nabou is a Perl script which can be used to monitor changes to your system. It provides file integrity checking, and can also watch crontabs, suid files and user accounts for changes. It stores all data in standard dbm databases.
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tcpspy is a linux administrator's tool that logs information about incoming and outgoing TCP/IP connections: local address, remote address and, probably the most useful feature, the user name. The current version allows you to include and exclude certain users from logging - this may be useful if you suspect one of the users on your system is up to no good but do not want to violate the privacy of the other users.
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Samhain (development branch) is a file system integrity checker that can be used as a client/server application for centralized monitoring of networked hosts. Databases and configuration files can be stored on the server. In addition to forwarding reports to the log server via authenticated TCP/IP connections, several other logging facilities (e-mail, console, tamper-resistant log file, and syslog) are available. Tested on Linux, AIX 4.1, HP-UX 10.20, Unixware 7.1.0, and Solaris 2.6. The devel version has more advanced features, but not all options are thoroughly tested yet.
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