FreeBSD NAT redirect_port

/etc/rc.conf

gateway_enable="YES"
firewall_enable="YES"
firewall_type="OPEN"

nat_enable="YES"

nat_interface="em1" 

/etc/rc.firewall

For automatic configuration edit /etc/rc.firewall, search for ${firewall_type}=[Oo][Pp][Ee][Nn] and in section  firewall_nat_enable add following two lines.

${fwcmd} nat 1 config if ${firewall_nat_interface} redirect_port tcp 192.168.100.252:80 80
${fwcmd} add 50 nat 1 ip4 from any to any via ${firewall_nat_interface}