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While I can connect to mobile broadband with Ubuntu 22.04 and I can transmit packets (ping 8.8.8.8 works fine), DNS resolution doesn't work. The result is that I can't use the internet connection.

This is the output of nslookup google.de:

;; connection timed out; no servers could be reached

I've tried fiddeling with the settings in /etc/systemd/resolved.conf as suggested here. Setting a fallback DNS server doesn't work at all. Overriding the DNS server does (once I restart systemd-resolved). The problem is that I can't set a fixed DNS server, as I use VPNs extensively and I need to use a DNS server for name resolution within the VPN. The DNS server is set via network-manager, so I don't want to hardcode it.

The content of /etc/resolv.conf is:

nameserver 127.0.0.53
options edns0 trust-ad
search .

Does anyone have any idea how to fix this? The Q&A I've referenced above is pretty old and I'd image the bug in the packet that is discussed there has been fixed since then.

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In my case the main issue is that somehow systemd-resolved ignoring it's own /run/systemd/resolve/resolv.conf. Because when I am connecting my VPN I see that the correct nameserver being added there. But it makes no difference whatsoever. Probably some misconfiguration or I do not understand how it is supposed to work.

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I figured out the cause of this. It was not the DNS lookup after all, but it's a bug in Ubuntu's network-manager module that was introduced in 1.35.91-1. It's described here. The problem is that ipv6 isn't enabled correctly for the mobile broadband connection and the DNS server that is automatically configured in systemd-resolved is an ipv6 server only. That's why it's unreachable, but at least it's configured correctly.

Unfortunately I can't downgrade to a network-manager version that still works, as the Ubuntu 22.04 apt repo doesn't include an older version.

Setting a static global DNS server in /etc/systemd/resolved.conf works as a workaround after all. It doesn't seem to override the VPN's DNS server.

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