apt-get install debsums
I forget the name every time (confused with
debsigs-verify), so I'll try to blog it.
It's a cool stuff. It checks every installed file against its MD5 sum in the dpkg database.
It's good for a quick consistency check in your machine in case you suspect that some hardware failure or system crash corrupted some of your files.
It won't protect against malicious attacks, of course, as the attacker can easily change the dpkg database, or a kernel rootkit can easily show debsums the unchanged version of files.
But it's a cool thing to have installed. Also, when you install a package that doesn't have the md5sums informations in it, it would regenerate them at install time.