My code for AX.25 is part of the gensio library. The individual files are at:

https://github.com/cminyard/gensio/blob/master/lib/gensio_ax25.c

https://github.com/cminyard/gensio/blob/master/lib/gensio_ax25_addr.c

The gensio library is a whole framework for all kinds of stream and packet I/O. But you should be able to pull out the middle of the file that has the state machine, pretty much unmodified, and use it.

One thing that might be more interesting is I documented a bunch of bugs in the spec in the comments at the top of the gensio_ax25.c file.

Anyway, it's available if you want it. I'd be willing to split out the state machine if you wanted something that was individually maintainable.

-corey