Sometimes you need a TCP proxy - and proxying TCP should be a rather trivial task. There are many TCP proxies out there, e.g. the simpleproxy which even has a Debian package but is rather old (last release around 2005) and limited (no IPv6 support, no persistent configuration file or init script).

A better solution is provided by tcpproxy from spreadspace.org. It’s latest release dates from 2010, it supports IPv6 and IPv4-to-IPv6 cross connecting (and vice versa), has a working init script and a persistent configuration file with a nice clear syntax. Works out of the box in my current v4-to-v4 use case.

If you’re looking for Debian or Ubuntu packages you may get covered here with nearly every flavor of them (squeeze, wheezy, etc).