"A crude yet effective intrusion detection system such as Tripwire can alert systems administrators to possible intrusion attempts by periodically verifying the integrity of a server's file systems. Systems intruders will often use trojan binaries for login, su, ps, and ls, etc. to cover their tracks and keep a low profile on the system. Under normal circumstances even astute systems administrators may not observe the intrusion because the trojan binaries mimic the system binaries so well."
"One tried and true method to alert systems administrators of unexpected file system alterations is to use a software package such as Tripwire to keep a database of checksums on the file sizes of critical system files. Depending on the configuration, Tripwire can notify appropriate personnel if a critical file or directory is modified or deleted."
"By using a strong checksum method similar to MD5, Tripwire can identify with absolute certainty whether or not a file has been modified, unlike similar programs that use weaker algorithms such as CRC to calculate checksums."
