FreeBSD firewall, nat, kernel

[Dennis Olvany]

 From a fresh install, a working nat should only require a few commands. 
Kernel compilation is not necessary.

kldload ipfw
kldload ipdivert
sysctl net.inet.ip.forwarding=1
dhclient xl0
natd -dynamic -n xl0
ipfw add divert natd ip from any to any via xl0
ipfw add allow ip from any to any
ifconfig rl0 192.168.100.253/24

To make the config permanent, you just need to use the rc equivalents of 
those commands.

/etc/rc.conf

firewall_enable="yes"
firewall_type="/etc/ipfw.rules"
gateway_enable="yes"
ifconfig_xl0="dhcp"
ifconfig_rl0="192.168.100.253/24"
natd_enable="yes"
natd_interface="xl0"

/etc/ipfw.rules

add divert natd ip from any to any via xl0
add allow ip from any to any