HowToDetectAndResolveIpConflicts
From UoWiki
NB: system used: Freebsd 5.4, but it shouldn't matter
1. Detect IP conflicts
There's something wrong with your network. You can't figure it out. Your host is up one minute, down the next, it's pissing you off. Finally, you get to the terminal and do
prompt% less /var/log/messages Jun 25 12:17:45 myhost kernel: arp: 00:11:22:33:44:55 is using my IP address 10.0.0.5! Jun 25 12:27:46 myhost last message repeated 546 times
If this is in your logs, the problem is that there's a rogue host on your network, with the same IP address as your host, which is making lots of problems and is naughty.
2. Locate rogue host
The task now is to find the rogue host. You do this by finding out which host on your network has the mac address 00:11:22:33:44:55. You can do this by pinging all the hosts on your network (you could do this using a pingscan from a tool like [nmap ], so that there's a record in your arp table. Once you've done this, do:
prompt% arp -a (10.0.0.10) at 00:11:22:33:44:55 on fxp0 (ethernet)
Ok! Now we know, the host 10.0.0.10 is the rogue, it's obviously advertising the wrong IP address every now and again. If you don't recognise that IP address, or you don't know who'se box that is (and it's definitely on your network) run a portscan on it using nmap ( prompt% nmap 10.0.0.10 ) or something. But don't do that if it's a public IP address that you do not administrate yourself. A portscan can be interpreted as an attack.
3. Switch it off!
So, once you've found out where that box is, go turn it off. Conflict resolved!
