artificial ignorance: how-to guide

Marcus J. Ranum
Tue, 23 Sep 1997 23:06:06 +0000

By request, here's a quick how-to on log scanning via
artificial ignorance. :) It assumes UNIX and the presence
of a good grep - you could use other stuff if you wanted to
but this is just an example.

Setting up a filter is a process of constant tuning. First
you build a file of common strings that aren't interesting,
and, as new uninteresting things happen, you add them
to the file.

I start with a shell command like this:

cd /var/log
cat * | \
	sed -e 's/^.*demo//' -e 's/\[[0-9]*\]//' | \
	sort | uniq -c | \
	sort -r -n > /tmp/xx

In this example "demo" is my laptop's name, and I use
it in the sed command to strip out the leading lines of
syslog messages so that I lose the date/timestamps. This
means that the overall variation in the text is reduced
considerably. The next argument to sed strips out the
PID from the daemon, another source of text variation.
we then sort it, collapse duplicates into a count, then
sort the count numerically.

This yields a file of the frequency with which something
shows up in syslog (more or less):
 297  cron: (root) CMD (/usr/bin/at)
 167  sendmail: alias database /etc/aliases.db out of date
 120  ftpd: PORT
  61  lpd: restarted
  48  kernel: wdpi0: transfer size=2048 intr cmd DRQ
 ... etc

In the example on "demo" this reduced 3982 lines of
syslog records to 889.

Then what you want to do is trim from BOTH ends of
the file and build an "ignore this" list. In this example, I
don't care that cron ran "at" OK so I'd add a regexp
cron.*: (root) CMD (/usr/bin/at)
That's a pretty precise one. :)

At the bottom of my file there were about 200 entries
that looked like:
   1  ftpd: RETR pic9.jpg
   1  ftpd: RETR pic8.jpg
   1  ftpd: RETR pic7.jpg
   1  ftpd: RETR pic6.jpg

Clearly these are highly unique events but also not
interesting. So I add patterns that look like:
ftpd.*: RETR
ftpd.*: STOR
ftpd.*: CWD
ftpd.*: USER

Now, you apply your stop-list as follows:
cat * | grep -v -f stoplist | \
	sort, etc --

This time I get 744 lines. Putting a pattern in that
sendmail.*: .*to=

Drops it down to 120 lines. Just keep doing this and
pretty soon you'll have a set of patterns that make your
whole syslog output disappear. You'll notice that in the
early example I had a warning from sendmail because
the aliases database was out of date. Rather than putting
a pattern for that, I simply ran newalias. Next time my
aliases database is out of date, my log scanner will tell

System reboots are cool, too. My log shows:
  48  kernel: wdc2 at pcmcia0: PCCARD IDE disk controller
  48  kernel: wdc1 at pcmcia0: PCCARD IDE disk controller
  48  kernel: wdc0 at isa0 iobase 0x1f0 irq 14: disk controller
  48  kernel: wd0 at wdc0 drive 0: sec/int=4 2818368*512

Those will be pretty much static. So I add those exact
lines. Now they won't show up whenever the system
boots. BUT I'll get a notification if a new SCSI drive
is added, or (I did this deliberately!):

kernel: fd0c: hard error writing fsbn 1 of 1-19 (fd0 bn 1; cn
kernel: fd0: write protected

Oooh! Some bad boy trying to step on my tripwire file!

kernel: changing root device to wd1a

..interesting. My pattern was for wd0a!

I used to run this kind of stuff on a firewall that I used
to manage. One day its hard disk burned up and my
log scan cheerfully found these new messages about
bad block replacement and sent them to me. :) The
advantage of this approach is that it's dumb, it's
cheap -- and it catches stuff you don't know about

Once you've got your pattern file tuned, put it in
cron or whatever, so it runs often. The TIS Gauntlet
has a hack I wrote called "retail" which I can't
unfortunately release the code for, but is easy to
implement. Basically, it was like tail but it remembered
the offset in the file from the previous run, and the
inode of the file (so it'd detect file shifts) - the trick is
to keep one fd open to the file and seek within it,
then stat it every so often to see if the file has grown
or changed inode. If it has, read to EOF, open the new
file, and start again. That way you can chop the end
of the log file through a filter every couple seconds
with minimal expense in CPU and disk I/O.

I'm sure there are lots of fun ways this simple trick
can be enhanced -- but just in its naive form I've found
it quite useful. I wish I had a program that helped me
statistically build my noise filters, but in general I find
it's about a 2 hour job, tops, and it's one you do once
and forget about.

Marcus J. Ranum, CEO, Network Flight Recorder, Inc.