[Bug 1649931] Re: systemd-networkd needs to ensure DNS is up before network-online.target

[Steve Langasek]

>> For systemd portion, we carry the
>>  systemd-networkd-resolvconf-update.service as a delta from Debian

> I'm not sure what you mean here; the systemd package in zesty is currently
> in sync with Debian.

Ok, it appears that post-16.04, systemd-networkd-resolvconf-
update.service is dropped from the package in favor of
debian/extra/units/systemd-resolved.service.d/resolvconf.conf, so this
patch does not apply.

I have verified that there is no ordering bug post-16.04, provided that
the changes to resolvconf are applied. /lib/systemd/system/systemd-
resolved.service.d/resolvconf.conf causes systemd-resolved to notify
resolvconf of its nameserver, if /run/resolvconf/enable-updates exists.
/run/resolvconf/enable-updates exists if resolvconf.service is started.
resolvconf.service is started Before=network-pre.target; network.target
is After=network-pre.target; systemd-resolved.service is
After=network.target.  So only the change to resolvconf is needed for
yakkety and beyond.

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https://bugs.launchpad.net/bugs/1649931

Title:
  systemd-networkd needs to ensure DNS is up before network-
  online.target

Status in resolvconf package in Ubuntu:
  Fix Released
Status in systemd package in Ubuntu:
  New
Status in resolvconf source package in Xenial:
  New
Status in systemd source package in Xenial:
  New

Bug description:
  Currently resolvconf and systemd-networkd don't ensure DNS has been
  configured before allowing network-online.target to be reached.

  This was discussed in https://launchpad.net/bugs/1636912 however it
  was not a regression since there aren't any users of networkd + DNS
  early in boot at this time, it was requested that we move this DNS
  issue to a separate bug.

  
  [SRU]
  Fix: switch resolvconf.service to run Before=network-pre.target and add Wants=network-pre.target.  Add a Before=network-online.target to systemd-networkd-resolvconf-update.service to ensure we update /etc/resolv.conf with DNS config prior to reaching network-online.target.

  Regression potential: Low. networkd is not widely being used outside
  of netplan/snappy in xenial.

  
  Test Case:
    lxc launch ubuntu-daily:xenial x1
    lxc exec x1 /bin/bash

    # make sure you're on systemd-229-4ubuntu13
    apt update && apt install -y systemd

    # enable networkd and netplan
    apt install -y nplan
  cat <<EOF > /etc/netplan/nplan.yaml
  network:
    version: 2
    ethernets:
      eth0:
        dhcp4: true
  EOF
    sed -i.orig -e 's/^source/# source/' /etc/network/interfaces
    
    netplan generate
    
    # make sure cloud-init.service uses networkd
    sed -i.orig -e '/After=networking.service/a After=systemd-network-wait-online.service' /lib/systemd/system/cloud-init.service

    reboot

    # check that the order of execution with:
    journalctl -o short-precise --unit resolvconf.service --unit network-online.target --unit systemd-networkd-wait-online.service --unit systemd-networkd-resolvconf-update.service

    # the order should be:
      1. resolvconf:  systemd[1]: Started Nameserver information manager.
      2. systemd-networkd-wait-online.service:  systemd[1]: Starting Wait for Network to be Configured...
      3. systemd-networkd-resolvconf-update.service: systemd[1]: Started Update resolvconf for networkd DNS.
      4. network-online.target: systemd[1]: Reached target Network is Online.

  === BAD OUTPUT ===
  On a failing system, Reached target Network is Online occurs before (1, 2, or 3) above, like this output:

  Dec 15 19:18:15.233443 x4 systemd[1]: Started Nameserver information manager.
  Dec 15 19:18:15.797857 x4 systemd[1]: Starting Wait for Network to be Configured...
  Dec 15 19:18:15.799573 x4 systemd-networkd-wait-online[145]: ignoring: lo
  Dec 15 19:18:15.804949 x4 systemd-networkd-wait-online[145]: ignoring: lo
  Dec 15 19:18:15.805079 x4 systemd-networkd-wait-online[145]: ignoring: lo
  Dec 15 19:18:29.100305 x4 systemd[1]: Starting Update resolvconf for networkd DNS...
  Dec 15 19:18:29.101870 x4 systemd[1]: Started Wait for Network to be Configured.
  Dec 15 19:18:29.102144 x4 systemd[1]: Reached target Network is Online.
  Dec 15 19:18:29.212842 x4 systemd[1]: Started Update resolvconf for networkd DNS.

  
  === GOOD OUTPUT ===
  On a passing system, Reached target Network is Online occurs after 1, 2, and 3.

  Dec 15 19:28:42.548545 x4 systemd[1]: Started Nameserver information manager.
  Dec 15 19:28:43.144389 x4 systemd[1]: Starting Wait for Network to be Configured...
  Dec 15 19:28:43.146155 x4 systemd-networkd-wait-online[145]: ignoring: lo
  Dec 15 19:28:56.081487 x4 systemd[1]: Started Wait for Network to be Configured.
  Dec 15 19:28:56.100353 x4 systemd[1]: Starting Update resolvconf for networkd DNS...
  Dec 15 19:28:56.124005 x4 systemd[1]: Started Update resolvconf for networkd DNS.
  Dec 15 19:28:56.124555 x4 systemd[1]: Reached target Network is Online.

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