[Bug 1918141] Re: nfs-server.service needs name resolution and network online

[Niklas Edmundsson]

First off, nfs-server starts but doesn't export the unresolvable hosts.
The failures do show in systemctl status nfs-server.

To reproduce in a VM you'll likely need something that causes
network.target to be fulfilled but actual network traffic to not be
forwarded until a few seconds later when network-online.target is
fulfilled. Exactly how to do this varies between setups, but adding a
unit that fulfills network.target shouldn't be too hard, delaying the
network startup in a meaningful way is the tricky one. In any case, I
recommend systemd-analyze plot > boot.svg to get an overview of the boot
timings.

If I find time today I can take a stab at figuring something out that
emulates the behavior we see on physical hardware, for these machines we
see between 8 to 10 seconds for systemd-networkd-wait-online.service to
complete, with the corresponding delay in network-online.target
fulfillment. I think the record I've seen on physical hardware was 30+
seconds on first boot when doing a major OS upgrade trigging a more
involved firmware download and NIC restart...

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https://bugs.launchpad.net/bugs/1918141

Title:
  nfs-server.service needs name resolution and network online

Status in nfs-utils package in Ubuntu:
  Confirmed

Bug description:
  nfs-server.service has insufficient dependencies to start correctly in
  a setting where the nfs exports list contains DNS host names (not in
  local hosts file) or netgroups served via network (for example sssd).

  Typical failures listed by systemctl status nfs-server.service are:

  Mar 08 14:16:52 server.example.com exportfs[844]: exportfs: Failed to resolve client1.example.com
  Mar 08 14:16:52 server.example.com exportfs[844]: exportfs: Failed to resolve client2.example.com

  
  Our workaround is to add the appropriate dependecies in /etc/systemd/system/nfs-server.service.d/dependencies.conf like so:

  ----------------------8<----------------------------
  [Unit]
  # nfs-server.service runs exportfs on startup, thus we need to be able to
  # do host and netgroup lookups which requires network to be online.
  After=network-online.target nss-lookup.target nss-user-lookup.target
  ----------------------8<----------------------------

  While nfs-server.service do depend on network.target, that only means
  that the network has been configured. On physical hardware it can take
  significantly longer for the network to come online (8+ seconds for
  our 10G NICs). Also note that we configure static IP:s via systemd-
  networkd, things might behave differently when using DHCP, network-
  manager etc. In any case, depending on network.target is almost always
  wrong, and network-online.target is usually the right one.

  nss-lookup.target is needed to ensure that DNS resolution works, and
  nss-user-lookup.target is the best approximation to ensure that
  netgroup resolution via sssd or equivalent works. Usually things "just
  works" even without these dependencies, but to ensure correct startup
  they should be present.

  It should be noted that this seems to once have been fixed in Ubuntu,
  but has been lost along the way for quite some time. When googling I
  find for example https://code.launchpad.net/~ubuntu-
  branches/ubuntu/wily/nfs-utils/wily-201507271018/+merge/265946 that
  fixes the network.target vs network-online.target dependency, but it
  has since been lost in the wind it seems.

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