[Bug 1850704] Re: networkd doesn't set MTUBytes if interface is already up

[Steve Langasek]

Hello Dan, or anyone else affected,

Accepted systemd into bionic-proposed. The package will build now and be
available at
https://launchpad.net/ubuntu/+source/systemd/237-3ubuntu10.33 in a few
hours, and then in the -proposed repository.

Please help us by testing this new package.  See
https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how
to enable and use -proposed.  Your feedback will aid us getting this
update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug,
mentioning the version of the package you tested and change the tag from
verification-needed-bionic to verification-done-bionic. If it does not
fix the bug for you, please add a comment stating that, and change the
tag to verification-failed-bionic. In either case, without details of
your testing we will not be able to proceed.

Further information regarding the verification process can be found at
https://wiki.ubuntu.com/QATeam/PerformingSRUVerification .  Thank you in
advance for helping!

N.B. The updated package will be released to -updates after the bug(s)
fixed by this package have been verified and the package has been in
-proposed for a minimum of 7 days.

** Tags removed: verification-failed verification-failed-bionic
** Tags added: verification-needed verification-needed-bionic

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https://bugs.launchpad.net/bugs/1850704

Title:
  networkd doesn't set MTUBytes if interface is already up

Status in systemd:
  Fix Released
Status in systemd package in Ubuntu:
  Fix Released
Status in systemd source package in Bionic:
  Fix Committed

Bug description:
  [impact]

  if a networkd .network file specifies a [Link] section with
  MTUBytes=XXX set, networkd will only apply that mtu if the interface
  is down when networkd starts; if the interface is already up, the mtu
  won't be applied.

  [test case]

  on a bionic system, create a .network file like:

  [Match]
  Name=ens8

  [Link]
  MTUBytes=6666

  then, reboot.  The interface should be set correctly with that mtu:

  $ ip l show ens8
  3: ens8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 6666 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
      link/ether 52:54:00:30:4c:1e brd ff:ff:ff:ff:ff:ff

  
  now, manually change the interface back to 1500 mtu, and restart networkd, then recheck the mtu:

  $ ip l show ens8
  3: ens8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 6666 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
      link/ether 52:54:00:30:4c:1e brd ff:ff:ff:ff:ff:ff
  $ sudo ip l set mtu 1500 dev ens8
  $ ip l show ens8
  3: ens8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
      link/ether 52:54:00:30:4c:1e brd ff:ff:ff:ff:ff:ff
  $ sudo systemctl restart systemd-networkd
  $ ip l show ens8
  3: ens8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
      link/ether 52:54:00:30:4c:1e brd ff:ff:ff:ff:ff:ff

  [regression potential]

  low, but any regression would likely involve failure to correctly set
  the configured mtu.

  this is needed only in bionic, it's fixed in disco and later already.

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