Intrusion Detection: Tripwire

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When someone breaks into a system, they will usually try to gain control by making their own changes to system administration files, such as password files. They could create their own user and password information, allowing them access at any time, or simply change the root user password. They could also replace entire programs, such as the login program, with their own version. One method of detecting such actions is to use an integrity checking tool such as Tripwire or AIDE to detect any changes to system administration files. AIDE (Advanced Intrusion Detection Environment) is an alternative to Tripwire. It provides easy configuration and detailed reporting. Neither are included with Red Hat.

An integrity checking tool works by first creating a database of unique identifiers for each file or program to be checked. These can include features such as permissions and file size, but also, more important, checksum numbers generated by encryption algorithms from the file's contents. For example, in Tripwire, the default identifiers are checksum numbers created by algorithms like the MD5 modification digest algorithm and Snefru (Xerox secure hash algorithm). An encrypted value that provides such a unique identification of a file is known as a signature. In effect, a signature provides an accurate snapshot of the contents of a file. Files and programs are then periodically checked by generating their identifiers again and matching them with those in the database. Tripwire will generate signatures of the current files and programs and match them against the values previously generated for its database. Any differences are noted as changes to the file, and Tripwire then notifies you of the changes.

Note 

You can also check your log files for any suspicious activity. See Chapter 27 for a discussion on system logs. /var/log/messages in particular is helpful for checking for critical events such as user logins, FTP connections, and superuser logins.

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