An Introduction to FreeS/WAN, Part II

Connect two private LANs securely with a FreeS/WAN tunnel that runs on your existing firewall systems.
Firewalls and NAT

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.

Listing 4. iptables Rules to Allow IPSec

Conclusion

With that, you're ready to connect your network securely and cheaply to those of your vendors, partners and acquaintances. Good luck!

Resources

Mick Bauer (mick@visi.com) 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).

______________________

White Paper
Linux Management with Red Hat Satellite: Measuring Business Impact and ROI

Linux has become a key foundation for supporting today's rapidly growing IT environments. Linux is being used to deploy business applications and databases, trading on its reputation as a low-cost operating environment. For many IT organizations, Linux is a mainstay for deploying Web servers and has evolved from handling basic file, print, and utility workloads to running mission-critical applications and databases, physically, virtually, and in the cloud. As Linux grows in importance in terms of value to the business, managing Linux environments to high standards of service quality — availability, security, and performance — becomes an essential requirement for business success.

Learn More

Sponsored by Red Hat

White Paper
Private PaaS for the Agile Enterprise

If you already use virtualized infrastructure, you are well on your way to leveraging the power of the cloud. Virtualization offers the promise of limitless resources, but how do you manage that scalability when your DevOps team doesn’t scale? In today’s hypercompetitive markets, fast results can make a difference between leading the pack vs. obsolescence. Organizations need more benefits from cloud computing than just raw resources. They need agility, flexibility, convenience, ROI, and control.

Stackato private Platform-as-a-Service technology from ActiveState extends your private cloud infrastructure by creating a private PaaS to provide on-demand availability, flexibility, control, and ultimately, faster time-to-market for your enterprise.

Learn More

Sponsored by ActiveState