Comment 0 for bug 1891215

# Issue description:

On 2 Linode VMs that are used as lxd hosts, we noticed that /run/systemd/resolve/*resolv.conf were re-created quite frequently (~ once per second). We noticed because of the log noise from lxd's dnsmasq instance using inotify to watch the target of /etc/resolv.conf (which points to the stub-resolv.conf in our case). This was (wrongly) reported as a lxd bug (https://github.com/lxc/lxd/issues/7765) until it became apparent it was more likely to be a problem with systemd(-resolved)?.

The log noise is the observable problem that would be nice to see addressed:

  root@lxd02:~# uptime
   17:55:48 up 9:52, 1 user, load average: 0.18, 0.11, 0.05
  root@lxd02:~# journalctl -b0 | grep -cF dnsmasq
  158609

Upon further investigation, it seems that systemd-resolved re-creates the resolv.conf and stub-resolv.conf files whenever an IPv6 RA is received.

1) One can observe that by setting systemd-resolved's service in debug mode:

$ sudo systemctl edit systemd-resolved

and in the editor that is opened, add and save this content:

[Service]
Environment=SYSTEMD_LOG_LEVEL=debug

then restart systemd-resolved and watch the logs scroll by with:

$ journalctl -fu systemd-resolved

3) In another terminal, watch the files be recreated with:

watch -d -n 0.1 stat /run/systemd/resolve/stub-resolv.conf

3) In yet another terminal, run a packet capture and watch "ICMP6, router advertisement" messages come by:

sudo tcpdump -ni eth0 icmp6

You will see that every time a RA packet comes in, resolved's journal will log this:

  Aug 11 17:33:55 lxd02 systemd-resolved[15368]: Sent message type=signal sender=n/a destination=n/a path=/org/freedesktop/resolve1 interface=org.freedesktop.DBus.Properties member=PropertiesChanged cookie=244 reply_cookie=0 signature=sa{sv}as error-name=n/a error-message=n/a

And the stat monitoring terminal will blink to highlight the new inode and timestamps of the freshly replaced stub-resolv.conf file.

# Additional information:

root@lxd02:~# lsb_release -rd
Description: Ubuntu 20.04.1 LTS
Release: 20.04

root@lxd02:~# apt-cache policy systemd
systemd:
  Installed: 245.4-4ubuntu3.2
  Candidate: 245.4-4ubuntu3.2
  Version table:
 *** 245.4-4ubuntu3.2 500
        500 http://archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     245.4-4ubuntu3 500
        500 http://archive.ubuntu.com/ubuntu focal/main amd64 Packages

root@lxd02:~# uname -a
Linux lxd01 5.4.0-42-generic #46-Ubuntu SMP Fri Jul 10 00:24:02 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux