iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
iptables -A FORWARD -i eth1 -j ACCEPT
and how does it get security patches more often than Debian or FreeBSD or OpenBSD?
As an answer to the second, I checked the site and the last release was 51 weeks ago and the last beta was 3 weeks ago. Compare that to Debian archive runs a couple times a day, has a couple hundred devs, and a couple mirrors, and full security teams.
Here's a list to security related bugs that Debian has patched since the last beta of smallwall.
https://www.debian.org/security/
There's 27 debian security patches since the most recent smallwall beta, not all related to a firewall installation of course.