Comment 9 for bug 1817903

> I do not use NetworkManager as I use fixed ip, so the ip, etc. and dns is/was
> specified in /etc/network/interfaces

ah, ok, so you are not using systemd-networkd then, ok that makes sense since you upgraded from pre-bionic where ifupdown/resolvconf are the default. In bionic they have been replaced with just systemd-networkd (and netplan for configuration) but upgrades keep ifupdown/resolvconf.

> So it seems to that the issue is really that systemd-resolve is adding the options
> to resolv.conf when it shouldn't interfere

no - systemd-resolved does not add anything to the /etc/resolv.conf file, it *only* sets up its privately-managed files, under /run/systemd/resolve/. If /etc/resolv.conf is symlinked to either the stub or normal systemd-resolved file, then it will pick up all systemd-resolved's config (since it's just a symlink, of course) but systemd won't edit an existing /etc/resolv.conf.

> systemd-resolve isn't checking /etc/network/interfaces for dns-nameservers....

it never does. e/n/i is 100% ifupdown, completely separate from systemd-networkd.

What's happening here is resolvconf has a systemd service that pulls "original" config from systemd-resolved:

$ cat /lib/systemd/system/resolvconf-pull-resolved.service
[Unit]
ConditionPathExists=/run/resolvconf/enable-updates
ConditionFileIsExecutable=/sbin/resolvconf

[Service]
Type=oneshot
ExecStart=+-/bin/sh -c 'cat /run/systemd/resolve/stub-resolv.conf | /sbin/resolvconf -a systemd-resolved'

This service is triggered whenever the systemd-resolved stub conf file changes:

$ cat /lib/systemd/system/resolvconf-pull-resolved.path
[Path]
PathChanged=/run/systemd/resolve/stub-resolv.conf
PathExists=/run/systemd/resolve/stub-resolv.conf

[Install]
WantedBy=systemd-resolved.service

This service is just wrong; the stub-resolv.conf file is *strictly* for use as a /etc/resolv.conf symlink when systemd-resolved is managing the system's dns. There is another file, /run/systemd/resolve/resolv.conf, that is intended for use by other local applications that control the system's dns but also want to know about any dns info that systemd-resolved might know about; this file leaves out the local stub resolver's address (127.0.0.53) and the edns0 option.

As a temporary workaround until resolvconf can be fixed, can you try reinstalling resolvconf, and then disable it from pulling in the systemd-resolved stub configuration:

$ sudo apt install resolvconf
$ sudo systemctl disable resolvconf-pull-resolved.path
$ sudo systemctl disable resolvconf-pull-resolved.service
$ sudo reboot

After reboot, you should then have a /etc/resolv.conf that does not contain the 127.0.0.53 local stub resolver nor the edns0 option.

Let me know if that works. Thanks!