In order for any of this to work, you must have your kernel configured correctly. If you don't know how to build your own kernel, then you should read the Kernel HOWTO. You'll need to make sure that the following kernel options are turned on in addition to basic networking. I use a 2.0.38 kernel in my system.

For 2.0 kernels:

For 2.2 kernels:

First, we write firewall filter rules that allow our users to access our internal nets, while restricting them from accessing the outside internet. This sounds strange, but since the users already have access to the internet, why let them use the tunnel to access the net? It wastes both bandwidth and processor resources.

The filter rules that we use depend upon which internal nets we use, but translate to: "Allow traffic coming from our VPNs that is destined for our internal nets to go there." So how do we do that? As always, it depends. If you are running a 2.0 kernel, you use the tool called ipfwadm, but if you are using a 2.2 kernel, you use the utility called ipchains.

To set the rules with ipfwadm, run it with options similar to the following:

# /sbin/ipfwadm -F -f
# /sbin/ipfwadm -F -p deny
# /sbin/ipfwadm -F -a accept -S 192.168.13.0/24 -D 172.16.0.0/12

To set the rules with ipchains, run it with options similar to the following:

# /sbin/ipchains -F forward
# /sbin/ipchains -P forward DENY
# /sbin/ipchains -A forward -j ACCEPT -s 192.168.13.0/24 -d 172.16.0.0/12

For those using 2.2 kernels, please read Section 1.3, “IPv4 Forwarding and 2.2 kernels”.

Now that users are allowed to access our nets, we need to tell the kernel where to send the packets. On my system, I have two ethernet cards, one is on the external network, while the other is on the internal network. This helps keep things secure, as outbound traffic is masqueraded by the gateway, and any incoming traffic is filtered and routed by the Cisco Router. For most setups, the routing should be simple.

Next, route all traffic destined for the private networks out the internal interface, and all other traffic out the external interface. The specific routing commands depend on which internal nets you are using. Below is an example of what they might look like. These lines are of course in addition to your basic routes for your local nets. I also doubt that you are using all 3 groups of internal numbers:

Assuming that 172.16.254.254 is the internal gateway:

# /sbin/route add -net 10.0.0.0 netmask 255.0.0.0 gw 172.16.254.254 dev eth1
# /sbin/route add -net 172.16.0.0 netmask 255.240.0.0 gw 172.16.254.254 dev eth1
# /sbin/route add -net 192.168.0.0 netmask 255.255.0.0 gw 172.16.254.254 dev eth1

One additional note on routing. If you are using two way routing for say, a remote office, then you will need to do one more thing. You need to set up routes on the server that point back to the client. The easiest way to accomplish this is to run a cron job every minute that quietly sets back routes. If the client is not connected, route will just spit out an error (that you've conveniently sent to /dev/null.)