SSL Encrypting Syslog with Stunnel
Written by Rainer Gerhards (2005-07-22)
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 (another 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, our syslogd.
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 your system.
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 follows:
- rsyslog forwards message to stunnel local portal at port 61514
- local stunnel forwards data via the network to port 60514 to its remote peer
- 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 this paper.
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 its insecurity.
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 foreground=yes
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 /etc/rsyslog.conf:
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 udp.
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.
Copyright (c) 2005 Rainer Gerhards and Adiscon.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license can be viewed at http://www.gnu.org/copyleft/fdl.html.