It is often the case that a system behind the firewall needs to be able to access a remote network through Virtual Private Networking (VPN). The two most common means for doing this are IPSEC and PPTP. The basic setup is shown in the following diagram:
A system with an RFC 1918 address needs to access a remote network through a remote gateway. For this example, we will assume that the local system has IP address 192.168.1.12 and that the remote gateway has IP address 192.0.2.224.
If PPTP is being used, there are no firewall requirements beyond the default loc->net ACCEPT policy. There is one restriction however: Only one local system at a time can be connected to a single remote gateway unless you patch your kernel from the 'Patch-o-matic' patches available at http://www.netfilter.org.
If IPSEC is being used then only one system may connect to the remote gateway and there are firewall configuration requirements as follows:
ACTION SOURCE DESTINATION PROTOCOL PORT CLIENT
PORTORIGINAL
DESTDNAT net:192.0.2.224 loc:192.168.1.12 50 DNAT net:192.0.2.224 loc:192.168.1.12 udp 500
If you want to be able to give access to all of your local systems to the remote network, you should consider running a VPN client on your firewall. As starting points, see http://www.shorewall.net/Documentation.htm#Tunnels or http://www.shorewall.net/PPTP.htm.
Last modified 12/21/2002 - Tom Eastep
Copyright © 2002 Thomas M. Eastep.