Subject: Re: systemd-resolved ignoring /etc/resolv.conf custom nameservers after Bookworm upgrade

Le 05/03/2026 … 03:47, Vahid Shaik a ‚critÿ:

Hi all,

Hello,

Please take into consideration that I am far from competent for anything network related.

[...]

1. Setting DNS= and FallbackDNS= in /etc/systemd/resolved.conf ? works temporarily but reverts after reboot on one of my machines

ArchLinux Wiki advise to create a /etc/systemd/resolved.conf.d/dns_servers.conf file to indicate custom DNSes: https://wiki.archlinux.org/title/Systemd-resolved#Manually

2. Symlinking /etc/resolv.conf to /run/systemd/resolve/resolv.conf ?
gives me the "upstream" servers but breaks .local resolution

From what I understand (wrongly?), that the other way around: "[...]systemd-resolved maintains the /run/systemd/resolve/resolv.conf
file for compatibility with traditional Linux programs. This file may be symlinked from /etc/resolv.conf and is always kept up-to-date,
containing information about all known DNS servers.[...]" https://www.freedesktop.org/software/systemd/man/latest/systemd-resolved.service.html#/etc/resolv.conf

3. Disabling systemd-resolved entirely ? works but feels like fighting
the system

Why? I don't know why but there seem to be a general misconception that systemd-resolved is the default for headless machines in the Debian
ecosystem.
Paragraph 5.2.4 of Bookworm Release Notes states: "[...]Note that systemd-resolved was not, and still is not, the default DNS resolver in Debian[...]" https://www.debian.org/releases/bookworm/amd64/release-notes/ch-information.en.html#systemd-resolved

--- PyGate Linux v1.5.12
* Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)

Subject: Re: systemd-resolved ignoring /etc/resolv.conf custom nameservers after Bookworm upgrade

On Thu, 2026-03-05 at 01:58 +0000, Vahid Shaik wrote:

1. Setting DNS= and FallbackDNS= in /etc/systemd/resolved.conf ?

?

works temporarily but reverts after reboot on one of my machines

You have to find out what "reverts" it & how (was that config
overwritten or ignored?).

And when systemd-resolved is managing resolv.conf, you have to make
sure nothing else is trying to do the same.

2. Symlinking /etc/resolv.conf to /run/systemd/resolve/resolv.conf ?

?

gives me the "upstream" servers but breaks .local resolution

Assuming by .local you mean Multicast DNS (mDNS), then systemd-resolved
can handle that or not, depending on the MulticastDNS setting in /etc/systemd/resolve.conf (if disabled there, you'll need nss-mdns to
resolve .local names through mDNS). Of course, if your .local host
names are regular DNS entries in Pi-hole (not a good idea), then you'll probably have to disable mDNS entirely.

3. Disabling systemd-resolved entirely ? works but feels like
fighting the system

For those who've dealt with this ? what's the recommended Debian

way

to handle custom DNS with systemd-resolved? Should I stick with
resolved and configure it properly, or is disabling it and managing resolv.conf manually still a valid approach on Bookworm?

Both options should still work.

The systemd-resolved.service(8) manpage documents how systemd-resolved interacts with /etc/resolv.conf, so that's the best place to start.

Also, whatever manages your network connection might be trying to set
custom DNS server for that connection, e.g. provided by DHCP or NM
static configuration or VPN setup. When using systemd-resolved, the
command 'resolvectl status' will show the global & per-link DNS servers systemd-resolved is using. Per-link (=connection) settings override
global ones.



--
Jan Claeys

(please don't CC me when replying to the list)

--- PyGate Linux v1.5.12
* Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)

Subject: Re: systemd-resolved ignoring /etc/resolv.conf custom nameservers after? Bookworm upgrade

On Wed, 2026-03-04 at 22:43 -0500, Robert Heller wrote:

If systemd-resolved is installed and running, you completely lose
control of /etc/resolv.conf -- editing /etc/resolv.conf is not
effectual at all.

That's not (entirely) true, systemd-resolved can work with resolv.conf
as explained in the systemd-resolved.service(8) manpage under the '/ETC/RESOLV.CONF' header.


--
Jan Claeys

(please don't CC me when replying to the list)

--- PyGate Linux v1.5.12
* Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)