At first I thought this was a typo (
), but it appears these are Virgin Media’s UK-based DNS servers. Are they your ISP? They are probably providing this value to your gateway, which is in turn giving it to the rest of your network.
You may or may not be able to change this default in your router admin interface (depends on who owns the gateway, you or your ISP). Some people appear to be rather unhappy with the Virgin DNS server according to this.
Tailscale sets this value. Setting up “MagicDNS” in your tailnet (or possibly other DNS settings like “Override Local DNS” or “Split DNS” according to this thread: DNS Stuck on 100.100.100.100 · Issue #3817 · tailscale/tailscale · GitHub) can set this as the DNS server.
This is not a real nameserver, it’s just a placeholder value Tailscale uses. It’s no problem when connected to your tailnet, because Tailscale figures out what DNS server should be used based on the configuration you have specified. In your case however, there is something about setting the static IP in the DietPi-Config menu that is breaking Tailscale.
See here, on DHCP you have a Tailscale IP address (100.79.14.71):
On the static setting this value is missing. Something about the DietPi-Config method is removing it, which means you are off of the tailnet and the 100.100.100.100 nameserver can no longer be reached.
If you want this device to have a static IP address, a better way to accomplish that would be to configure a DHCP reservation in your router (or whatever serves DHCP on your network) and leave the DietPi-Config configuration as it is.
Another option would be to change the DNS server (you can do it in the DietPi-Config menu while you are in there setting the static IP address), but this may affect your ability to use Tailscale. A simple tailscale up might get you back on the tailnet, but if not you might have to uninstall/reinstall after setting the static IP.