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Does anyone know what could cause my Mac (macOS 11.6.4) to 'spontaneously' change its hostname from the normal setting (say, mycomputer) to what looks like some kind of UUID (of the format xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx where each x is a hex digit)? I've seen when the normal computer name gets a number appended, like mycomputer (2), but this looked completely different.

I first noticed it when I saw that the prompt in Terminal had changed from the usual username@mycomputer ~ % to username@xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx ~ %

Running hostname in Terminal returned xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx.fritz.box (my router is a FRITZ!Box)

However:

  • System Preferences > Sharing > Computer Name: still showed mycomputer
  • The FRITZ!Box router itself still showed mycomputer and not xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx in the list of devices on the network (and I couldn't see anything obvious in the router's logs about any changes)
  • Trying to ping xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx or ping xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx.fritz.box did not succeed, while ping mycomputer did.

I wondered if the hostname had been changed to the UUID and then already changed back to the original mycomputer (which could be why everything looked normal in the bullets above), but new Terminal windows still showed the UUID as the hostname.

In the end, clicking on System Preferences > Network > Ethernet > Advanced... > Renew DHCP Lease seemed to fix it. The hostname changed back to mycomputer in the Terminal prompt and the output from the hostname command changed back to mycomputer.fritz.box.

Although everything seems back to normal now, I've never seen this before. I am curious about what might have happened (and whether I need to be concerned). Does anyone know?

skipjack
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  • This is expected and normal behavior for DNS – bmike Feb 21 '22 at 08:51
  • Thanks @bmike! I (think I) understand the explanations under those links, but I'm still confused because this is on my home router (not a public network) and the UUID is not the name of another old/known client. I'm curious where the UUID-like name came from, and why. Is it possible that the Mac requested that name for some reason, or is it more likely to be behaviour on the router side (I'm surprised because the router should default to names which look like PC-192-168-xxx-xxx). – skipjack Feb 21 '22 at 13:20
  • I wonder is there is a site that covers router configuration. For our side, once the airport are really on topic - is there a user manual explaining how to set up your dns and network on the Fritz box? Maybe edit in details how you set up DHCP on the Mac (anything not default) and anything you know about the router details perhaps into the body of the question. – bmike Feb 21 '22 at 13:22

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