Encrypting syslog with stunnel
In this paper, I describe how to encrypt syslog
messages on the network. Encryption
is vital to keep the confidiental content of syslog messages secure. I describe the overall
approach and provide an HOWTO do it with the help of
rsyslogd and stunnel.
Syslog is a
clear-text protocol. That means anyone with a sniffer can have
a peek at your data. In some environments, this is no problem at all. In
others, it is a huge setback, probably even preventing deployment of syslog
solutions. Thankfully, there is an easy way to encrypt syslog communication. I
will describe one approach in this paper.
The most straigthforward solution would be that the syslogd itself encrypts
messages. Unfortuantely, encryption is only standardized in
RFC 3195. But there
is currently no syslogd that implements RFC 3195's encryption features,
so this route leads to nothing. Another approach would be to use vendor- or
project-specific syslog extensions. There are a few around, but the problem here
is that they have compatibility issues. However, there is one surprisingly easy
and interoperable solution: though not standardized, many vendors and projects
implement plain tcp syslog. In a nutshell, plain tcp syslog is a mode where
standard syslog messages are transmitted via tcp and records are separated by
newline characters. This mode is supported by all major syslogd's (both on Linux/Unix
and Windows) as well as log sources (for example,
EventReporter for Windows
Event Log forwarding). Plain tcp syslog offers reliability, but it does not
offer encryption in itself. However, since it operates on a tcp stream, it is now easy
to add encryption. There are various ways to do that. In this paper, I will
describe how it is done with stunnel (an
other alternative would be IPSec, for example).
Stunnel is open source and it is available both for Unix/Linux and Windows.
It provides a way to
use ssl communication for any non-ssl aware client and server - in this case,
Stunnel works much like a wrapper. Both on the client and on the server machine,
tunnel portals are created. The non-ssl aware client and server software is
configured to not directly talk to the remote partner, but to the local
(s)tunnel portal instead. Stunnel, in turn, takes the data received from the
client, encrypts it via ssl, sends it to the remote tunnel portal and that
remote portal sends it to the recipient process on the remote machine. The
transfer to the portals is done via unencrypted communication. As such,
it is vital that
the portal and the respective program that is talking to it are on the same
machine, otherwise data would travel partly unencrypted. Tunneling, as done by stunnel,
requires connection oriented communication. This is why you need to use
tcp-based syslog. As a side-note, you can also encrypt a plain-text RFC
3195 session via stunnel, though this definitely is not what the
protocol designers had on their mind ;)
In the rest of this document, I assume that you use rsyslog on both the
client and the server. For the samples, I use Debian.
Interestingly, there are
some annoying differences between stunnel implementations. For example, on
Debian a comment line starts with a semicolon (';'). On
Red Hat, it starts with
a hash sign ('#'). So you need to watch out for subtle issues when setting up
Overall System Setup
In ths paper, I assume two machines, one named "client" and the other named "server".
It is obvious that, in practice, you will probably have multiple clients but
only one server. Syslog traffic shall be transmitted via stunnel over the
network. Port 60514 is to be used for that purpose. The machines are set up as
- rsyslog forwards message to stunnel local portal at port 61514
- local stunnel forwards data via the network to port 60514 to its remote
- stunnel listens on port 60514 to connections from its client peers
- all connections are forwarded to the locally-running rsyslog listening
at port 61514
Setting up the system
For Debian, you need the "stunnel4" package. The "stunnel" package is the
older 3.x release, which will not support the configuration I describe below.
Other distributions might have other names. For example, on Red Hat it is just "stunnel".
Make sure that you install the appropriate package on both the client and the
server. It is also a good idea to check if there are updates for either stunnel
or openssl (which stunnel uses) - there are often security fixes available and
often the latest fixes are not included in the default package.
In my sample setup, I use only the bare minimum of options. For example, I do
not make the server check client cerficiates. Also, I do not talk much about
certificates at all. If you intend to really secure your system, you should
probably learn about certificates and how to manage and deploy them. This is
beyond the scope of this paper. For additional information,
http://www.stunnel.org/faq/certs.html is a good starting point.
You also need to install rsyslogd on both machines. Do this before starting
with the configuration. You should also familarize yourself with its
configuration file syntax, so that you know which actions you can trigger with
it. Rsyslogd can work as a drop-in replacement for stock
sysklogd. So if you know
the standard syslog.conf syntax, you do not need to learn any more to follow
At the server, you need to have a digital certificate. That certificate
enables SSL operation, as it provides the necessary crypto keys being used to
secure the connection. Many versions of stunnel come with a default certificate,
often found in /etc/stunnel/stunnel.pem. If you have it, it is good for testing
only. If you use it in production, it is very easy to break into your secure
channel as everybody is able to get hold of your private key. I didn't find an
stunnel.pem on my Debian machine. I guess the Debian folks removed it because of
You can create your own certificate with a simple openssl tool - you need to
do it if you have none and I highly recommend to create one in any case. To
create it, cd to /etc/stunnel and type:
openssl req -new -x509 -days 3650 -nodes -out
stunnel.pem -keyout stunnel.pem
That command will ask you a number of questions. Provide some answer for
them. If you are unsure, read
http://www.stunnel.org/faq/certs.html. After the command has finished, you
should have a usable stunnel.pem in your working directory.
Next is to create a configuration file for stunnel. It will direct stunnel
what to do. You can used the following basic file:
; Certificate/key is needed in server mode
cert = /etc/stunnel/stunnel.pem
; Some debugging stuff useful for troubleshooting
debug = 7
accept = 60514
connect = 61514
Save this file to e.g. /etc/stunnel/syslog-server.conf. Please note that the
settings in italics are for debugging only. They run stunnel
with a lot of debug information in the foreground. This is very valuable while
you setup the system - and very useless once everything works well. So be sure
to remove these lines when going to production.
Finally, you need to start the stunnel daemon. Under Debian, this is done via
"stunnel /etc/stunnel/syslog.server.conf". If you have enabled the debug
settings, you will immediately see a lot of nice messages.
Now you have stunnel running, but it obviously unable to talk to rsyslog -
because it is not yet running. If not already done, configure it so that it does
everything you want. If in doubt, you can simply copy /etc/syslog.conf to /etc/rsyslog.conf
and you probably have what you want. The really important thing in rsyslogd
configuration is that you must make it listen to tcp port 61514 (remember: this
is where stunnel send the messages to). Thankfully, this is easy to achive: just
add "-t 61514" to the rsyslogd startup options in your system startup script.
After done so, start (or restart) rsyslogd.
The server should now be fully operational.
The client setup is simpler. Most importantly, you do not need a certificate
(of course, you can use one if you would like to authenticate the client, but
this is beyond the scope of this paper). So the basic thing you need to do is
create the stunnel configuration file.
; Some debugging stuff useful for troubleshooting
debug = 7
accept = 127.0.0.1:61514
connect = 192.0.2.1:60514
Again, the text in italics is for debugging purposes only. I suggest
you leave it in during your initial testing and then remove it. The most
important difference to the server configuration outlined above is the "client=yes"
directive. It is what makes this stunnel behave like a client. The accept
directive binds stunnel only to the local host, so that it is protected from
receiving messages from the network (somebody might fake to be the local sender).
The address 192.0.2.1 is the address of the server machine. You must change it
to match your configuration. Save this file to /etc/stunnel/syslog-client.conf.
Then, start stunnel via "stunnel4 /etc/stunnel/syslog-client.conf". Now
you should see some startup messages. If no errors appear, you have a running
client stunnel instance.
Finally, you need to tell rsyslogd to send data to the remote host. In stock
syslogd, you do this via the "@host" forwarding directive. The same works with
rsyslog, but it suppports extensions to use tcp. Add the following line to your
Please note the double at-sign (@@). This is no typo. It tells rsyslog to use
tcp instead of udp delivery. In this sample, all messages are forwarded to the
remote host. Obviously, you may want to limit this via the usual rsyslog.conf
settings (if in doubt, use man rsyslog.con).
You do not need to add any special startup settings to rsyslog on the client.
Start or restart rsyslog so that the new configuration setting takes place.
After following these steps, you should have a working secure syslog
forwarding system. To verify, you can type "logger test" or a similar smart
command on the client. It should show up in the respective server log file. If
you dig out you sniffer, you should see that the traffic on the wire is actually
protected. In the configuration use above, the two stunnel endpoints should be
quite chatty, so that you can follow the action going on on your system.
If you have only basic security needs, you can probably just remove the debug
settings and take the rest of the configuration to production. If you are
security-sensitve, you should have a look at the various stunnel settings that
help you further secure the system.
Preventing Systems from talking directly to the rsyslog Server
It is possible that remote systems (or attackers) talk to the rsyslog server
by directly connecting to its port 61514. Currently (Jule of 2005), rsyslog does
not offer the ability to bind to the local host, only. This feature is planned,
but as long as it is missing, rsyslog must be protected via a firewall. This can
easily be done via e.g iptables. Just be sure not to forget it.
With minumal effort, you can set up a secure logging infrastructure employing
ssl encrypted syslog message transmission. As a side note, you also have the
benefit of reliable tcp delivery which is far less prone to message loss than
I would appreciate feedback on this tutorial. If you have additional ideas,
comments or find bugs (I *do* bugs - no way... ;)), please
let me know.
- 2005-07-22 * Rainer Gerhards * Initial Version created
- 2005-07.26 * Rainer Gerhards * Some text brush-up, hyperlinks added
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