pppd + precompiled-active-filter

OpenWRT has long had some patches against pppd 2.4.4 for what’s called ‘Precompiled Active Filter’.  This is a file, which contains the libpcap compiled filter on what should be considered ‘active’ or ‘interesting’ traffic.  In the Cisco world, it’s the access-list you reference with dialer-list and dialer-group commands.

The default file that’s shipped tends to ignore icmp requests and a few other odds and ends, and is either never used, or works for everyone else.  Sadly, I needed to change it.

Finding #1)  tcpdump -ddd not icmp and not udp 123 and …. generates a precompiled file nicely

Finding #2)  After several hours - this is not the right way to generate it.  Files generated do not do what you think they do.

Finding #3)  Included in the buildroot is a tool from http://www.fli4l.de/en/home/news/ called pfc.c which will build the correct filters.  That’s the Floppy Linux ISDN… Floppies and ISDN - something I haven’t run into for several years.

Building it (gcc -o pfc -lpcap pfc.c) gives you a nice little tool to generate them using command line libpcap filter syntax - and guess what - they work!

October 15, 2008 · Filed Under Random

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