Something repeatedly puts old nameservers into `resolv.conf`

After I use Android thethering to give Internet to my NixOS 22.05 laptop, sometimes (certainly after suspend/resume), something keeps generating the phone as nameservers into /etc/resolv.conf, even though the phone has been detached for days:

# Generated by resolvconf
nameserver 192.168.110.175          # <- the phone
nameserver 192.168.1.1              # <- my real wifi nameserver
nameserver 2001:1620:2777:1::10     # <- more stuff from the phone
nameserver 2001:1620:2777:2::20     # <- more stuff from the phone
options edns0

Because the “bad” nameserver is first, it makes all my network connections hang on the DNS queries for many seconds, making browsing and SSHing a total pain.

Only rebooting fixes it.

Any idea where this information could get stuck?

I’m using nm-applet which uses wpa-supplicant for the Wifi.

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I can also reliably trigger the problem by switching between my two wifi networks (2.4 GHz and 5 GHz) in nm-applet.

For both, the settings are set to Automatic (DHCP) and no custom nameservers are defined in the NetworkManager GUI.

Where could this info be lingering?

Which resolver daemon are you using?

If it’s any other than systemd-resolved, is systemd-resolved still running?

No, systemd-resolved is not running, and as far as I can tell, services.resolved.enable defaults to false.

I don’t think any resolver is running; doesn’t network-manager just update /etc/resolv.conf directly?

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