master. The master side accepts input from your telnet program and passes them to telnetd on the remote side. As you might guess, the side that has to listen to the master is the slave. The slave side of the pseudo-terminal serves as stdin, stdout, and stderr for the remote application.

Figure 0-5 Pseudo-ttys

Similar in functionality to telnet is rlogin. The server for rlogin, is rlogind, and like telnetd, is started by inetd. One of the primary differences is that, if configured, rlogind can provided a connection without the normal login procedures.

The functionality of rlogind is very similar to that of telnetd. Pseudo-terminals are allocated and the slave portion becomes the stdin, stdout, and stderr. During login, rlogind uses an authentication procedure called "host equivalence", which sets up remote machines as being "trusted". If rlogind on the destination machine authenticates the source machine, the user is automatically logged in. If the authentication fails, the user must go through the normal login procedure. How to set up host equivalence, we'll get to later.

TCP connections are not the only ones that are managed by inetd. Basically all network connections are. This can be understood if we go back to the telephone operator analogy. If the operator (inetd) is also the receptionist, we can then think of TCP connections as incoming telephone calls and UDP packets as incoming letters. Like incoming phones calls, the receptionist is responsible to route the incoming mail to the right person. (This is a valid analogy as like regular mail, there is nothing to guarantee the delivery of the message in the letter, although with TCP connections you can ask your partner to resend the message.) Like TCP connections, UDP daemons are "listening" on specific ports. Also like TCP, connections these well-known ports are listed in /etc/services.

One common UDP connection is the routing daemon: routed. Routed supplies (as you might have guessed) routing information in the form of routing packets. If your system is serving as a router, then routed periodically sends copies of its routing tables to other machines.

One key difference is that routed is not actually started by inetd. Instead, it is normally started as the system is entering run-level 2 through the /etc/rc2.d/S85tcp script. When it starts, routed makes the assumption that it will forward packets between all interfaces on the system. This only includes those that are "up" and does not include the loopback driver. (The loopback driver is a special TCP/IP interface that simply loops the packets back to the local machine. Hence the name.) Routed then transmits a REQUEST packet on each of these interfaces and waits for a RESPONSE packet for any other hosts. Potentially there are other machines on the network that are also sending REQUESTS packets, so routed can also respond to them.

The response routed gives is based on information it has in it's routing tables. This contains information about known routes, including how far away the destination machine is in turns of hops or intermediary machines. When routed receives a RESPONSE packet, it uses the information contained in that packet to update it's own routing tables. Look at the routed(NADM) man-page for more information.

DNS - Finding Other Machines

If you have TCP/IP installed, by default, you machine is set up to use the /etc/hosts file. This is a list of IP address and the matching name of the machine. When you try to connect to another machine, you can do it either with the IP address or the name. If you use the name, the system will look in the /etc/hosts file and make the translation from name to IP address. The only real drawback with this scheme is that every time a machine is added or removed from the network, you have to change the /etc/hosts file on all the affected machines.

Those you of that have had to administer large networks know that updating every /etc/hosts filelike this can be a real pain. There is always at least one that you forget or you mis-type the name or address and have to go back and change it on every machine. Fortunately, there is hope.

Provided with both ODT and OpenServer is a hostname/IP address database called the Berkeley InternetName Domain (BIND) service. Instead of updated every machine in the network, there is a Domain Name System (DNS) server that maintains the database and provides the client machines with information about both addresses and names. If machines are added or removed, there is only one machine that needs to get changed. This is the Name Server. (Note: The SCO documentation translates DNS as Domain Name Server. Most every other reference I have found calls it the Domain Name System. I have seen some references call it Domain Name Service. Since we know what it is, I'll just call it DNS.)

So, when do you use DNS over the /etc/hosts file? Well, it's up to you. The first question I would ask is "Are you connecting to the Internet?" If the answer is "yes", "maybe" or "someday" then definitely set up DNS. DNS functions somewhat like directory assistance from the phone company. If your local directory assistance doesn't have the number, you can contact one in the area you are looking. If your name server doesn't have the answer, it will query other name servers for that information. (assuming you told it to do so.)

If you are never going to go into the Internet, then the answer is up to you. If you only have two machines in your network, the trouble setting up DNS is not worth it. On the other hand, if you have a dozen or more machines, then setting it up makes life easier in the long run.

There are several key concepts that need to be discussed before we dive into DNS. The first is DNS, like so many other aspects of TCP/IP, is client-server oriented. We have the name server containing the IP addresses and names which serves information to the clients. Next, we need to think about DNS operating in an environment similar to a directory tree. All machines that fall under DNS can be thought of as files in this directory tree structure. These machines are often referred to as nodes. Like directories and file names, there is a hierarchy of names with the tree. This is often referred to as the domain name space.

A branch of the DNS tree is referred to as a domain. A domain is simply a collection of computers that are managed by a single organization. This organization can be a company, university or even a government agency. The organization has a name that it is know by to the outside world. In conjunction with the domains of the individual organizations, there are things called top-level domains. These are broken down by the function of the domains under it. The top level domains are:

COM - Commercial

EDU - Educational

GOV - Government

NET - Network

MIL - Military

ORG - Non-profit organizations

Each domain will fall within one of these top-level domains. For example, there is the domain sco, which falls under the commercial top-level domain. It is thus designated as sco.COM or sco.com. The domain assigned to the White House is whitehouse.gov. The domain assigned to the University of California at Santa Cruz is ucsc.edu. (Note that the dot is used to separate the individual components in the machine's domain and name)

Keep in mind that these domains are used primarily within the US. While a foreign subsidiary might belong to one of these top-level domains, for the most part, the top level domain within most non-US countries is the country code. For example the geographical domain Germany is indicated by the domain abbreviations de (for Deutschland). There are examples, however. I do know some German companies within the com domain. There are also geographic domains within the US, such as ca.us for California as compared to just .ca for for Canada. This is often for very small domains or non-organizations, such as individuals.

Figure 0-6 Internet domains

Within each domain, there may be sub-domains. However, there doesn't have to be. You usually find sub-domains in larger domains in an effort to break down the administration into smaller units. For example, if you had a set of machines that was for use by Or, if SCO Tech Support had a sub-domain it might be support.sco.com.

Keep in mind that these are just the domain names, not the machine, or node name. Within a domain there can be (in principle) any number of machines. A machine sitting on the desk in the oval office might be called boss1. It's full name, including domain would be boss1.pres.whitehouse.gov. A machine in SCO Support called darkstar would then be darkstar.support.sco.com. This might be extended further if there was a further division within support so that the TEAM support analysts had their own sub-domain, the machine might be darkstar.team.support.com. In principle, this can go on quite a ways. Up to now, I have only seen a machine name with five components: the machine name, two sub-domains, the company domain and then the top-level domain. On the other hand, if there was no support sub-domain, and everything was under the sco.com domain, the machine's name would be: darkstar.sco.com.

You may often see the fully-qualified domain name (FQDN) of a machine listed like this:

darkstar.sco.com.

Including the trailing dot(.). That dot indicates the root domain. This has no name other that root domain or .(read "dot"). Very similar to the way the root directory has no name other than root or /. In some cases this dot is optional. However, there are cases where is it required and we'll get to those in the section on configuring DNS.

Like files, it is possible that two machines have the same name. The only criteria for files is that their full path be unique. The same applies to machines. For example, there might be a machine darkstar at the whitehouse. (Maybe Bill's a closet Dead Head) It's FQDN would be darkstar.whitehouse.gov. This is obviously not the same machine as darkstar.sco.com any more than 1033 Main Street in Santa Cruz is the same as 1033 Main Street in Annsville. Even something like darkstar.support.sco.com is different from darkstar.sales.sco.com.

A zone is a grouping of machines that may, or may not, be the same as a domain. This is the set of machines over which a particular name server has authority and maintains the data. In our example above, there might be a zone for support, even if there was no sub-domain. On the other hand, there might be a team.support.sco.com domain, but the zone is still sco.com. Therefore, zones can be sub-ordinate or superior to domains. Basically, zones are used to make the job of managing the name server easier. Therefore, what constitutes a zone depends on your specific circumstances.

In the SCO implementation of DNS, there are five types of servers: primary, secondary, caching-only, slave and client. Although the SCO doc list the last one as a server (they also call it a remote server), I hesitate to use that designation. A remote server doesn't serve anyone but itself. It must use the DNS software to resolve all requests through a another server.

A primary server is the master server for one or more DNS zones. Each server maintains the database files, and is considered the authority for this zone. It may also periodically transfer data to a secondary server, if one exists for that zone.

DNS functions are carried out by the Internet domain name server, named. When it starts, named reads it's configuration file to determine what zones it is responsible for and in which files the data is stored. By default, the configuration file /etc/named.boot. However, named can be started with the -b option to specify an alternate configuration file. Normally, named is started from /etc/rc2.d/S85tcp.

SCO DNS provides a stub server. This is used when a primary server delegates a portion of it's name space (like a sub-zone) to another server. The primary server for the zone needs to know the name service information about the files within that sub-zone. One way is serving as a secondary server to that sub-zone or serving as a stub server. Note that the concept of a stub server is new to OpenServer so you cannot designate an ODT machine be a stub server. A stub server is like a secondary server, except that it only caches the NS and SOA records. With this information it know what machine is the name server and how long the information is valid.

For example, the primary server for the sco.com domain needs to know about the machines within the support.sco.com domain. It could server as a secondary server to the support.sco.com domain, whereby it would maintain all the records for the machines within that sub-domain. If, on the other hand, it servers as a stub server, the primary for the sco.com need only know how to get to the primary for the support.sco.com sub-domain. Note here, that it is possible for a server to be primary in one zone and secondary in another.

By moving responsibility to the sub-zone, the administrator of the parent zone, does not need to concern him or herself with changing the configurations files when a machine is added or removed within the sub-zone. As long as the address of sub-zone primary server remains matches the stub server entry all is well.

A secondary server takes over for the primary, should the primary go down or be otherwise inaccessible. A secondary server maintains copies of the database files, and "refreshes" them at predetermined intervals. If it cannot reach the primary to refresh it's files, it will keep trying at (again) predetermined intervals. If after another predetermined time, the secondary still cannot reach the primary, the secondary considers it's data invalid and flushes it.

Caching-only servers saves data in a cache file only until that data expires. The expiration time is based on a field within the data that is received from another server. This is called the time-to-live. Time-to-live is a regularly occurring concept within DNS.

A slave server can be a primary, secondary, or caching-only server. If it cannot satisfy the query locally, it will pass, or forward, the request to a fixed list of forwarders (forwarding server), rather than interacting directly with the primary name servers of other zones. These request are recursive, which means that the forwarder must answer either with the requested information or saying it doesn't know. The requesting machine then asks the next server, then the next and then the next until it finally runs out of servers to check or gets an answer. Slave servers never attempt to contact servers other than the forwarders.

The concept of recursive request is in contrast to iterative requests. Here the queried server either gives an answer or tells the requesting machine where it should look next. For example, darkstar asks, iguana, the primary server for support.sco.com for some information. In a recursive query, iguana asks, boomer, the primary server for sco.com and passes the information back to darkstar. In a iterative query, iguana tells darkstar about boomer, and darkstar then goes asks boomer. This process of asking name servers for information, whether recursive or iterative is called resolution.

Keep in mind that there is client software running on the server. When an application needs information, the client DNS server asks the server for the information, despite the fact that the server is running on the same machine. Applications don't access the server directly.

There is also the concept of a root server. These are severs located at the top of the domain tree and maintain information about the top-level zone. Root servers are positioned at the top, or root, of the DNS hierarchy, and maintain data about each of the top-level zones.


Your Own IP Address

If you have a network that is completely disconnected from the rest of the world, then there is no need for you to adhere to any of these conventions. You might be a commericial organization, but still want to use the EDU domain. Nothing prevents you. There is also nothing preventing you from using IP addresses that are used by some other organization. However, once you decide to connect to another organization or the Internet at large, you need to ensure that both your names and IP address are unique.

To ensure you use a unique name and network, the best thing is to contac the Network Information Center, or NIC. Via email, they can be contact at [email protected] The telephone is 1-800-365-3642 or 1-703-802-4535. By regular mail, use:

DDN Network Information Center

14200 Park Meadow Drive, Suite 200

Chantilly, Virginia 22021

If you are not in the United States, the NIC can still provide you with a contact in your area.

If you would like to have one machine that connects to the internet, but have other machines that cannot. One solution is to use one of the IP address defined in RFC 1597. This RFC describes the need for "private" address and lists a range of class A, B and C addresses that can be used internally within a company.

Some routers will filter out this addresses automatically, other require that they be so configured. This allows you to not only limit access to and from the internet, but also limits the need for unique addresses. If you only have a handful of machines that need Internet access, some internet providers will sub-net a Class C address and assign you a small block of addresses. See the RFC for more details.

NFS

The Network File System (NFS) is an industry standard means of being able to share entire filesystems among machines within a computer network. As with the other aspects of networking, the machines providing the service (in this case the filesystem) are the servers and the machines utilizing the service are the clients. Files residing physically on the server appear as if they are local to the client. This enables file sharing without the hassle of copying the files and worrying about which one is the more current.

One difference that NFS filesystem have over "conventional" filesystem is that it is possible to allow access to a portion of a filesystem, rather than the entire one. The directory you want the server to make available are said to be exported.

Normally, filesystem under NFS are mounted just like any other. The same options apply and they can be mounted either automatically at boot through entries in /etc/default/filesys or manually by the system administrator. This can be a problem sometimes as the server might not be active when the client boots. Even if the server is active, mounting NFS filesystems on boot-up slows down the boot process.

Figure 0-7 An NFS mount

A solution that SCO has provided is automount and as its name implies is can automatically mount NFS filesystems. Once configured, any command or program that accesses a file or directory on the remote machine within the exported directory forces the mounting to occur. The exported directory remains mounted until it is no longer needed.

To aid in preventing conflicting access requests to file, SCO provided the Network Lock Manager (NLM). This consists of a device driver and several daemon programs that support record and file locking. Remote Execution (REX) is a service that allows users to export their user environments to remote server in order to execute commands. Commands that can access files in the user's current directory and allow the execution of interactive processes such as full-screen editors.

Keep in mind that NFS is not a stand-alone protocol. It relies on the TCP/IP protocol suite in order to function correctly. NFS uses UDP and not TCP. Therefore, it is up to the programs using NFS to ensure the reliability of the data and not just assume the information coming across is correct. NFS will tell you if the data are there by setting the appropriate failure indicators. This causes your program (such as cp) to fail.

Under ODT 3.0, filesystems are made available for remote mounting by using the exportfs command. Normally this happens at boot, however the system administrator can run it at anytime. When run, exportfs uses information contained in the /etc/exports file to determine which directories and with what characteristics.

The /etc/exports file is a simple ASCII file and additions or changes can be made with any text editor. There is one line for each directory you want to export. The left side is the full path of the directory you want to export and the right side is options you want to apply. For example, you can limit access to the directory to just one machine or make the directory read only. If changes are made, exportfs needs to be run again to make changes take effect.

If your site is on the Internet, the /etc/exports file can be a problem. If you export a filesystem, but do not specify who has access, then everyone does. If you are on the Internet, everyone who can access your machine can mount your filesystems. Therefore, I recommend that you explictely list which machines can mount the exported filesystems.

OpenServer provides a graphical interface to this functionality in the form of the Filesystem Manager of SCOAdmin. SCOAdmin can also be used to start the NFS daemons if they are not already running as well as mount and unmount the remote filesystems.

I learned the hard way that configuring a remote filesystem to be mountable through /etc/default and automount are mutually exclusive. If both are configured, automount is disabled. This caused me quite a few headaches as I was first learning NFS. You can also user automount with the Network Information System (NIS) in that automount maps are exported along with the other NIS maps. This ensure that each NIS system automatically gets access to the same NFS filesystems. (NIS is a method of automatically exchanging files between different machines, which we'll get into in a moment.)

The term exporting is used to describe how NFS makes local directories available to remote systems. These directories are then said to be exported. Therefore, an exported directory is a directory that has been made available for remote access. Sometime the term importing is referred to the process of remotely mounting filesystems, although mounting is more commonly used.

If you can access a filesystem under OpenServer, you can access it under NFS. This is because the access to the file is a multi-step process. When you first access a file (say opening a text file to edit it). The local system first determines that this is an NFS mounted filesystem. NFS on the local system then goes NFS on the remote system to get the file. On the remote system, NFS tries to read the file that is physically on the disk. It is at this point that it needs to go through the filesystem drivers. Therefore, if the filesystem is supported on the remote system, NFS should have no problem accessing it.

Once a filesystem has been exported, the client sees the filesystem as an NFS filesystem and therefore what type it is really irrelevant. The filesystems support by OpenServer as of this writing are: AFS and EAFS, DOS, DTFS, High Sierra (CD-ROM), HTFS, ISO-9660 (CD-ROM), SCO Gateway for NetWare, Rockridge CD-ROM, S51K and XENIX.

There are a couple of limitations with NFS. First, although you might be able to see the device nodes on a remote machine, you cannot access the remote devices. Think back to the discussion on the kernel. The device node is a file that is opened by a device driver to gain access to the physical device. It has a major and minor number that point and pass flags to the device driver. If you open up a device node on a remote system, the major and minor numbers for that device node point to drivers in the local kernel.

Also, NFS-ity is not transitive. That is, you cannot access an NFS file system that is mounted by another machine. Keep in mind, too, that the newer filesystems in OpenServer can have more inodes than those in earlier releases. Since the number of inodes possible is few on older systems, an ODT machine mounting an OpenServer machine via NFS might not see every file.

The Flow of Things

There are two daemon processes that provide the NFS services on the server. These are mountd and nfsd. Mountd is responsibility for checking access permissions to the exported filesystem. When a clients tries to mount a filesystem, mountd return a pointer to the filesystem if the client has permission to mount it.

The workhorse on the server side is the nfsd daemon. It has the responsibility of handling all filesystem requests from the clients. Once a filesystem has been mounted, all access to the data on that remote filesystem is made through nfsd. Remember that you could be exporting directories and not just entire filesystems. Therefore it's better to say that access to the mount point and below is made through nfsd.

On the client side is biod, which handles block reads. Biod is the Block IO Daemon. Handling just block I/O is an important aspect. If a process is doing operations that do not require block IO, then there little performance loss by having them each do their own RPC (remote procedure call) system calls. However, when you start moving blocks around, NFS needs to ensure things don't get jammed up and will use the kernel buffer cache. (RPC is a away to execute procedures or functions on a remote machine as if their were local to your machine.)

Also key to this who process is the portmapper, portmap. The portmapper converts TCP/IP port numbers to RPC program numbers. What this means is that when the NFS starts up, it registers its port with the local portmap process. The clients access the server by asking the portmapper on the server for the port number of nfsd and mountd. The port number is then used on all subsequent RPC calls.

In principle, mounting a remote filesystem is like mounting a local one. The general syntax is:

mount <options> <filesystem> <mountpoint>

One of the primary differences is that since we are an NFS filesystem, we have to explicitly tell mount by using the '-f NFS' option. We can also include other options such as '-r' for read only. Let's assume that we have an our two machines scoburg and siemau. On siemau is an NFS filesystem that we want to mount from scoburg. Assuming that the proper entries exist in the /etc/exports file on siemau, the command on scoburg might look like this:

mount -f NFS siemau:/usr/data /data

Like other filesystems, the local mount command parses the command into tokens and ensure that entries don't already exist in the mount table (/etc/mnttab) for either the filesystem or the directory. Realizing that this is a remote filesystem, mount gets the IP address for siemau (by whatever means are configured on the local machine) and gets the port number of mountd on siemau. The mount command then passes mountd the pathname of the requested directory (/usr/data).

Now it's the server's turn. To makes sure it can service the request, mountd must first check /etc/exports for the requested filesystem. In this case /usr/data. If scoburg is permitted, mountd passes back what is called a file handle, or pointer. Now the mount back on scoburg uses that file handle and the mount point (/data) as arguments the mount() system call. Finally, an entry is place in the local mount table.

There are two primary NFS configuration files: /etc/exports and /etc/default/filesys. The /etc/exports file exists on the server and list those files and directories that can accessed by remote hosts. It can also be configured to allow or deny access to specific hosts. Since these are filesystem, you can manage anything mounted by NFS through /etc/default/filesys. This allows you to mount remote filesystems at boot or in any way you can with a "normal" filesystem. One advantage NFS has over local filesystems is that you can configure them to be mounted only when you need them. That is, if the files in the directories are not used, the connection is not made. However, if the files are needed, the connection is automatically made. This is the concept of automounting, which we will get into later.

Accessing files across a network present a whole new set of problems that need to be addressed. One of the important things is that particularly when using NFS, each user and group has to be unique throughout the network. Well, to be quite honest, saying "has to be unique" is not entirely true. "Should be" would be more accurate. Remember that an SCO system only sees the UID as a number. If you have the same LOGNAME on two machines, but two different UIDs, both systems will sees these as two separate users. This can run you into trouble when copying files between the two systems. When systems are running NFS, NIS comes in handy to prevent such mismatches.

SCO NFS also gives you the ability to lock files to prevent multiple users from accessing it. The mechanism used allows the system to more effectively clean-up in the event of a system crash or other abnormal shutdown. This is handled by lockd and statd. Lockd is the Network Lock Manager and is responsible for file and record locking. Lockd runs on both the client and the server, processing both lock requests and lock releases as well as recovers (or at least attempts to recover) in the event of a system crash.

It is the responsibility of the Network Status Monitor (statd) to provides application processes with host status information. Like lockd, statd runs on both the client and server machines to monitor each other's status (as well as their own). Part of the status that is monitored is that of locks. When a state change occurs, lockd need to be told, which is done by the local statd.

Let's assume that a client were to go down improperly, such as crashing or somehow the network connection dropped. Well, it's too late now for the client statd to do anything. However, when it comes back up, the client statd can tell the server statd "Hey, I just crashed." Since any process that had a lock on a server file no longer exists, the server lockd can free those locks. When the server comes back up after a crash, it's statd needs to tell the statd on the clients. The client statd's need to tell their lockd's which can then "reclaim" the locks still on the server.

Automounter

The /etc/exports file contains entries for directories that can be exported

to NFS clients. This file is read automatically by the exportfs(NADM)

command. If you change this file, you must run exportfs(NADM) for the

changes to affect the mount daemon's operation.


Only when this file is present at boot time does the NFS startup script

execute exportfs(NADM) and start the NFS file-system daemon, nfsd(NADM),

and the mount daemon, mountd(NADM).

Automount provides you with the ability to mount NFS filesystems only when you need them, automatically. They are automatically mounted by automount, hence the name. Actually, conventional NFS mounted filesystems can also be mounted automatically in the sense that you can configure them in /etc/default/filesys and they are automatically mounted as the system boots. Automount filesystems, on the other hand, are mounted when a user tries to access files or directories under the mount point. Also, if the files or directories are not accessed within a specific time, (five minutes by default) they are unmounted, thereby saving network resources. When booting you also save time since the system is waiting to connect to a remote machine that possibly could be down.

One disadvantage that automount has compared to traditional NFS mounts is the automounted directories cannot be dynamically added. When changes are made to the configuration, automount must be restarted. Now, this doesn't require a reboot of the system, however it is one extra step. In my opinion this is more than compensated by the fact that whereas traditional NFS mounts are hard coded (meaning one machine:one mount point), automount can be configured to attempt to mount a secondary system if the first one is not available.

Keep in mind that the server side is oblivious to the fact that the request is coming from automount. As far as it knows it is just your normal every day NFS mount, therefore automounter can be used with system that don't know about it.

Don't think of automount as your only means of mounting NFS filesystem just because of it's advantages. If you are constantly accessing certain filesystems, then you gain nothing by making them automounted. In fact, you might lose something since each time the connection is made, you need to wait. If mounted in the conventional manner, then you only need to wait once. If you have filesystems that are accessed regularly, but others that are accessed only on occasion, you simply mount some at boot and the rest via automount.

A common use of automount is with NIS. Filesystems mounted via automount are maintained in files, called maps. NIS is used to distribute configuration files from the NIS server across the net to the NIS clients. Why not include the automount maps in the set of files that is being distributed? This could be useful if you wanted to have all the documentation on a single machine to save space and access to the doc is made through automount. Since doc is not being constantly access, this saves the problem of having the filesystem containing the doc be continually mounted. Another use is when you want each user to have the same home directory no matter where they are. If mounted by automount and distributed via NIS, every time they logged in, no matter on what machine, they would have the same home directory. Granted, there is the problem of not being able to access their home directory if the server is down. However, that problem still applies when login into a single machine.

The automount daemon, is normally started at boot up via the NFS start-up script, /etc/rc2.d/S89nfs, (The name may be slightly different) however, it can be started by hand. Usually, automount is started with the -f flag and the name of it's configuration file, which by convention is /etc/auto.master. This contains a list of mount points and where the configuration information for that mount is found. (We'll talk more about these files in the chapter on configuring your network

In reality, automount behaves very similarly to traditional NFS mounts. The system knows that the specified directory is an NFS mount point. When something is accesses on the other side of the mount point, the automount daemon reacts to the request basically the same way nfsd does with a normal NFS filesystem. The automount daemon then checks the mount table (/etc/mnttab) to see if the filesystem is already mount and mounts it if it isn't. Once the file system is mounted, requests are handle normally.

One point of interest is the mount points themselves. File systems mounted via automount are not where you think they are. Instead, all automount filesystem are mounted to sub-directories under /tmp_mnt and symbolic links point to these directories from what we see as the mount points. Because of the use of symbolic links, you need to be careful when changing directories up through the mount point when using automounter.

Like other filesystems, an entry is maintained in the system mount table (/etc/mnttab) for all filesystem that have been mounted with automounter. When the timeout has expired (five minutes by default), automounter removes the entry from /etc/mnttab, but still maintains a copy in its memory. This copy is updated whenever mounting or unmounting a filesystem.

There are four types of automount maps: master, direct, indirect and built-in. At first, they are a source of a fair bit a confusion, at least with me and many people I know. A direct mount points to a real directory. The direct maps are used to specify direct mounts. Each of these maps contains a separate entry for each direct mount point. If you need to, (or are looking for something to do) you can create nested maps. This is where direct maps point to other maps. The contents of each entry is the full path of the mount point, mount options and the path to the remote directory (or nested map).

An indirect mount points to a virtual directory, which is managed by automount and doesn't really exists. At least that's the way the SCO doc describes it. So, what does it mean? Let's take an example of each and hopefully that will clarify things.

First, let's look at what happens with a direct mount. Let's say we want to set up the system so that help files were accessed through automount on the machine scoburg. This would then be the filesystem scoburg:/usr/lib/scohelp and for simplicity's sake, let's say we are mounting it on the local /usr/lib/scohelp directory. When local user runs scohelp, the mount point is crossed and automount goes into action.

The first thing it does is to create the mount point if it doesn't already exist. Remember the real mount-point is not where we think it is. Instead, in this case, it would be /tmp_mnt/usr/lib/scohelp, which automount may created if necessary. Next, the filesystem is mounted onto /tmp_mnt/usr/lib/scohelp, as if you had issued the command:

mount -f NFS scoburg:/usr/lib/scohelp /tmp_mnt/usr/lib/scohelp

Since it may be the case that /usr/lib/scohelp does not yet exist, automount would then create a symbolic link from /tmp_mnt/usr/lib/scohelp to /usr/lib/scohelp. Now, whenever you change directories or access a file in /usr/lib/scohelp, you are really accessing them on scoburg.

An indirect mount, works different. (Obviously, since why have two things that behave the same). Let's say that I not only want to have the help files on a remote machine, but the man-pages as well and we wished to mount them under /usr/lib, as /usr/lib/man (so we know where everything is, for example). We could create multiple direct mounts or we could use the concept of indirect mounts. Basically, what appears to happen is the same as with direct mounts. First, a mount point is created under /tmp_mnt if necessary. Next, automount mounts the remote directory onto the mount point under /tmp_mnt. Finally, the symbolic link is created. Note that automount only does this when something under the mount point is accessed. If, in our example, /usr/lib/man was accessed only it would get mounted, but /usr/lib/scohelp is untouched.

The interesting thing is that using indirect mounts, the directory /usr/lib/man and /usr/lib/scohelp do not exist. If you were to do a listing of /usr/lib, these two directories would not be there. Because of the fact these directories don't really exist, you can make changes to indirect maps without having to restart automounter.

As I mentioned before the master map points to the files containing the other three maps. It contains the name of the mount point and which file to look in for the configuration information, as well as any mount options. By default this is /etc/auto.master.

Because the mount point directory does not exists for indirect mounts, the map file contains just the name of the parent and not the full path. Therefore, the indirect map is dependent on the master map to tell it where to mount the filesystem.

Built-in automount maps are used to save time and effort if many machines are going to be accessed. One of the maps is -hosts, which is used to mount all exported file systems from all known hosts. Using this option you cannot choose where you want the filesystems mounted as they are all mounted as a unit. All are mounted to a single mount point, which is /net, by convention, but you can choose whatever one you want.

All directories mounted from a single server are mounted in the same fashion. First, there is a sub-directory based on the name of the server from which the filesystems are exported. For example, the filesystems mounted from scoburg would be under /net/scoburg. Next, all directories from that server are mounted at the same time whenever any directory is accessed. Lastly, every directory is mounted with the same options, or default options if none are specified.

The -passwd map is a way of getting automount to automatically mount each user's home directories, to that machine where they login in. The limitation is that all home directories must reside on the same machine. In order to create automount home directories that are on different machines, you will need to create either direct or indirect mounts.

NIS

One of the major problems administrators have with networked systems is the administration of users and user accounts. In the "before time", the system administrator had to create accounts on multiple system if users needed access to each of them. Shell scripts could be written to automate the task, but there was always the possibility of users not be copied to every system. The administrator then had to go back and make sure that the user in question got copied everywhere.

Then along came the Network Information Service, or NIS. NIS is a system service that forms part of SCO's NFS package. Although supplementing each other, running NIS does not mean you have to mount remote filesystems, nor does mounting remote filesystems mean you have to run NIS. Like DNS, the primary function of NIS is to ensure consistency of information across the machines within a network. Also, like DNS, there is a central machine that is the "master" of all the others in that all the information is maintained on this one system and then propagated to the others.

As we progress through the inner workings on NIS, you'll notice that many of the utilities and databases begin with the prefix 'yp.' This is historical and dates from the time when NIS was referred to as Sun "Yellow Pages" or simply "YP." The problem is that "Yellow Pages" is a registered trademark in Great Britain and can't be used without permission, so Sun (the developer) renamed it to NIS.

Like NFS, NIS works in a client-server fashion to propagate the information. The server, or master, has the source copies of several key files that it then copies to the other servers. Servers that simply receive the files for their own purposes and do not propagate them further are referred to as slaves or "copy-only" servers. In the following discussion I will referred to both slaves or copy-only servers as "clients." First, I get bugged by the term slave and second it's easier to type. If there is a distinction to be made, I will be sure to make it. Keep in mind that true clients do not manage any information themselves. That is, the do not receive any files from a server.

One important aspect is the ability to expand the list to cover other files. By default, the following files are propagated:

/etc/passwd

/etc/group

/etc/hosts

/etc/ethers

/etc/networks

/etc/netgroup

/etc/protocols

/etc/services

Under NIS, there is the concept of a domain. While this is comparable to an Internet domain, an NIS domain simply defines a group of machines sharing information. An NIS domain may be composed of machine entirely within a single Internet domain or even within an MMDF domain. However, there is no other connection between these types of domains, other than the label. NIS domains can spread across or encompass the other kinds of domains as well. Normally the domain name is established when NIS is first configured, but can also be set using the domainname command. If the Internet domain name matches the NIS domain name, this is just a coincidence.

When you initialize NIS, the information for the various files is compiled into several databases. These databases are referred to as maps, which are then propagated to the clients. Once on the client side, these maps are then converted back to their original ASCII format.

Each of these maps is a database itself, called an ndbm database as they are created using system calls to the ndbm library. When the maps are received by the client (slave or copy-only server) they are converted back to the ASCII files and are available locally. These files exist in pairs in the /etc/yp/<domainname> directory. The .dir files are "directories.” These are bitmaps of indices used by the DBM database. The .pag files are the data files. Because of the nature of the files, you cannot simply copy them from one machine to another by hand. The databases contain both a time stamp and the identify of the source. Using the timestamp, the client can determine if the information it gets is newer than what it already has. By identifying the master as well, the client can request an update directly.

When NIS is initialized the original files with the same name as NIS file are backed up in sub-directories under /usr/lib/nisrt. For example, there will be a /usr/lib/nisrt/etc directory that contains the files (among others) group and passwd. Certain binaries are copied into these "parallel" directories as well. One such file is /bin/passwd. This is copied into /usr/lib/nisrt/bin and then /bin/passwd is linked to /etc/yppasswd.

Security and NIS

For the most part, this is a contradiction of terms. Where NIS is used to open up the network, security is used to close it. This means that NIS is substantially more difficult to administer when using either high or improved security. This is because the TCP will not allow NIS to modify either /etc/passwd or /etc/group in either of these modes. Because you are propagated password and group information, systems running NIS cannot really be C2 secure. Servers, for that reason, cannot be configured at the higher security levels and run NIS. On the other hand, clients and copy-only servers can. They can receive maps, but not propagate them. If operating in a higher security level, the maps cannot be translated into their ASCII forms.

If you were familiar enough with the way the TCB it put together, you could come up with a way to override these safeguards. Just as you could if you were not running NIS. The big question is "Why?" If you implement a higher level of security, then you want to use it. Circumventing the built-in restriction to "make things easier" is comparable to having root's password taped to the console monitor. If you do decide to do it and either C2 or NIS breaks, you are on your own. This is unsupported and you were told not to do it.


Serial Network Protocols

Serial Line Internet Protocol (SLIP)

The first protocol in the TCP family to be developed to run across a serial line was the aptly name Serial Line Internet Protocol (SLIP). One advantage that SLIP provides over PPP is simply its age. There are still many computers that are running comparatively old systems and some do not understand PPP. As a result, the only way to make a serial line TCP connection was SLIP.

Basic to SLIP is the concept of a link, or SLIP link. A SLIP link is a serial communication path between the two computers. It is over this link that the data is transferred as serialized IP packets. That is, the systems sends the IP packets to the SLIP driver, which then encodes them to be able to be sent across the serial line. In principle, just like PPP. Also like PPP, SLIP can be configured to use either a single serial line connecting the two machines, or the machines can be connected via mode across telephone lines.

Improvements have been made to SLIP between ODT and OpenServer. Originally, you were required to have specific IP addresses defined for each end of the link. Although this did not mean that each side has a permanent IP address, but rather the IP address had to be defined as the link was being created. The SLIP implementation in OpenServer provides for dynamic IP address allocation at connect time. It still supports dedicated IP address allocation, as well.

Links are established using the slattach command which takes as parameters the local connection (either as a tty name or a UUCP site name), the IP address of both sides of the connection and other IP and serial line attributes for this link. When the link is established, SLIP has created a network interface, which functions in principle the same way as the network interface you have with a Ethernet card, or PPP for that matter. Like the other types of network interfaces, a SLIP interface can be monitored with netstat and accessed using telnet, rlogin, ftp or any other network utility.

The SCO implementation of SLIP in OpenServer provides for up to 64 simultaneous serial network links. This includes both SLIP and PPP (which we'll talk about shortly). Each of these connections can be initiated from either side and on an "as-needed" basis. Therefore, if you only have one serial line, this can be used for either UUCP, SLIP or PPP.

There are two ways to make the link "inactive", but only one way I would recommend. The first is to simply mark it down using ifconfig. The problem is that this simply makes the link inaccessible, but does not remove it. Since the link is still there, the connection does not stop. Therefore if this link is on a phone line, you can experience some unexpectedly high telephone bills. The safer alternative actually removes the link. This is done by simply killing (i.e. with kill -15) the slattach process. This removes the SLIP interface, so please make sure that there are no more processes accessing the interface as you are liable to have angry users banging on your door.

The primary command when using SLIP is slattach or SLIP Link attach. SLIP is attached to a serial port and creates either a dedicated, dynamic incoming or dynamic outgoing link. Each end of the link is given a network (IP) address, based on the parameters passed to slattach. Once communication is establish, the link functions essentially the same as any other network connection. Applications on both sides of the link can communicate, just as if it were Ethernet, for example. Unlike UUCP that also uses serial lines, multiple processes can be using the connection at the same time.

The newest SCO implementation of SLIP provides a means to identify and filter out specific types of IP packers. This is accomplished through the /etc/pppfilter program, which, as you might have guessed from the name, is shared with PPP. This file is created either when the SLIP STREAMS stack is installed or by PPP and, by default, only contains comments until you add the filters you want.

Like the other network protocols, SLIP resolves names either by the /etc/hosts file or using the Domain Name Service (DNS). The newest implementation also can be configured to connect to sites listed in /usr/lib/uucp/Systems.

A dedicated SLIP connection is one in which communication is made between hosts over a specific (dedicated) link. Either side may initiated the link, however this is (usually) across the same line. This type of connect can only be used with a serial line that is always up and dedicated to a single connection. This can include regular serial cables or leased telephone lines

A dynamic incoming line is paired with a dynamic outgoing line on the remote line (or vis-versa). The incoming line must accept the login and password information forwarded by UUCP to establish the connection. This type of connection allows the local system to accept incoming SLIP connection requests just like normal logins. That is the calling system connections to the destination machine and must get past getty. Therefore, this type of SLIP link requires a login account on the side being called. When SLIP is configured using the Network Configuration Manager, a slip account is created. Rather than a normal shell, this account gets a special shell script that initiates the slattach.

On the calling machine, that is, the side initiating the SLIP link an entry must exist for the destination machine in the /usr/lib/uucp/Systems file. This must include the slip account name and password for the remote side.

SLIP packet filtering By default, all packets are allowed through a particular interface. Packet filtering allows you to control which packets pass through the SLIP interface and which do not. It is possible to filter out packets on several different criteria. For example, you may choose to allow only TCP or ICMP packets or you may want to disallow TCP and UDP packets. You could also choose to filter out packets going to a particular destination or those coming from a particular source. Depending on your needs, you can configure filtering for each link individually talk about netstat and other network tools

Point-to-Point Protocol (PPP)

SCO PPP is another means by which network connections can be made across serial lines. Despite the limitations that might be imposed by serial lines, PPP supports the same programs under TCP/IP that are supported with other media. PPP has certain advantages over faster network connections, such as Ethernet, in that it does not require the specialized hardware. In addition, PPP can be configured to dial up across a modem and make the connection only when necessary. This allows network connections to anywhere in the world without the cost of other methods. PPP provides advantages over other serial line protocols like UUCP in that multiple, concurrent sessions are possible as well as mixing programs such as telnet and ftp.

The way PPP works is that the serial lines are linked to the IP layer through what is called the PPP STREAMS stack. This stack consists of several different modules: packet filter module, the PPP driver, asynchronous HDLC protocol module, and the STREAMS message to clist conversion driver.

Like the other protocols at the physical layer, PPP is responsible for encapsulating the IP packages for transmissions and unpacking them up receipt. Each connection (local host to remote host) is referred to as a PPP link. Because this link represents the physical connection, Each PPP link requires a unique IP address on both sides of the link. Since this is a network link, both sides need to be on the same network.

Like any other network connection, each layer of PPP must be installed. This includes starting the PPP daemon (pppd). Normally, pppd is started by the /etc/rc2.d/S85tcp (which is linked to /etc/tcp). However, the PPP configuration file (/etc/ppphosts) must be present for pppd to start. Starting pppd does not necessarily mean that connections are made.

Use of the same PPP link requires only that the applications use the same IP address to specify the remote host. By using different IP addresses to specify the same remote host, two applications can also use individual PPP links to the same host, provided a separate serial line is available for each separate address and both addresses have unique PPP network interfaces. Once a PPP link is established, it remains active until the administrator marks the interface down or the idle timer expires. You can use ifconfig to mark an interface down. The idle timer is set in the PPP link configuration.

The version of PPP provide with OpenServer has many improvements over its predecessor. This includes support for up to 64 simultaneous links; communication initiated in either direction; dynamic acquisition of the ports (accessed as needed, thereby allowing lines to be shared with UUCP); dynamic IP addressing, including IP address pooling; dynamic reconfiguration which does not require relinking the kernel or rebooting the machine; packet filtering; Negotiation of link and IP parameters. Authentication of the local host by the remote and of the remote host by the local. PPP MIB support (for SNMP).

SCO PPP provides two methods of authentication in the form of two protocols: Password Authentication Protocol (PAP) and Challenge-Handshake Authentication Protocol (CHAP). If authentication is enable, the administrator can choose either one of these methods (provided the other side understand also supports it). The authentication is on a per-link basis so you can enable it for some, disable it for others, as well a choose the authentication method you want.

Mailers

MMDF

If you haven't made changes and are using mail locally, then you are using MMDF. MMDF stands for Multi-channel Memorandum Distribution Facility and is the default mail system used on SCO UNIX systems. It is a collection of programs that allows relatively complex mail configurations to be quickly established and easily maintained.

SCO has a built-in mechanism (mkdev mmdf) that allows to you automatically configure MMDF. Often the limitations of this automatic procedure require administrators to go beyond the basics. When this happens, you need to know how the parts of MMDF fit together and interact with the rest of the system.

In this section, we're going to talk about the basics of MMDF. This is more than just an overview of the functionality. We're going to get into the interactions between the various files so that if something goes wrong, you will be in a much better position to find the cause of the problem.

Before we get into things we need to first talk about a couple of terms: Mail Transport Agent (MTA) and Mail User Agent (MAU). The user agent is the program that the user sees, such as /bin/mail. It hands off the message to the transport agent, which does the processing and sends it on its way. The default transport agent on SCO UNIX systems is MMDF as is independent of which user agent is being used. (I will be using the terms "transport agent" and "user agent" because I feel these abbreviations tend to be distractions.)

MMDF provides transport through several different methods. The method used is referred to as a channel. Sets of machines are grouped together in domains, which may or may not be equivalent to the Internet domains we talked about earlier. Usually (by default), the MMDF domains are based on what channel is used to get to each machine. However, other criteria such as geographical location is possible.

When a user wants to send a message, he or she starts up whatever program is normally used (the user agent), compose their message and send it off. The user agent hands the message off to the submit program. Submit then finds the destination machine by looking through the configured domains and then sends the message into the appropriate channel. Once in the appropriate channel, the deliver program starts the program that will actually do the transfer.


INSERT FILE: 10_7.CDR

Figure 0-8 The Flow of MMDF

When mail reaches the other end the process is reversed. The receiving end of the transport agent hands the message to deliver to be processed further. If the message is destined for a user on that machine, it is place in that machine's local channel. If not, it is processed further and sent through the channel appropriate for the next hop.

There are three channels that are primarily used on SCO systems: local, uucp and smtp. The local channel is used for mail between users on the same machine and is configured by default, even if you've never touched MMDF. The uucp channel (logically) uses UUCP to transfer mail between remote machines. MMDF hands the message to UUCP along with a remote execution command to sent the job through the remote mail system. The smtp (Simple Mail Transport Protocol) channel is used on systems running TCP/IP.

Prior to SCO UNIX Version 3.2. Release 4.0, all of the configuration files had to be created by hand. This meant that there were the inevitable errors and calls to support. With SCO UNIX 3.2.4.0, a new utility was introduce to ease the MMDF configuration: mkdev mmdf.

Despite it's relatively simple nature, the mkdev mmdf script is able to configure MMDF with very little trouble. Even if you have a relatively complex setup, mkdev mmdf can be used to configure your system.

The mkdev mmdf script is fairly straight forward in what it does. You are prompted for information about you local system such as its name, who should root's mail get send to and so on. You are then asked questions on the different channels you intend to use. As you answer, the mkdev mmdf script is storing the information you input and will use it later to build the appropriate files. In OpenServer, the mkdev mmdf script actually starts the MMDF Manager. This is much more extensive that the old mkdev mmdf script. Since mkdev mmdf in OpenServer calls the MMDF Manager, I will just refer to the script for simplicity's sake.

Knowing what to answer is not always easy. However, going through the mkdev mmdf step-by-step will do little for your understanding of what is happening, nor will it allow you to go beyond the basic configuration or troubleshoot problem systems. In order to do that, we need to look out how MMDF is laid out and how the files interact.

You might want to log in as either root or mmdf and go into the /usr/mmdf directory. This way you have complete access to all the files we need to look at. Let's start first with an overview of the directory structure. There are two places you will find MMDF files: /usr/mmdf and /usr/spool/mmdf. You only need to look at the spool files when problems arise and you need to start tracking it own. For that reason, I am going to put off talking about them until later.

The /usr/mmdf directory is the heart of the MMDF sub-system. This contains all the binaries and configuration files that mmdf uses, plus the log files it writes to as it does it work. Is this directory is a least one file: mmdftailor. (If you've run mkdev mmdf at least once then there is also mmdftailor-). This is the central MMDF configuration file. Most of the information provided by this file is a result of our responses to questions when we run mkdev mmdf.

Although this file is built by mkdev mmdf, it is important to know the structure of this file should this be misconfigured or you want to enhance your system. Therefore, we are going to take a quick look at it. I will be addressing the entries that you re likely to find on your system and not give you extensive details on what could be there. For details on each of the individual entries, take a look at the mmdftailor(F) man-page.

By default, the mmdftailor is broken down into four functional areas. Provided they appear in the right order in relationship to each other, there is no absolute order that things must appear. In fact, in SCO UNIX Release 3.2 Version 2, the arrangement was much different that the current release. The order I will be describing below is simply the convention used by SCO. Open Server changed the layout of mmdftailor, once again. Entries are grouped by their type and not by channel. However, the concepts we will address here are the same.

The top portion is the administrative portion. Here, we define the description of our system, such as the host and domain name. It is here that you would define such things as an alternate location for your users' mailboxes (normally it's /usr/spool/mail) or set up name hiding. Name hiding is where you hide the name of the machine where the mail originated and simply indicate the domain. We get into this more in the section on configuring MMDF.

The next section is used for aliasing. This is where you define the files, or tables, to look through when aliasing user names addresses. This is useful when you want to send mail to people at remote sights without having to remember long addresses. The MTBL lines tell us the names of the alias files to use. The name= entries is a kind of alias that will be used in the ALIAS lines to configure that particular alias file. Keep in mind that the name= line must appear before the others as this defines the alias entry that will be used later.

The ALIAS lines point to files (through the name= entry) that are to be used when establishing user aliases. If you look, each line points back to a different alias file. Here only the name of the file is listed, as the system knows to look in /usr/mmdf/table. In addition to the file name, there are configuration options for the ALIAS entries.

The next section is where the individual channels are defined and configured. Like alias, there are MTBL entries for each channel, with aliases defined by the name= entries. There is one defining the domain file and one defining the channel file. The behavior is the same as for the alias file as an alias is defined for each of the domain of channels files and reference made to the alias. Along with the file name entries there are entries defining the domain specific characteristics (MDMN) and the channel specific characteristics (MCHN).

Even if configured correctly, there are two channels that do not have a corresponding domain. These are the badhosts channel and the baduser channel. The badhost channel is used whenever you have addressed mail to a address that your local machine does not know about. If configured, the system will try to send to a machine that you have defined as your badhosts machine. This is not necessarily a bad thing. All it means is that your local machine does not know how to get there. If your bad hosts does, then all is well.

This is commonly used when your email is routed through another host. Your machine need not know how to deliver anything other than local mail (or mail internal to your company). There may be only one machine in your company that knows how to get to outside. All mail directed to address outside the company goes through this one machine. If that machine is listed as your badhosts machine and it know how to go to the rest of the Internet, you will have no problem delivering mail.

The baduser machine works in the same fashion, but for users that the local machine does not know about. If you were to mail to a user (without any address) that your machine didn't know about, MMDF could be configured to send it to a baduser machine. This, however, is normally only used within a company. Often there is a central mail server that knows about all the users. (and maybe all the machines) in the company. Any user that the other machines don't know about end up going here. This machine could also be the badhosts machine for the company, which in turn has a badhosts machine outside of the company.

At the bottom of mmdftailor file is where logging and locking is normally configured. Locking is used to prevent concurrent access to a user's mailbox in order to keep the mailbox consistent. Logging is just as the name suggests.

Although details of each of these are listed in mmdftailor, I must mention one aspect of logging. By default, logging is only turned on for fatal errors (FAT). This is normally sufficient. If you are having mail problems, you may often need to turn on full trace (FTR) logging. Be careful! If this is turned on a single correctly addressed message will increase the log file by 10K! If you have a busy system, you can quickly have log files that are several megabytes in size.

When doing some MMDF testing with on my system while in support, I came in one Monday morning to find some very disturbing messages on my screen. Sometime over the weekend my hard disk had filled up and I was now getting messages that the root had disk was full. When I finally got it, I discovered that I had a 45MB MMDF log file!

That about covers the basic entries that you find in mmdftailor. There a dozens of value that can be set, most of which are not set by default or have default values defined within MMDF. Since they are all listed in the mmdftailor(F) man-page, I won't go into details about them here. Note that in OpenServer, no logging or locking is defined by default, so each takes on a default value.

Within the /usr/mmdf directory, there are several sub-directories. As you would expect, the bin directory is where the binary programs are kept. This is where both the deliver and submit programs reside. The remaining programs serve various functions as MMDF is running it's course. Two important ones are checkque and checkaddr, both of which we discuss in more detail in the section on configuring MMDF.

The chans directory contains the channel programs that deliver calls to send message along through the correct channel. The log directory is just as its name implies it is where MMDF writes logging information. Also as its name implies, the tmp directory is for temporary files that MMDF creates as it's going about its business.

The last directory, table, is where the address oriented configuration information is kept. There are three type of files of importance here: alias files, domain files and channels files. As it is processing mail, MMDF used the contents of these files to determine where mail should go.

In our discussion of mmdftailor, we talked about the ALIAS, MCHN and MDMN entries. As you see from their corresponding MTBL entries, these relate to the files in /usr/mmdf/table. For example, let's take the MTBL entry:

MTBL name=lalias, file="alias.list", show="Mailing list aliases"

This refers to a file alias.list Within the /usr/mmdf/table directory there is a file alias.list. In this line:

MTBL name=locdom, file="local.dom", show="Local Domain"

There is a reference to the file local.dom. There is also a file local.dom in the /usr/mmdf/table directory. As I mentioned above, the name= entry is an alias for that file. In subsequent lines, you see that alias being used.

The alias files (alias.*) are used in several instances. For example, if you have someone to whom you regularly send mail and they have a long address that is annoying to type in. You could create an alias for this person that was easier to type. Perhaps you wish to send to one address, but have it be received by 10 people. This can be done with alias files as well. Note that in OpenServer, the breakdown on the alias files is substantially different, although the principles are the same. We go into more details in the section on configuring MMDF.

Next are the domain files (*.dom). The domain files contain the names of every machine you want to send mail to and any form of their address that you might want to use. By convention the machines are group by the channel that is used to connect to that machine. However, it is also possible to group them according to other criteria such as geographical location.

The channels files (*.chn) contain the address that will be used to connect to this machine. Since addresses are dependent on the channel being used, the convention here too is that the names of these files is also the name of the channel.

To understand the flow of things, it is important to understand the relationship of these three files to the MMDF system. To do that we need to side track again. When you run mkdev mmdf, it calls a program (dbmbuild) that combines the data from these files in the order they appear in mmdftailor into a database. When you mail something, MMDF doesn't need to look through all of the files, just this one database.

Since all that is being done is a look-up in this database, it is important to maintain the correct format. All three sets of files have the same format:

alias: real-name

Although I use the words "alias" and "real-name," this format also applies to both the channel and the domain files. The left side is basically what you want to call it and the right side is what it "really" is. How this applies to domain and channel files we'll get into in a moment.

A thing to note is that if there is an entry in an alias file that matches a name, the address will be translated to the alias even if there is a valid user with that name. For example, assume there is a user 'jim' on the system. If there was an entry in the alias.user file:

jim: [email protected]

All mail sent to jim would end up going to [email protected] instead of the local user jim..

At first this may seem a little odd. However, there is some logic behind it. This is the mechanism that allows mail sent to the system users mmdf, uucp, etc to end up going to root. If the aliases didn't work like this, then there would be no way to redirect mail from uucp or mmdf to root. Also, if you have accounts on several machines, you can get your mail on just one.

If there are no defined alias that match, MMDF attempts to deliver the mail locally. If there is a user by that name on the local system, all is well. If not, MMDF will attempt to deliver it into the baduser channel. If there is one defined, the processing continues. If there is no baduser channel defined, you end up with getting you message sent back with an error saying that this is an unknown user.

If there is a host or domain name with that address, MMDF bypasses the alias entries and looks directly in the domain entries. Because there is no rule as to where machine names go, MMDF must look through all of the domain entries the database.

To understand how the search is accomplished, let's not look at the database, but rather the source files: the *.dom and *.chn files. Although this is not what happens in reality, it is what happens conceptually and helps understanding what is happening. In essence, MMDF searches the left side of all the configured domain files until it finds a match.

Note that you often need to specify every way you wish to refer to a machine. For example, if you wanted to contact SCO's support bulletin board machine, sosco, you would need entries for sosco, sco.com and sosco.sco.com. (Assuming you wanted to be able to use all off them to contact sosco. If you wanted, you could have a entry that simply said 'sco' that pointed to sosco as well.)

Once MMDF has found the entry in the domain file, it begins searching all the channel files. Again, this is because there is no rule that says where machine names can go. Theoretically, you could have a machine listed in the local.dom file, but the address is in uucp.dom. Once it has found a match in a channel file, it is able to send the message through the appropriate channel.

Note again that this "search" of the *.dom and *.chn files does not actually take place. The database is searched. This description is used only as an illustration of what "appears" to happen. The advantage of this method is that once this database is created, MMDF usually needs to only look through a single, relatively small file, instead of several, larger ones.

After parsing the database and determining what channel the message needs to be sent through, the message is place in the appropriate queue directory. These are located in /usr/spool/mmdf/lock/home. There is one directory of the form q.<channel> for each <channel> configured. In addition, there are two other directories here: msg and addr.

The files in q.<channel> are linked to the files in addr and contain administrative information about the message like the sender, recipient and the queue. At first, it may seem odd to have duplicate files, like this. However, this makes sense when you consider the different delivery modes, or "priorities" you can configure MMDF for.

In the MCHN entry for each queue in the mmdftailor is a mod= entry, which specifies the delivery mode. There are several different kinds of modes, including "imm" for immediate and "reg" for regular. Immediate means just that. Messages will be passed along to the transport agent immediately. Even if deliver is not running! Regular means that the message will sit in the queue until delivery starts up again. (The default is once every 10 minutes.)

Although it makes sense for local mail to be sent immediately, it may not make sense for others, depending on the circumstances. Take for example a situation where your only contract to the Internet is via modem through a provider that is not in your local calling area. Once the remote modem answers, the clocks starts ticking. The phone company doesn't care that you are trying to negotiate the protocol. All it cares about is the fact that the phone line is actually being used. If mail through this channel were sent immediately, every message would have to reconnect and waste time. Instead, you queue the messages up, run delivery by hand and then waste the time needed to connect only once. An alternative would be to set the deliver program to run at larger intervals, such as every 120 minutes.

What does this have to do with the links in q.<channel> and addr? Well, if delivery is running on its own, by default it delivers through every channel. To save time, all it needs to do is look through a single directory rather than several. On the other hand, if you want to deliver mail through a single channel, MMDF does not have to search through the files meant for other channels.

The messages themselves, which includes all the header information, is stored in the msg directory. This are essentially the same thing you would see when reading your mail. When deliver runs (either as a daemon or starting it by hand) it searches through the various directories here, looking for messages to send. It is at this point that the message is handed off to the channel program. In the case of local mail, deliver passes it off to /usr/mmdf/chans/local which appends the message to the user's mailbox. If the message is to be sent through UUCP, delivery starts up /usr/mmdf/chans/uucp which creates an UUCP transfer job as well as a UUX remote execution job which will start rmail on the other end.

Sendmail

When you compare sendmail to MMDF, it is like comparing shell scripting to C programming. MMDF is quick to configure and easy to maintain. Although you can manager fairly complex mail systems with MMDF, there are some things that you just can't do. Just like there as some thing you cannot do with shell scripts that you can with C. Because it is the default mailer and requires you to answer just a few simple question to get a base installation working, MMDF is, by far, the most common mail system on SCO machines.

Because of this and the intrinsic complexity of sendmail, I felt that a detailed discussion of sendmail was beyond the scope of this book. If you would like to learn more, both the ODT and OpenServer doc provide you with a good introduction. There is also a Nutshell Handbook from O'Reilly & Associates called sendmail by Bryan Costales with Eric Allman and Niel Rickert that goes into substantial details about sendmail. (It's almost 800 pages!!)

I would like to apologize to those of you who are disappointed by me excluding sendmail from our discussion. However, in my own defense I need to simply say that space considerations prevent my from including this as well as several other topics. Plus you can't please all the people all the time. Instead, I am attempting to please as many people as possible.


Mosaic and the Web

It was difficult to decide where to put this topic. You can't have access to the Web without networking, however, it looses much of it's impact unless you are using a graphical interface like X. Because the Web a network of machines accessed in a common manner, I figured the networking chapter would be the best place to talk about it. I think this is a good choice since, there are character based programs that do not require X.

So what is the Web. Well, as I just mentioned, it is a network of machines. Not all machines on the Internet are part of the Web, but we can safely say that all machines on the Web are part of the Internet. The Web is the shortened version of World Wide Web, and as its name implies it connects machines all over the world.

Created in 1989 at the internationally renowned CERN research lab in Switzerland, the Web was originally begun as a means on linking physicists from all over the world. Because it is easy to use and integrate into an existing network, the Web has grown to a community of tens of thousands of sites with millions of users accessing it. With the integration of Web access software, on-line services have opened the Web up to millions of people who couldn't have used it before.

What the Web really is, is a vast network of inter-linked documents, or resources. These resources may be pure text, but can include images, sound and even videos. The links between resources are made through the use of the concept of hypertext. Now, hypertext is not something new. It has been used for years in on-line help systems, for example, like those in MS-Windows' programs and more recently SCOHelp. Certain words or phrases are presented in a different format (often a different color or maybe underlined). These words or phrases are linked to other resources. When you click on them, the resource that is linked is called up. This resource could be the next page, a graphics image, or even video.

Resources are loaded from their source by means of the hypertext transfer protocol, http. In principle, this is very much like ftp, in that resources are files that are transferred to the requesting site. It is then up to the requesting application to make use of that resource, such as display and image or playing an animation. In many cases, files are actually retrieved using ftp instead of http and the application simply saves the file on the local machine.

The application that is used to access the Web is called a Web browser. What is used in OpenServer and the Global Access (Web-in-a-Box) product for ODT 3.0 is Mosaic from the National Center for Supercomputing Applications (NCSA). SCO also incorporated the NCSA Mosaic into SCOHelp, providing it with the same hypertext capabilities as Mosaic. Built into SCOHelp is a "Web Browser Mode" which gives you the same functionality as the stand-alone Mosaic, which can also be run as /usr/bin/X11/Mosiac. Mosaic is available from the sosco bulletin board at SCO or on the SkunkWare CD as TLS033. There is a character based version called Lynx, which is available from the same sources as TLS045.

Web resources are provided by Web Servers. A Web Servers is simply a machine running the http daemon. In the case of SCO, this is scohttpd. Like other network daemons, scohttpd receives requests from a Web client (such as Mosaic) and sends it the requested resource.

Like the ftp daemon, scohttpd, is a secure means of allowing anonymous access to your system. You can define a root directory, which, like ftp, prevents users from going "above" the defined root directory. Access to files or directories can be defined on machine basis and you can even provided password control over files.

By default, the OpenServer installation sets up the scohttp server, which starts the scohhtp daemon using the configuration files in /var/scohttp. Like TCP, there is actually a script that is used to interface to the scohttp daemon. This is /etc/scohttp. There are several options that you can pass to it that will start, stop, enable, disable query the status or cleanup log files. The daemon itself is /var/scohttp/scohttpd. One argument it is passed is the location of its configuration files. By default these are the files in /var/scohttp/conf.

When scohttpd starts, it reads its configuration files and begins listening for requests from a document viewer (one that uses the HTTP protocol). On OpenServer, this is scohelp, man and Mosaic. If you are running ODT, then only Mosaic is available. When a document is requested, scohttpd checks for the file relative to the DocumentRoot (defined in /var/scohttp/conf/srm.conf). By default this is set to /usr/lib/scohelp. It is best to leave it like that. Otherwise, scohelp may not function correctly. If you want to make references to your documents, you can use symbolic links to point to places other than the DocumentRoot. For details on the configuration files, take a look at the scohttp(ADM) man-page.

Web pages are written in the Hyptertext Markup Language (HTML). This is "plain-text" file that can edited by any editor, like vi. Recently, as a result of the increasing popularity of the Web, several commercially available HTML editors have become available. The HTML commands are similar, and also simpler, that those used by troff. In addition to formatting commands, there are build in commands that tell the Web Browser to go out and retrieve a document. You can also create links to specific locations (labels) within that document. Access to the document is by means of a Uniform Resource Locator (URL).

There are several types of URLs that perform different functions. Several different program can be used to access these resources such as ftp, http, gopher, or even telnet. If you leave off the program name, the Web browser assumes that it refers to a file on your local system. However, just like ftp or telnet you can specifically make references to the local machine. I encourage using absolute names like that as it makes transferring Web pages that much easier.

For more details on HTML, take a look at the various HTML scripts in both /usr/man and /usr/lib/scohelp. You can load them into either Mosaic or SCOHelp. You can then select View Source from the File menu. Then compare the output to the source. (You need to set SCOHelp to Web Browser Mode from the Options menu). There is also a very large section on Writing HTML documents in Using Help.

Mosaic has some interesting features that allow you to move around quickly. First, Mosaic keeps track of where you have been. By using the Back button or the Back entry from the Navigate menu, you can backtrack through the pages you already visited. There is also a Forward (and a Forward menu entry) that allows you to move forward through the documents you visited.


If you have some places that you visit often, there is no need to trace through the route that brought you there or remember the URL. Instead, MOSAIC will remember this for you in the form of a "Hot List'. A "Hot List" is nothing more that a list of hot places that you've visited. There is a entry under the Navigate menu to add entries to the hot list as well as to jump directly to the document. The hot list is stored in $HOME/.mosaic-hotlist-default.

All that you need to access the Web is an Internet connection. If you can do ftp and telnet, then you can probably use the Web. So, assuming you have a Web browser and an Internet connection. The question is where do you go? The question is comparable to "Given a unlimited value plane ticket, where do you go on vacation?" The sky is the limit.

As I mentioned, the convention is that the Web server's machine name is www.domain.name. To access their home page, the URL would be http://ww.domain.name. For example, to get to SCO's home page, the URL is http://www.sco.com. In order to keep from typing so much, I will simply refer to the domain.name and you can expand it out the rest of the way. In some cases, where the convention is not followed, I'll give you the missing information.

Some sites are commercially oriented such as unidirect.com. This is from the company Unidirect, which provides mail-order UNIX products. The fedex.com site will allow you to track your package. If you are up late, doing some major net surfing, you might want to check out pizzahut.com. Here you can order a pizza on-line. However, as of this writing it was extremely limited in the areas it delivers.

Shortly after SCO came out with their Global Access product for ODT, the comet Schumaker-Levy 9 was making history by plowing into the backside of Jupiter. The Jet Propulsion Laboratory has a Web site, on which they regularly updated the images of Jupiter. I still remember my friends asking me if I had seen the "lastest" images. If they were more than three hours old, I would shrug them off as ancient history.

If you are interested in free software (did I say the magic word?), check out www.cdrom.com. You can download gigabytes worth of games and utilities and GIFs and ray-traces and source code and full-text copies of Alice in Wonderland. Most of these are available from sites spread out over the Internet. It's really nice to have them all it one place. The machine www.cdrom.com is the Web server for Walnut Creek CD. Although you could spend weeks downloading their archives, you don't need to. The CDROMs they offer are very reasonably priced. Use the Web site or you can even ftp to ftp.cdrom.com and get access to many of the CDs that they offer. I liked some of those that I found so useful, I subscribed. This saves you about $10 per CD and you get quarterly updates of many of them.

Another place that contains similar information is InfoMagic. While their offering is similar to Walnut Creek CD, InfoMagic does provide a few that Walnut Creek doesn't. They can be reached at www.informagic.com or email them at [email protected]

One of the CDs that I found very useful was the Internet Info CDROM. This contains a wealth of information about the Internet. This includes standards that are applicable to the Internet like IEEE and RFCs. There are also Frequently Asked Questions (FAQ) from some of the Usenet newsgroups.

The issue of Usenet newsgroups opens up a whole can of worms. Without oversimplifying too much, we could say that Usenet was the first, nation-wide on-line bulletin-board. Whereas the more commercial services like CompuServe store their messages in a central location, Usenet is based on the "store and forward" principle. That is, messages are stored on a message and forwarded to the next at regular intervals. If those intervals are not all that often, it may be hours or even days before messages are propagated to every site.

Messages are organized into a hierarchical, tree structure, very much like many things in UNIX. (although you don't have to be running a UNIX machine to be accessing Usenet. Groups range from things like rec.arts.startrek.fandom to alt.sex.bondage to comp.unix.admin.

Although I would love to go into more details, this really goes beyond the scope of this book. Instead, I would like to recommend Using UUCP and Usenet by Grace Todino and Dale Dougherty, and Managing UUCP and Usenet by Tim O'Reilly and Grace Todino, both from O'Reilly and Associates. In addition, there is a relatively new book that goes into more details about how Usenet is organized, what newsgroups are available and some general information about behavior and interaction with other when participating in a Usenet sendmail. This is Usenet Netnews for Everyone by Jenny Fristrup, from Prentice Hall.

If you would a good chucke, then check out iguana.images.com.


Network Technologies

Ethernet

SCO supports two of the major network types: Ethernet and token-ring. Ethernet could be labeled as the great grand-father of all the other network types. It was developed in the 1970s by Xerox for linking computers to printers. Although not very wide spread at first, Ethernet has since expanded to be (perhaps) the most widely spread type of network.

The principle behind Ethernet is called Carrier Sensing, Multiple Access with Collision Detection (CSMA/CD). What this means is that every machine on the net sits quietly listening for messages. When one of the machines needs to talk, it waits for a pause and jumps in to send its message. What if two machines simultaneously see the pause and start to send? Well, a collision occurs. This is detected by both machine which wait a random amount of time before they will try again. Although the random amount of time could be the same for both machines, it doesn't happen too often and each machine eventually gets to send its message. The one that didn't get it's turn will see that the other one is talking and wait.

Because there is no guarantee that a specific machine will ever get a turn on the net, this type of mechanism is referred to as a probabilistic access system, since each machine will probably get access to the system someday. Keep in mind that the busier a network is, the greater the chance for collisions and the greater the likelihood that there will be more waiting. This does not mean that more machines mean more collisions. If I am sitting at my machine doing all of my work locally, then the traffic on the network cause by my machine is minimal. However, once I make a connection, the traffic increases.

Ethernet appears in several different forms, depending on it's physical characteristics. Primarily, these fall into the IEEE specification 802.3, with an average speed of 10MHz. One thing I need to point out is that the original specification developed at Xerox is not what most people think about when they think about Ethernet. Rather it is the IEEE 802.3 standard.

The most popular ways Ethernet appears is 10Base5 (Thicknet), 10Base2 (Thinnet) and 10Base-T (Twisted-Pair). The general format of these labels is StypeL, where S is the speed of the cable in megahertz, type is the transmission system, in this case baseband versus broadband and the L is the maximum length of the cable in 100 meters. I have also heard that the last number indicates the thickness of the cable in tenths of an inch. Thicknet, as one would guess, is thicker than thin net, but both are coax cable. Twisted pair is similar is format to normal phone cable, but may often have eight separate wires.

Often times, the topology (layout) of your network is dependent on what kind of cable you are using. Because it requires a central hub, twisted-pair is usually laid out in a star, with the hub at the center. This is a star topology. Thin- and thickwire are usually be spread out in a line, or linear topology. This is also called a bus topology.

Token-Ring

Token-ring, developed by IBM, is embodied in the IEEE standard 802.5. The key concept in this type of network is the idea of a token. This is similar to a baton in a relay race when each machine must receive the token before it is allowed to go. If a particular machine has nothing to send, it simply passes the token on to the next machine. If it does have something to send, the message is "linked" with the token before it is send. By seeing the token linked to the message, the other machines know that the token is in use and pass it along to the destination. When the destination machine gets the entire bundle, it puts the token back on the network, with a tag to indicate that it received the packet. It is then passed to the originator as an acknowledgment. The originator then passes the token along to the next machine.

This scheme provides guaranteed access to the network since every machine will eventually get the token. Even if the originator of one packet has more to send, once it gets its acknowledgment back, it must pass the token along. If no others want to send anything, then it can come back to the first machine. However, the others were given the chance to send something. This method also provides reliability since the destination machine sends the packet back with an acknowledgment.

ISDN

For most of the life of electronic/electrical communication, the primary method of communication has been the telephone. As a result, there exists a network of cables and connection throughout the world that dwarfs the Internet in both number of connections and miles of wire. Wouldn't it be wonderful if we could take advantage of the already existing network? Well, we can. This comes to us in the form of a system called Integrated Services Digital Network, or ISDN.

ISDN is one of the fastest growing technologies, particularly in Europe. Local telephone companies are offering it as a replacement (or addition) to conventional telephone lines. As of this writing, the German phone company is offering cash incentives for businesses and private individuals to switch to ISDN. The primary benefit (at least to most end users) is that you can simultaneously send data across the same lines as your phone. For example, while you are speaking to a customer, you can be faxing them the spec sheets on your latest product across the same phone line. Although such functionality for both partners requires ISDN connections on both ends, your phone could be talking to their conventional phone and your fax could be talking to their conventional fax. However, from your office to the phone company is a single telephone connection.

If both sides are using ISDN, they need to be communicating in a fashion similar to a network (like with TCP/IP or IPX/SPX). Therefore, both sides know who is calling. Image getting a call from a customer and having your database automatically call up the record on that customer, even before you pick up the phone! This ability to integrate all these different services from voice to fax to data communication gives ISDN its name.

The key concept in ISDN is the idea of a digital data pipe between the end device and the service provider. Note that I didn't say between the two participants. This allows the service provide (the phone company) to switch between the ISDN connection on one side to the analog connection on the other. At the receiving end (your office) will be something similar to a switch box. As the packets come in from the service provider, this switch box will route the packets to the appropriate device. Each device is set with a particular ID number. This works conceptually the same way as SCSI IDs.

As of this writing, three types of connections have been standardized. The first is often referred to as the "basic" rate as it provides the necessary service for basic users such as homes and small businesses. This provides two 64 kbps channels for voice or data and one channel for "out-of-band" signaling. The "primary" service provides 23 voices or data channels, instead of just two. In Europe, this is increased to 30 channels. The third type provides a 4 KHz analog phone channel along with a 8 or 16 kbps data channel. This allows you to use your old analog phone along side the new ISDN device.

SCO Network Drivers

In ODT, network drivers used a concept called Link Level Interface, LLI. With LLI, the network protocol stacks sat directly on top of the LLI layer. This required that the LLI drivers had a certain knowledge of the workings of the protocol stack that was being used.

OpenServer introduced a totally new concept called MAC Driver Interface, MDI. MDI is now isolated from the protocol stacks by the Data Link Provider Interface (DLPI) layer. Therefore, new drivers can be added simply by interfacing them with the AT&T DLPI standard. On the other end, new protocols can be added and old ones modified. Again, simply by interfacing them to the DLPI standard.

Another advantage is that since much of the work of the LLI drivers is taken over by the DLPI layer, MDI drivers are 30-50% smaller. Despite that fact that the driver is smaller, the MDI drivers allow for more detailed driver statistics. Another advantage is that there is only one open on the device. (when the card is initialized) This decreases the likelihood of hangs and other problems, which increases the reliability of the interface.


Stay Tuned

Believe it or not, that's not all. Networking is more than just understand what parts are there. You need to be able to configure it correctly to make it work. In this chapter we just covered the concepts involved. Later, we're going to go into more details about how you configure your network software. So, stay tuned.



Next: The Computer Itself

Index

Copyright 1996-1998 by James Mohr. All rights reserved. Used by permission of the author.

Be sure to visit Jim's great Linux Tutorial web site at http://www.linux-tutorial.info/