Linux server security
From Gender and Tech Resources
Installing intrusion detection
In hostile environments such as server environments, using a FIA makes sense only if it is installed, fully configured and initialised at the very first boot after an installation from scratch, before ever connecting to the internet or doing anything else. It takes only one attack to install a backdoor. Installing aide or tripwire after such an event would guarantee that the backdoor remains just as open as the day an intruder installed it. For more on aide vs tripwire, see Linux applications: Intrusion detection, and for an example install of aide, see Linux security: Installing intrusion detection.
Remote commands and procedure calls
ssh, rssh, scp and sftp.
Remote Procedure Calls (RPC), Portmapper
Network Wrappers (PAM)
Blocking ICMP and look invisible to ping.
Traffic Control (TC) and TC New Generation (TCNG)