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Further resources

In learning about routing for Linux I found this book to be very useful:

Linux Routers, A Primer for Network Administrators by Tony Mancill Publisher: Prentice Hall ISBN 0-13-009026-3

Linux Advanced Routing and Traffic Control (iproute) manual: http://lartc.org/

Linux documentation project networking howto: a href="http://www.tldp.org/HOWTO/NET3-4-HOWTO.html">http://www.tldp.org/HOWTO/NET3-4-HOWTO.html

The Debian reference manual has a chapter on networking: http://www.debian.org/doc/manuals/reference/ch-gateway.en.html

Commands

Tcpdump

The 'tcpdump' command is your best friend when it comes to setting up and debugging network problems.

ping

Ping is always there for you, on any Linux or Windows system. It won't always work, sometimes firewall rules block it. But using it with tcpdump is invaluable.

Suggested use: Set up one system to ping the destination, and run 'tcpdump -n -i interface icmp' on the destination system and/or a router system in between. The interface should be something like eth0 or eth1, the '-n' turns off DNS name lookups, and 'icmp' limits the traffic dumped to icmp protocol, which is what ping uses.

It's very satisfying to see ping responses start coming back when you finally get the routing set correctly.

Traceroute

The 'traceroute' command will give you a blow by blow account of the path between your machine and a remote machine. Most Linux distros don't install it by default; install the package named 'traceroute'. Just type 'traceroute' followed by a remote hostname. You will see a listing of the route packets have to take to get to the remote location. (If you only see lines with * * * there is probably a firewall in the way.)

MS Windows has the same command renamed 'tracert'.