12 Routing and other problems

Hint: DON'T RUN routed!

The PPP software will not replace the default route if you have one already set when you run pppd. This is done to prevent people from destroying their default route to the ethernet routers by accident. A warning message is written to the system log if the defaultroute parameter is not performed for this reason.

The remote system is not configured for `IP forwarding'. It is an RFC requirement that this option NOT be enabled by default. You must enable the option. For Linux systems, you will need to build the kernel and specify that you want IP forwarding/gatewaying.

The remote computers need a route back to you just as you need a route to them. This may be accomplished by one of four methods. Each has advantages and limitations. You need to do one and only one of these.

• Use a host route. At each host on the remote system, add a host route to your Linux IP address with the gateway being the terminal server that you use for your local access. This will work if you have a small number of host systems and a simple network without bridges, routers, gateways, etc.
• Use a network route. Subdivide the remote IP addresses so that your local Linux IP address and the remote terminal server address and the remote terminal server's ethernet address is on the same IP domain. This will work if you have the IP addresses to spare. It will work very well if you have a Class-B IP domain and can afford to put the all of the remote addresses on the same IP domain. Then add a network route on each of the gateways and routers so that any address of the remote network is sent to the terminal server. Most configurations have many hosts but few routers. (At sii.com, we have over 300 active host systems with only 3 routers.)
• Use gated on all of the gateways and on the terminal server. This will cause the terminal server to broadcast to the gateways that it can accept the frames for your IP address. Since the hosts will have a default route to one of the gateways, the gateways will generate the ICMP re-direct frame and the specific host will automatically add its host route.
• Use proxy ARP on the terminal server. This will only work if your remote IP address is in the same IP domain as one of the domains for the network cards.

There is no clear solution. You must choose one of these.

If you wish to ping your own system then use the loopback address of 127.0.0.1.

You may be able to ping the remote address. However, some terminal servers may not allow this as the address may be 'phony' to them. It depends upon their environment.

In general, don't try to ping either address. Choose a third address which is well known to be available on the remote network such as the name server IP address.