Understanding Rootkits
Source: O'Reilly Network
L33tdawg: This is a pretty good article and is well worth a read if you're new to the world of rootkits.
A rootkit is a collection of tools an intruder brings along to a victim computer after gaining initial access. A rootkit generally contains network sniffers, log-cleaning scripts, and trojaned replacements of core system utilities such as ps, netstat, ifconfig, and killall. Although the intruders still need to break into a victim system before they can install their rootkits, the ease-of-use and the amount of destruction they cause make rootkits a big threat for system administrators.
The main purpose of a rootkit is to allow intruders to come back to the compromised system later and access it without being detected. A rootkit makes this very easy by installing a backdoor remote-access daemon, such as a modified version of telnetd or sshd. These will often run on a different port than the one that these daemons listen on by default.
