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segFaultCreator

Member

Registered: 2021-11-04

Posts: 7

[SOLVED] systemd-resolved switches to fallback dns

I have an issue with my DNS configuration with systemd.

My configuration has worked flawlessly for the past year, when it suddenly stopped working. I think it might come from a recent update (around the 6th of October), although I can't pinpoint it to a particular version.

/etc/resolv.conf is symlinked to /run/systemd/resolve/stub-resolv.conf

When I connect to my university network, systemd-resolved correctly picks up the DNS servers as well as the search domains from the access point.
But the, for some reason, after a few seconds, it switches to the fallback DNS servers

Switching to fallback DNS server 1.1.1.1#cloudflare-dns.com.

It's a big problem from me, because the uni does not allow DNS queries to pass through the firewall, it's also an issue at work for services available on the LAN only.

Is there a configuration or a timeout that must be increased somewhere ?

Currently my only solution is to set the DNS using resolvctl using a while loop. It works but it's far from ideal.

Thank you !

Last edited by segFaultCreator (2022-10-10 14:24:18)

schard

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From: Hannover

Registered: 2016-05-06

Posts: 2,152

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Re: [SOLVED] systemd-resolved switches to fallback dns

What does systemd-resolved's journal say?

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segFaultCreator

Member

Registered: 2021-11-04

Posts: 7

Re: [SOLVED] systemd-resolved switches to fallback dns

Oct 07 12:55:07 carbon-x1 systemd-resolved[50208]: wlan0: Bus client set DNS server list to: 10.193.64.16, 10.193.56.16, 10.193.65.16
Oct 07 12:55:07 carbon-x1 systemd-resolved[50208]: wlan0: Bus client set search domain list to: xy.ch, xz.ch, yz.ch, ad.yz.ch
Oct 07 12:55:07 carbon-x1 systemd-resolved[50208]: Switching to fallback DNS server 1.1.1.1#cloudflare-dns.com.
Oct 07 12:55:22 carbon-x1 systemd-resolved[50208]: Using degraded feature set UDP instead of UDP+EDNS0 for DNS server 2606:4700:4700::1111#cloudflare-dns.com.

seth

Member

Registered: 2012-09-03

Posts: 60,805

Re: [SOLVED] systemd-resolved switches to fallback dns

https://wiki.archlinux.org/title/system … d#Fallback - last paragraph.
Alternatively just use a resolver/config that's not systematically dumb.

The fallback however likely happens because your universities DNS doesn't respond (in time), possibly because it's overloaded/underdimensioned and in this case

the uni does not allow DNS queries to pass through the firewall

=> https://wiki.archlinux.org/title/DNS_over_HTTPS_servers

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segFaultCreator

Member

Registered: 2021-11-04

Posts: 7

Re: [SOLVED] systemd-resolved switches to fallback dns

Alright so the issue was coming from the DHCP  server distributing an IPv6 address way before distributing an IPv4 one.
The DNS were already configured at this point, causing systemd-resolved to try to contact the (IPv4 addressed) DNS servers without having an IPv4 configured for the machine.

I disabled IPv6, which I don't use anyway, in the DHCP client by adding

noipv6rs
noipv6

to /etc/dhcpcd.conf

Last edited by segFaultCreator (2022-10-10 14:25:42)