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Network Security Hacks by Andrew Lockhart

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Verify File Integrity and Find Compromised Files

Use Tripwire to alert you to compromised files or verify file integrity in the event of a compromise.

One tool that can help you detect intrusions on a host and also ascertain what happened after the fact is Tripwire (http://sourceforge.net/projects/tripwire). Tripwire is part of a class of tools known as file integrity checkers, which can detect the presence of important changed files on your systems. This is desirable because intruders who have gained access to a system will often install what’s known as a root kit , in an attempt to both cover their tracks and maintain access to the system. A root kit usually accomplishes this by modifying key operating system utilities such as ps, ls, and other programs that could give away the presence of a backdoor program. This usually means that these programs will be patched to not report that a certain process is active or that certain files exist on the system. Attackers could also modify the system’s MD5 checksum program (e.g., md5 or md5sum) to report correct checksums for all the binaries that they have replaced. Since using MD5 checksums is usually one of the primary ways to verify whether a file has been modified, it should be clear that something else is sorely needed.

This is where Tripwire comes in handy. It stores a snapshot of your files in a known state, so you can periodically compare the files against the snapshot to discover discrepancies. With this snapshot, Tripwire ...

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