Packet filtering is something I've always hard a hard time
getting my head around. Not the basics; that's easy enough. It's
just the incredible level of detail, the difficulty of keeping it
all in your head at once.
And then, of course, there are all the different flavors: ipfw,
ipfilters, ipchains, and now iptables. It gets more than a little
confusing, and I've never taken the time for more than a cursory
look at any of them.
Well, time to change that. I needed to learn more about iptables
because the SME Server firewall/mail server
I used to sell uses this. So..
The basic idea of any packet filtering is to look at a network
packet and decide what to do with it: accept it as is and let it go
on its way, stop it dead, or change it in some way (which usually
involves sending it somewhere other than where it was originally
Chains and Tables
Iptables starts with three built in chains. You can add more
chains, (generally for convenience). Let's understand what it comes
It is important to first understand what packets these chains
If a packet comes from this machine (is generated by an
application running on this machine), it will go to the OUTPUT
A packet coming TO this machine traverses the INPUT chain
A packet going somewhere else uses FORWARD only.
THAT'S NOT HOW IPCHAINS WORKS. A packet going
somewhere else never sees INPUT with iptables. Similarly, a
forwarded packet never sees the OUTPUT chain with iptables. In some
ways this makes iptables easier to understand, but if you have the
ipchains flow stuck in your head, it makes it confusing.
Another major difference is that iptables is
stateful; that is, it keeps track of each connection. You
can look at connections by examining /proc/net/ip_connact. Here's a
little bit from a machine:
Note that you can see an ssh and an ftp
You need the ip_connact module to have iptables
understand the relationship between the control and data sides of
an ftp connection. If that makes no sense right now, you might want
to read the ftp section in /Security/dslsecure.html. This
module is also used by the nat translation module.
There are other differences. The
lists most of them (there's a lot of other good iptables help at
But back to chains: why would you want to add your own chains -
it looks like the standard three pretty well cover everything?
True, but you'd usually do that so that you can apply the rules you
make for the new chain to other chains. A quick example:
You create a chain called "mychain" and add a bunch of rules to
it. You want both the INPUT and the FORWARD chains to use those
iptables -N mychain
iptables -A mychain -m state --state ESTABLISHED,RELATED -j ACCEPT
# .. more rules ..
iptables -A mychain -j DROP
iptables -A INPUT -j mychain
iptables -A FORWARD -j mychain
That saves the effort of writing out the same rules for both
INPUT and FORWARD chains. It's also, unfortunately, why
professionally written iptables firewalls are so hard to
comprehend: you have to follow them back through chain after chain
to figure out what's really going on. One chain will list several
other chains as targets for its various rules, and those in turn
may list others - it can be hard to follow.
Now for some more confusion. You can have more than one network
card on the machine. That's the whole idea of a firewall: one
interface to the internet, one or more to the internal lan. A
packet coming in on the external interface may be a FORWARD to it,
and an INPUT to the lan side. Therefore, you may have to write more
than one rule to control the packet.
Tables make this even more confusing. This is straight from the
There are current three independent tables (which tables
are present at any time depends on the kernel configura�
tion options and which modules are present).
This option specifies the packet matching table
which the command should operate on. If the kernel
is configured with automatic module loading, an
attempt will be made to load the appropriate module
for that table if it is not already there.
The tables are as follows:
filter This is the default table. It contains the built-
in chains INPUT (for packets coming into the box
itself), FORWARD (for packets being routed through
the box), and OUTPUT (for locally-generated pack�
nat This table is consulted when a packet that creates
a new connection is encountered. It consists of
three built-ins: PREROUTING (for altering packets
as soon as they come in), OUTPUT (for altering
locally-generated packets before routing), and
POSTROUTING (for altering packets as they are about
to go out).
mangle This table is used for specialized packet alter�
ation. It has two built-in chains: PREROUTING (for
altering incoming packets before routing) and OUT�
PUT (for altering locally-generated packets before
Does you head hurt yet? Mine sure did. It gets worse: while
those three are probably all you have at most, you could have more.
You can find out by "cat /proc/net/ip_tables_names".
This also introduces another complication: if you want to list
the rules for the chains, you also need to specify the table. If
you just do "iptables -L -n" (don't forget the -n to avoid wasting
time asking DNS to resolve your internal addresses), you only get
the filter table. To get them all, do something like:
for i in `cat /proc/net/ip_tables_names`
echo "Table $i:"
iptables -L -n -t $i
Got that all digested? Good, because now we have to learn about
extensions. Look in /lib/iptables or /usr/lib/iptables.
You should find a bunch of libraries, here are just a few:
Each of these are things you can use in iptables rules. We used
the "state" module in the user defined chains example above. That's
great, but how do you use these things? Well, some of them are
documented in the "man iptables" page, but they are also self
documenting. Try these:
iptables -p tcp --help
iptables -m state --help
iptables -j LOG --help
How do you know whether to use -p, -j or -m? Honestly, it can be
a little confusing, but if one doesn't work, try another- you'll
find it by trial and error if no other way. When you are reading
someone else's rules, you may need at least this to understand what
their rule is trying to do. You may also find
Writing iptables rules
There is no way that I'm even going to attempt to write firewall
rules. I will, when necessary, add to or modify someone else's
rules to do something needful that they didn't include. The level
of knowledge necessary for that is substantially less than that
required for actual authoring. Even that can be daunting, however:
these things can be very complicated.
There are iptables firewall generators available on the net. Use
Google and search for "iptables
firewall". Some of these are pretty well documented, so you can
learn quite a bit more about iptables by studying them.
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© 2012-11-29 Tony Lawrence