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Adventures in DHCP


Internet Down
Working for a startup, office IT crises can be as important as production operations.

We all arrived in the office this morning to find out there was no internet access. Further debugging showed that all of the DHCP lease requests were failing. However, the internet connection was working if you manually configured your IP/DNS info.

We had installed a new DHCP/DNS server last Friday night, so we immediately suspected some failure there. Thankfully, we had the old server as a hot backup. Surprisingly, the hot backup server didn’t work either.

We fired up a network sniffer and saw that between all of the clients in the office network and the DHCP server, someone else was intercepting the DHCP discovery requests and responding negatively to every attempt before the real server could.

DHCP is a broadcast protocol, so it’s easy for a misconfigured machine or router to hijack valid network requests. All of the evidence pointed to a similar thing happening here.

At the same time, we had just opened up a new room in our offices and had wired drops put in. There was a new patch panel with wiring for several drops in the room. I was immediately suspicious of a new router/device in this area due to the timing. However, we didn’t find any new devices.

Mystery Solved
It turns out that someone had wired the new patch panel up to an existing Time-capsule/Airport hub, but plugged it into the Uplink port instead of a hub port. This caused the Airport to configure itself as a router, turn on DHCP forwarding to the dead network (the new room), and intercept all DHCP requests from the entire office.

Categories: Debugging, Operations
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