When I looked at the source code for various famous sniffers, I've noticed
that they all had all seperate .C files for interpreting various protocols.
Why not have a sniffer that can understand user-supplied protocol details?
Here it is.

When fired, Hafiye first visits each sub-directory under its knowledge-base
directory and opens to see whether it is a protocol knowledge-base file. If
so, It loads the necessary information from that file and places it into its
memory space. After constructing the supplied knowledge-base, Hafiye starts
looping for receiving packets. When a packet arrives, it demultiplexes the
layers according to its knowledge-base and prints protocol-based information.

Features
   - Multi Platform Support (Posix Compliant)
   - Customizable Protocol Definitions (Layer II, III and IV)
   - Customizable Packet Interpretation (Layer II, III and IV)

WWW: http://www.enderunix.org/hafiye/