Linux Firewalls, Third Edition

Summary

Virtual private networks are popular because they leverage existing infrastructure to provide a seamless network experience to end users. Numerous implementations of VPNs are available, taking advantage of the different protocols available for creating VPNs. As you would expect, Linux has several options available, including Openswan and PoPToP, among others.

Some problems exist when connecting VPNs through NAT-enabled firewalls. This is because IPSec creates a digital signature based on the IP header, which is altered during the NAT process.