On the same machine as sdeziel found the problem:
root@lxd02:~# lsb_release -rd
Description: Ubuntu 20.04.2 LTS
Release: 20.04
root@lxd02:~# apt-cache policy systemd
systemd:
Installed: 245.4-4ubuntu3.7
Candidate: 245.4-4ubuntu3.7
Version table:
*** 245.4-4ubuntu3.7 500
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 lxd02 5.4.0-77-generic #86-Ubuntu SMP Thu Jun 17 02:35:03 UTC 2021 x86_64
x86_64 x86_64 GNU/Linux
root@lxd02:~# date
Tue 29 Jun 2021 01:16:13 PM UTC
root@lxd02:~# tail -f /var/log/syslog
Jun 29 13:16:20 lxd02 dnsmasq[1896]: using nameserver 127.0.0.53#53
Jun 29 13:16:20 lxd02 dnsmasq[1896]: reading /etc/resolv.conf
Jun 29 13:16:20 lxd02 dnsmasq[1896]: using local addresses only for domain lxd
Jun 29 13:16:20 lxd02 dnsmasq[1896]: using nameserver 127.0.0.53#53
^C
root@lxd02:~# uptime
13:16:35 up 4 min, 1 user, load average: 0.74, 1.46, 0.74
Problem is still occuring.
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https://bugs.launchpad.net/bugs/1891215
Title:
systemd-resolved re-creates /run/systemd/resolve/*resolv.conf for
every IPv6 RA received
Status in systemd package in Ubuntu:
Incomplete
Bug description:
# 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
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