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O'Reilly Network: Detecting Local Filesystem Changes with Perl

Aug 27, 2000, 15:58 (0 Talkback[s])
(Other stories by David N. Blank-Edelman)

"It is not uncommon for system administrators to have to drop whatever they are working on to deal with the security problem du jour. Some of these problems involve serious breaches of security. In these cases, the first question asked is often, "What has the intruder done?" In my recently released O'Reilly book, Perl for System Administration, I begin the chapter on security and network monitoring with a discussion of some of the available Perl tools that can help answer this question. Here's an excerpt from that chapter, which deals with finding changes made to a local filesystem."

"Filesystems are an excellent place to begin our exploration into change-checking programs. We're going to explore ways to check if important files like operating system binaries and security-related files (e.g., /etc/passwd or msgina.dll ) have changed. Changes to these files made without the knowledge of the administrator are often signs of an intruder. There are some relatively sophisticated cracker tool-kits available on the Net that do a very good job of installing Trojan versions of important files and covering up their tracks. That's the most malevolent kind of change we can detect. On the other end of the spectrum, sometimes it is just nice to know when important files have been changed (especially in environments where multiple people administer the same systems). The techniques we're about to explore will work equally well in both cases."

"The easiest way to tell if a file has changed is to use the Perl functions stat() and lstat(). These functions take a filename or a filehandle and return an array with information about that file. The only difference between the two functions manifests itself on operating systems like Unix that support symbolic links. In these cases lstat() is used to return information about the target of a symbolic link instead of the link itself...."

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