A second look at the Gateway

Sometimes people get their IP and Gateway numbers backwards when they are setting up their computers. Do not ask me why? Just believe that this happens. In order to discover this problem Linux has a great server called arpwatch that watches the MAC addresses and IP addresses on your network line. You can run this program from root:

bash# arpwatch
bash# pine

The first command starts arpwatch, then the second one takes you into a mail reader. Press i to view your Inbox. Scroll to the bottom and wait for a few moments. When you think something might have happened press the down arrow a few times to see if any new messages are in you Inbox. Email messages should appear if you are on an active network. See if you have an email message with your gateway IP address. A message would look something like this:

Date: Thu, 14 Mar 2002 14:52:16 -1000
From: Arpwatch <arpwatch@bob.org>
To: root@bob.org
Subject: new station (dave.bob.org)

            hostname: dave.bob.org
          ip address: 1.2.3.8
    ethernet address: 0:10:20:30:2:f2
     ethernet vendor: Dell
           timestamp: Thursday, March 14, 2002 14:52:16 -1000

You should get a email message with the gateway's ethernet address. If you later get a different ethernet address you know that there might be another computer using the same IP address. It is also possible that the gateway machine has more than one ethernet address on the local network.