swatch: Automated Log Monitoring for the Vigilant but Lazy

Previously the Paranoid Penguin has pondered a plethora of powerful programs pursuant to protecting people's PCs from pernicious punks. [The right to excessive alliteration revocable at any time—Ed.] One important feature these tools share is logging; just as important as keeping system crackers out is knowing when they've tried to get in. But who's got the time or attention span to sift through scads of mostly innocuous log files on every system they administer, every single day?

swatch (the “Simple WATCHer”) does. swatch, written 100% in Perl, monitors logs as they're being written to and takes action when it finds something you've told it to look for. This simple, flexible and useful tool is a must-have for any healthily fearful system administrator.

There are two ways to install swatch. First, of course, is via whatever binary package of swatch, if any, your Linux distribution of choice provides. The current version of Mandrake has an RPM package of swatch, but none of the other more popular distributions (i.e., Red Hat, SuSE, Slackware or Debian) appear to include it.

This is just as well, though, because the second way to install swatch is quite interesting. swatch's source distribution, available from www.stanford.edu/~atkins/swatch, includes a sophisticated script called Makefile.PL. The script automatically checks for all necessary Perl modules and uses Perl 5's CPAN functionality to download and install any needed modules; it then generates a Makefile that can be used to build swatch.

After you've installed the required modules, either automatically from swatch's Makefile.PL script or manually (and then running perl Makefile.PL), Makefile.PL should return the following:

[root@barrelofun swatch-3.0.1]# perl Makefile.PL
Checking for Time::HiRes 1.12 ... ok
Checking for Date::Calc ... ok
Checking for Date::Format ... ok
Checking for File::Tail ... ok
Checking if your kit is complete...
Looks good
Writing Makefile for swatch
[root@barrelofun swatch-3.0.1]#

Once Makefile.PL has successfully created a Makefile for swatch, you can execute the following commands to build and install it:

make
make test
make install
make realclean

Since the whole point of swatch is to simplify our lives, configuring swatch itself is, well, simple. swatch is controlled by a single file, default $HOME/.swatchrc. This file contains text patterns in the form of regular expressions you wish swatch to watch for. Each regular expression is followed by the action(s) you wish swatch to take whenever it encounters that text.

For example, suppose you've got a web server, and you want to be alerted any time someone attempts a buffer-overflow attack by requesting an extremely long filename. By trying this yourself against the web server while tailing its /var/apache/error.log, you know that Apache will log an entry that includes the string “File name too long”. Suppose further that you want to be e-mailed every time this happens. Here's what you'd need to have in your .swatchrc file:

watchfor /File name too long/
    mail addresses=mick\@visi.com,
    subject=BufferOverflow_attempt

As you can see, the entry begins with a “watchfor” statement, followed by a regular expression. If you aren't proficient in the use of regular expressions yet (you are planning to learn regular expressions, aren't you?), don't worry: this can be as simple as a snippet of the text you want swatch to look for, spelled out verbatim between two slashes.

swatch will perform your choice of a number of actions when it matches your regular expression. In this example we've told swatch to send e-mail to mick\@visi.com, with a subject of BufferOverflow_attempt. Note the backslash before the @ sign; without it, Perl will interpret the @ sign as a special character. Note also that if you want spaces in your subject line, each space also needs to be escaped with a backslash, e.g., subject=Buffer\ Overflow\ attempt. Actions besides sending e-mail include those seen in Table 1.

Table 1. Some Actions swatch Can Take

For more details on configuring these and the other actions swatch supports, see the swatch(1) man page.

Let's take our example a step further. Suppose, in addition to being e-mailed about buffer-overflow attempts, you want to know whenever someone hits a certain web page, but only if you're logged on to a console at the time. In the same .swatchrc file, you'd add something like this:

watchfor /wuzza.html/
      echo=red bell 2

The event will then cause a beep and print to the console.

It's important to note you will only see these messages and hear these beeps if you are logged on the console in the same shell session from which you launched swatch. If you log out to go get a sandwich, when you return and log back in, you will no longer see messages generated by the swatch processes launched in your old session, even though those processes will still be running.

When in doubt add either a “mail” action or some other non-console-specific action, e.g., an “exec” action that triggers a script that pages you. Unless, that is, the pattern in question isn't critical.

Alert readers have no doubt noticed that the scenario in the previous example will work only for Apache installations in which both errors and access messages are logged to the same file. We haven't associated different expressions with different watched files, nor can we do so. But what if you want swatch to watch more than one log file?

No problem. While each .swatchrc file may describe only one watched file, there's nothing to stop you from running multiple instances of swatch, each with its own .swatchrc file. In other words, .swatchrc is the default but not the required name for swatch configurations.

To split the two examples into two files, therefore, you'd put the lines in the previous simple .swatchrc entry into a file called, say, .swatchrc.hterror, and the lines in the previous watchfor entry into a file called .swatchrc.htaccess.