systemd-resolved spams syslog whenever I'm disconnected

[Santiago Gala]

Every time I suspend my laptop and resume in a situation where I don't have connectivity
I get big syslog spam from systemd-resolved. See for instance yesterday when I went out:

Jun 18 18:48:16 chiron systemd[1]: Starting Suspend...
Jun 18 18:48:16 chiron systemd-resolved[1868]: Switching to fallback DNS server 8.8.8.8.
Jun 18 18:48:16 chiron systemd-resolved[1868]: Switching to fallback DNS server 8.8.4.4.
Jun 18 18:48:16 chiron systemd-resolved[1868]: Switching to fallback DNS server 2001:4860:4860::8888.
Jun 18 18:48:16 chiron systemd-resolved[1868]: Switching to fallback DNS server 2001:4860:4860::8844.
...

And the last messages of the "episode":

Jun 18 20:13:28 chiron systemd-resolved[1868]: Switching to fallback DNS server 8.8.8.8.
Jun 18 20:13:28 chiron systemd-resolved[1868]: Switching to fallback DNS server 8.8.4.4.
Jun 18 20:13:28 chiron systemd-resolved[1868]: Switching to fallback DNS server 2001:4860:4860::8888.
Jun 18 20:13:28 chiron systemd-resolved[1868]: Switching to fallback DNS server 2001:4860:4860::8844.
Jun 18 20:13:28 chiron systemd-resolved[1868]: Switching to fallback DNS server 8.8.8.8.
Jun 18 20:13:28 chiron NetworkManager[30674]: <info> [1497809608.0889] device (wlp2s0): supplicant interface state: associ
ated -> 4-way handshake

So this correspond to roughly 85 minutes of disconnection, during which I got...

$ grep "fallback DNS" /var/log/syslog.1 | wc -l
444066

This correspond to 5224 messages/minute, 87 messages/second

I had a few other episodes in the last week, including a 4h train roundtrip:

$ zgrep "fallback DNS" /var/log/syslog* | wc -l
1964217

I noticed the bug during the trip, when the SSD activity was slowing my machine in a clearly noticeable way.

I'm running:

$ lsb_release -rd
Description: Ubuntu 17.04
Release: 17.04

$ apt-cache policy $(dpkg-query -S $(which systemd-resolve ) | awk '{print $1}')
systemd:
  Installed: 232-21ubuntu3
  Candidate: 232-21ubuntu3
  Version table:
 *** 232-21ubuntu3 500
        500 http://es.archive.ubuntu.com/ubuntu zesty-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     232-21ubuntu2 500
        500 http://es.archive.ubuntu.com/ubuntu zesty/main amd64 Packages

[Launchpad Janitor (janitor)]

This bug was fixed in the package systemd - 233-8ubuntu2

---------------
systemd (233-8ubuntu2) artful; urgency=medium

  * Disable fallback DNS servers.
    This causes resolved to call-home to google, attempt to access network when
    none is available, and spams logs. (LP: #1449001, #1698734)
  * SECURITY UPDATE: Out-of-bounds write in systemd-resolved.
    CVE-2017-9445 (LP: #1695546)

 -- Dimitri John Ledkov <email address hidden> Wed, 28 Jun 2017 13:27:28 +0100

[Dimitri John Ledkov (xnox)]

Note disabling fallback DNS servers helps - https://bugs.launchpad.net/ubuntu/zesty/+source/systemd/+bug/1449001

Also upstream have downgraded this message to /silently/ switch between dns servers.

Disabling the fallback DNS servers will be cherrypick into zesty.

[Santiago Gala (sgala)]

Even if the log spamming disappears, I don't think switching DNS servers ~100 times per second will be a reasonable use of scarce battery power...

I mean, loosing connectivity is a common situation when a laptop is used while commuting, in a train, a bus, cafe or station, in which case the batteries are a scarce resource.

The least thing that could be expected about the underlying switching of servers is that there is some sort of exponential back-off, probably reset when routes to the servers change