netfilter is a framework for arbitrary packet mangling. So far, a new NAT system and packet-filtering system have been built on top of it, as well as compatibility modules for ipfwadm and ipchains. Netfilter is a work-in-progress, but should be fairly robust for non-exotic work.
You can download it from the following link: https://packetstormsecurity.com/files/download/11051/netfilter-0.1.14.tar.bz2
Source: https://packetstormsecurity.com/files/11051/netfilter-0.1.14.tar.bz2.html