click

This is the port of the Click Modular Router.

From the web page:

Click is a new modular software router for PCs developed by MIT
LCS's Parallel and Distributed Operating Systems group. Click
routers are flexible, configurable, and easy to understand at a
high level. They're also pretty fast (for software routers running
on commodity hardware); on a 700 MHz Pentium III, a Click IP router
can handle up to 435,000 64-byte packets a second.