An Introduction to FreeS/WAN, Part II
Naturally, you'll want to restrict what sorts of things hosts from the other network may do on your network and vice versa. I stated earlier that firewall rules on a Linux host running FreeS/WAN aren't necessarily any different from when they are without tunneling. This even holds true with NAT. When writing your firewall rules on each gateway, set up FORWARD, POSTROUTING and PREROUTING rules the same as if you weren't using IPSec—just be careful about interfaces. If you use -i and -o parameters, don't say “eth0” if you mean “ipsec0” (or “ipsec+” if you mean “all tunnel interfaces”). When in doubt, try to stick to IP addresses rather than interface names in your firewall rules.
In addition, make sure that no NAT is performed on tunneled packets. IPSec packets' headers are checksummed in the body of each packet's data field. Rewriting the IP header (e.g., by translating source or destination IPs) violates this message-digest, and weirdness will ensue. You can do NAT on packets as they leave the tunnel or before they enter it, but not while they're in the process.
Whatever else you do, you will need at least three new rules on each gateway to allow IPSec key negotiation and tunneling. In the INPUT and OUTPUT chains, you'll need to permit packets sent to UDP port 500, IP protocol 50 packets and IP protocol 51 packets. The relevant rules on both gateways would look like what is shown in Listing 4.
With that, you're ready to connect your network securely and cheaply to those of your vendors, partners and acquaintances. Good luck!
Mick Bauer (firstname.lastname@example.org) is a network security consultant for Upstream Solutions, Inc., based in Minneapolis, Minnesota. He is the author of the upcoming O'Reilly book Building Secure Servers with Linux, composer of the “Network Engineering Polka” and a proud parent (of children).