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

[Dan Streetman]

Public bug reported:

[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.

** Affects: systemd
     Importance: Unknown
         Status: Unknown

** Affects: systemd (Ubuntu)
     Importance: Undecided
         Status: Fix Released

** Affects: systemd (Ubuntu Bionic)
     Importance: Medium
     Assignee: Dan Streetman (ddstreet)
         Status: In Progress

** Also affects: systemd (Ubuntu Bionic)
   Importance: Undecided
       Status: New

** Changed in: systemd (Ubuntu)
       Status: New => Fix Released

** Changed in: systemd (Ubuntu Bionic)
     Assignee: (unassigned) => Dan Streetman (ddstreet)

** Changed in: systemd (Ubuntu Bionic)
   Importance: Undecided => Medium

** Changed in: systemd (Ubuntu Bionic)
       Status: New => In Progress

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

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

Status in systemd:
  Unknown
Status in systemd package in Ubuntu:
  Fix Released
Status in systemd source package in Bionic:
  In Progress

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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