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Mysterious Duplicate IP addresses

© April 2011 Bruce Garlock

Bruce Garlock

All machines on our 192.168.206.x subnet (mostly Mac's, with Windows servers) got the IP conflict message when they started up in the morning.

They all have static IP addresses

We do have a linux dhcp server running on that subnet, but it only hands out 2 IP addresses for barcode guns, and those are given the IP based on their MAC address, so each barcode gun always gets the same IP and can move to other buildings without reconfiguring them, as they run in dhcp mode.

Windows clients and servers didn't seem to get the IP conflict error

The error on each Mac OS X client, showed the same MAC address as the machine who was using that IP address. No matter what the IP (unique of course), they all showed the same MAC address, which is the MAC address of our backbone switch, which is at (different subnet). That switch does routing and has the VLAN for the .206 subnet.

As a work-around, I added a bunch more IP's to our dhcp server on the .200 subnet, and shutdown the dhcp server on the .206 subnet, so the clients would get IP addresses from the .200 subnet (which would then route everything back to the .206 subnet) This worked fine, but introduced a little lag to the .206 subnet.

Our main switch (the one that was listed as the machine who was using all those IP addresses) is a HP Pro-curve 2900-24G. We found a group specializing in HP switches to dial-in through our VPN this morning to see if he could track down and fix our problem. He poked around and the only thing he wanted to change was to take out the 'proxyarp' setting in the 2900-24G, and once he did that, the problem was solved.

This setting was in the switch ever since it was deployed some 3 years ago now, and the issue just happened yesterday. Kind of strange, but he said that there could have been a cable that got crimped or sometimes he sees OS updates cause strange issues like this.

See also Dealing with Duplicate IP addresses and Mysterious Duplicate IP problem solved

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Wed Apr 20 20:35:18 2011: 9462   TonyLawrence


I have a strange duplicate ip problem at another customer. This is a Win 2003 server that will think its IP is elsewhere on the network if it is rebooted for any reason.

They have no smart switches and of course this is a static IP.

The "fix" so far is to shut the box down, unplug it from the network and then bring it back up. After it is fully up, the plug the cable back in and it is fine.

It makes no sense - I don't think it is related to this proxyarp (for one thing, this problem has persisted across router, dhcp server and switch changes - not all switches, but the one it plugs into was changed) and it doesn't seem to be this (link) low level issue either.

So far, I'm baffled.


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