There IS such a thing as being too tidy

Pilot fish is called in to troubleshoot a relatively small network for a company whose network admin is on vacation.

"The problem was that roughly half of the computers were unable to connect to the rest of the network or the Internet," says fish. "They were throwing error messages like 'A duplicate name exists on the network.'"

Fish knows all the PCs are connected directly to one of two switches, which are linked together, with a single DHCP server handing out IP addresses. There's also one local gateway, with one main and several backup ISP links. The setup is simple -- too simple for a complicated failure. But something is clearly wrong.

But eventually fish notices something strange: The PCs with duplicate names have IP addresses from the wrong IP range. Aha, fish thinks -- a rogue DHCP!

But where is it?

"Really, the only place to look for another DHCP was the gateway that already hosted the main DHCP server," fish says. "The network didn't use any additional hardware except PCs and the gateway, and all the PCs were protected in a way that it wasn't possible to do anything to them that a simple reboot wouldn't set straight. There were no traces of anything rogue-like."

Fish keeps working on the problem -- and eventually he realizes he can look up the MAC address of the rogue DHCP server. From that he can at least identify what kind of equipment is doling out bad IP addresses.

And it works. The MAC address is for a cable modem -- which fish discovers plugged directly into a switch instead of the gateway.

That means the cable modem can't connect to its ISP's network. But that shouldn't be causing a problem, right?

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Except for one, um, helpful feature. "Since the cable modem was unable to establish a connection with its ISP network, the cable modem kept its DHCP server enabled on its Ethernet port," fish grumbles. "That serves as a diagnostic utility to help diagnose weak cable signal levels. It's disabled as soon as a cable connection is established.

"The network admin had the cable modem disconnected from the gateway on purpose, since it was out of service. It seems someone figured that it must be a good idea to plug it back in, using a free cable that was hanging from the rack.

"Nobody knew about that little intervention until sometime later, when the problem was already solved."

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Copyright © 2010 IDG Communications, Inc.

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