uredir (userspace UDP port redirector)

uredir is a small tool to redirect UDP traffic. It can be used as a
poor man's filtering tool, e.g. for small multihomed embedded systems
without a built-in firewall.

uredir forwards packets to a specified destination, remembering the
sender's address. Any packets received from the destination are in
turn forwarded to the sender.

uredir can be run standalone or via inetd. See uredir's man page for
details.