The first thing to always note is when your machine was rebooted. Since Linux is a robust and stable OS, the only times your machine should reboot is when you take it down for OS upgrades, hardware swapping, or the like. If your machine has rebooted without you doing it, that may be a sign that an intruder has compromised it. Many of the ways that your machine can be compromised require the intruder to reboot or power off your machine.
Check for signs of tampering on the case and computer area. Although many intruders clean traces of their presence out of logs, it's a good idea to check through them all and note any discrepancy.
It is also a good idea to store log data at a secure location, such as a dedicated log server within your well-protected network. Once a machine has been compromised, log data becomes of little use as it most likely has also been modified by the intruder.
The syslog daemon can be configured to automatically send log data to a central syslog server, but this is typically sent unencrypted, allowing an intruder to view data as it is being transferred. This may reveal information about your network that is not intended to be public. There are syslog daemons available that encrypt the data as it is being sent.
Also be aware that faking syslog messages is easy -- with an exploit program having been published. Syslog even accepts net log entries claiming to come from the local host without indicating their true origin.
Some things to check for in your logs:
su
entries or logins from strange places. We will discuss system log data later in the HOWTO.