[Bug 1698734] Re: systemd-resolved spams syslog whenever I'm disconnected

[Santiago Gala]

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

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Title:
  systemd-resolved spams syslog whenever I'm disconnected

Status in systemd package in Ubuntu:
  Fix Released
Status in systemd source package in Xenial:
  New
Status in systemd source package in Zesty:
  New
Status in systemd source package in Artful:
  Fix Released

Bug description:
  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

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