For official addresses on the inside: ftpsesame (OpenBSD + others), fetchable from
http://www.sentia.org/projects/ftpsesame/ (on FreeBSD: - ftp/ftpsesame)
configured via an anchor (sub ruleset), cut and paste from man ftpsesame
pftpx, the next generation ftp-proxy
http://www.sentia.org/downloads/pftpx-0.8.tar.gz, (on FreeBSD - ftp/pftpx) cut and paste from man pftpx