[Bug 1807273] Re: netplan not respecting mtu

[Steve Langasek]

A possible SRU regression has been reported against netplan.io
0.96-0ubuntu0.18.10.2 in LP: #1825206.  This version has been rolled
back to -proposed while the investigation is ongoing.

** Changed in: netplan.io (Ubuntu Cosmic)
       Status: Fix Released => Fix Committed

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https://bugs.launchpad.net/bugs/1807273

Title:
  netplan not respecting mtu

Status in netplan:
  Fix Released
Status in netplan.io package in Ubuntu:
  Fix Released
Status in netplan.io source package in Bionic:
  Fix Committed
Status in netplan.io source package in Cosmic:
  Fix Committed

Bug description:
  [Impact]
  DHCP configurations where custom settings (MTU) need to be applied. This is a subset of the changes presented in bug 1759014.

  [Test case]
  1) Configure netplan for the particulars of the network by configuring an appropriate dhcp{4,6}-override stanza:

  network:
    version: 2
    ethernets:
      engreen:
        dhcp4: true
        dhcp4-overrides:
          use-mtu: false

  Additionally, if so required, add a custom MTU to the configuration.
  e.g.

        mtu: 1700

  (See https://netplan.io/reference#dhcp-overrides for the available
  options)

  2) Run 'netplan apply' or reboot to have the configuration applied.
  3) Validate that the routes / DNS are properly ignored and/or replaced by the defined values.

  [Regression potential]
  Minimal; this adds new values to the configuration generated for networkd or NetworkManager. Existing configurations will remain unchanged, but new configurations using the dhcp{4,6}-overrides fields will benefit from additional flexibility.

  ---

  This is very similar to https://bugs.launchpad.net/bugs/1724895, but I
  decided to open this as i have tried what has succeeded for others.
  Thanks for any assistance.

  I'm trying to set the MTU to 1500 and using a match on mac address,
  but after a netplan apply or reboot, it does not change or stick. This
  is on a EC2 T2.Medium instance. Actual configuration from a dev
  server:

  network:
    version: 2
    ethernets:
      eth0:
        match:
          macaddress: 12:0f:ae:49:5d:06
        mtu: 1500
        dhcp4: true
        nameservers:
          search: [ devbuilds.vpc, ec2.internal ]

  $> cat /var/run/systemd/network/10-netplan-eth0.link
  [Match]
  MACAddress=12:0f:ae:49:5d:06

  [Link]
  WakeOnLan=off
  MTUBytes=1500

  $> cat /var/run/systemd/network/10-netplan-eth0.network
  [Match]
  MACAddress=12:0f:ae:49:5d:06

  [Network]
  DHCP=ipv4
  Domains=devbuilds.vpc ec2.internal

  [DHCP]
  UseMTU=true
  RouteMetric=100

  $> ip link show eth0
  2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9001 qdisc mq state UP mode DEFAULT group default qlen 1000
      link/ether 12:0f:ae:49:5d:06 brd ff:ff:ff:ff:ff:ff
  # manually set MTU
  $> sudo ip link set dev eth0 mtu 1500
  $> ip link show eth0
  2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT group default qlen 1000
      link/ether 12:0f:ae:49:5d:06 brd ff:ff:ff:ff:ff:ff
  $> sudo netplan generate
  $> sudo netplan apply
  $> ip link show eth0
  2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9001 qdisc mq state UP mode DEFAULT group default qlen 1000
      link/ether 12:0f:ae:49:5d:06 brd ff:ff:ff:ff:ff:ff

  The last netplan commands could be replaced with a reboot with the
  same result. This configuration seemed to help others, so hopefully
  i'm simply missing something or perhaps this is related to EC2?

  Attaching output of cloud-init

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