In the base system, load the daemon if you need it
/etc/rc.conf[.local]
ftpproxy_flags=""
/etc/pf.conf
anchor "ftp-proxy/*"
the redirection
pass in quick inet proto tcp to port ftp rdr-to 127.0.0.1 port 8021 pass in quick inet6 proto tcp to port ftp rdr-to ::1 port 8021
In OpenBSD 5.0 and newer, use divert-to instead:
pass in quick inet proto tcp to port ftp divert-to 127.0.0.1 port 8021 pass in quick inet6 proto tcp to port ftp divert-to ::1 port 8021
pass out proto tcp from $proxy to port ftp