I found a patch for tcpdump to work with ax25, but didn't tried it yet.
http://he.fi/archive/linux-hams/200509/0060.html

I tried the patch, it works well. It's worth a try. The patch did not
apply cleanly, however. I had to apply some parts of the patch
manually. For those who don't want to do that, I've made a tarball of
the patched sources:
http://guillaume.filion.org/tcpdump-3.9.3-ax25-1.tar.gz

Here's the short readme:
Full infos about this patched tcpdump available at:
http://he.fi/archive/linux-hams/200509/0060.html

To build this version of tcpdump:

cd libpcap-0.9.3
../configure
make
cd ../tcpdump-3.9.3
../configure
make
../tcpdump -eni ax0

73,
GFK's
--
Guillaume Filion, ing. jr
Logidac Tech., Beaumont, Québec, Canada - http://logidac.com/
PGP Key and mo http://guillaume.filion.org/