Apple recently released version 10.5.8 of Mac OS X and Mac OS X Server. As part of troubleshooting an email issue for one our server customers, I decided to run all the software updates and install the new 10.5.8. After installing and rebooting, and then running mailbfr to rebuild the email databases, I checked the server to see if everything was up and running.

While I had solved the email problem, the server reported a new error message in a sporadic fashion, periodically saying that the server’s serial number was invalid, and that there was another server running with the same serial number! I brainstormed about what could possibly be the problem, going from worst to best. Did someone take the serial number and set up their own server? Had the client’s serial number expired or something? I was imagining the worst!

My first step in figuring it out was to check the log files. You can do this on your own machine with the Console.app in the Utilities folder. Apple’s Server Admin program provides access to many important server logs directly in it’s own interface. Reading the logs, I noticed that it was reporting the IP address of the offending server with the duplicate serial number. I checked the IP address and was bemused to learn that that it was the IP of the second ethernet port on the same server. The server was complaining about itself!

My next step was to check the Apple Support forums and knowledge base, where I found I was far from alone in seeing this problem appear with 10.5.8. Apparently, this was only happening when both ethernet ports were configured within the same subnet. It was not affecting servers with only one active ethernet port, servers with two bonded ethernet ports (two ports acting as one, with one IP), or servers setup as gateways, where one port is part of the outside Wide Area Network, and the other is part of the private Local Area Network.

The simple fix was to turn off the other ethernet port, which I did, since it was just there as a spare. However, Apple posted a Knowledge Base article to specifically discourage setting both ports on the same subnet, a practice I will now follow with all my servers! Where I previously used one port as a spare, I will probably now use link aggregation to combine them and double the network bandwidth.

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