bsdproxy

bsdproxy is a generic, event-driven proxy designed specifically for the
BSD platform. It uses the kqueue()/kevent() system calls to determine
when to relay data from one side of the connection to the other. It also
uses GLib (http://www.gtk.org) data structures and memory management
functions to optimize steady-state performance (minimize unnecessary
memory allocation/deallocation).

bsdproxy has been used to proxy HTTP, HTTPS, telnet, and mysql without
any problems. It should be able to serve as a transparent proxy for
anything over a TCP/IP connection.