There is an excellent introduction here http://www.phparchitecture.com/howto_show.php?id=3&showall
The part that is missing for kernels (I am told up to 2.6.32) is the link to this: http://code.google.com/p/setvps/downloads/detail?name=Linux-2.6.18-mppe-mppc-1.4.patch&can=2&q=
Don’t forget to do a global search and replace of the linux version before patching,
If vi is your favorite editor then:
Later version 2.6 kernels and onwards require even later patches because of changes in the crypto API.
I couldn’t find one myself, so I made my own: a patch that works with kernel 3.2.21 that I have created is available here:
All original credits of course for the MPPC conversion go to Jan Dubiec of http://mppe-mppc.alphacron.de/ . All I’ve done is ported it to the new Crypto API (and mixed some of the ppp_mppe.c code in)!
If you try to apply this patch to a very late 2.6 kernel you should take note that the ppp files have moved into their own subfolder of the net directory with the advent of 3 series kernels. You will need to change the the folder paths to fix this.