Curtin vmtest verification for eoan using systemd from -proposed has run
successfully.

(neipa) ipv6_mtu % egrep "242-7ubuntu3.3" 
output/EoanTestNetworkMtu/logs/install-serial.log
[  136.520219] cloud-init[765]: Get:12 http://archive.ubuntu.com/ubuntu 
eoan-proposed/main amd64 libnss-systemd amd64 242-7ubuntu3.3 [126 kB]
[  136.527400] cloud-init[765]: Get:13 http://archive.ubuntu.com/ubuntu 
eoan-proposed/main amd64 udev amd64 242-7ubuntu3.3 [1202 kB]
[  136.693312] cloud-init[765]: Get:14 http://archive.ubuntu.com/ubuntu 
eoan-proposed/main amd64 libudev1 amd64 242-7ubuntu3.3 [76.9 kB]
[  136.701002] cloud-init[765]: Get:15 http://archive.ubuntu.com/ubuntu 
eoan-proposed/main amd64 systemd-sysv amd64 242-7ubuntu3.3 [9360 B]
[  136.722467] cloud-init[765]: Get:17 http://archive.ubuntu.com/ubuntu 
eoan-proposed/main amd64 libpam-systemd amd64 242-7ubuntu3.3 [130 kB]
[  136.743657] cloud-init[765]: Get:19 http://archive.ubuntu.com/ubuntu 
eoan-proposed/main amd64 systemd amd64 242-7ubuntu3.3 [3440 kB]
[  137.138458] cloud-init[765]: Get:20 http://archive.ubuntu.com/ubuntu 
eoan-proposed/main amd64 libsystemd0 amd64 242-7ubuntu3.3 [262 kB]
[  142.884502] cloud-init[765]: Preparing to unpack 
.../libnss-systemd_242-7ubuntu3.3_amd64.deb ...
[  142.887869] cloud-init[765]: Unpacking libnss-systemd:amd64 (242-7ubuntu3.3) 
over (242-7ubuntu3.2) ...
[  142.924747] cloud-init[765]: Preparing to unpack 
.../udev_242-7ubuntu3.3_amd64.deb ...
[  142.962745] cloud-init[765]: Unpacking udev (242-7ubuntu3.3) over 
(242-7ubuntu3.2) ...
[  143.165029] cloud-init[765]: Preparing to unpack 
.../libudev1_242-7ubuntu3.3_amd64.deb ...
[  143.168604] cloud-init[765]: Unpacking libudev1:amd64 (242-7ubuntu3.3) over 
(242-7ubuntu3.2) ...
[  143.206268] cloud-init[765]: Setting up libudev1:amd64 (242-7ubuntu3.3) ...
[  143.272647] cloud-init[765]: Preparing to unpack 
.../systemd-sysv_242-7ubuntu3.3_amd64.deb ...
[  143.275554] cloud-init[765]: Unpacking systemd-sysv (242-7ubuntu3.3) over 
(242-7ubuntu3.2) ...
[  143.594537] cloud-init[765]: Preparing to unpack 
.../libpam-systemd_242-7ubuntu3.3_amd64.deb ...
[  143.599803] cloud-init[765]: Unpacking libpam-systemd:amd64 (242-7ubuntu3.3) 
over (242-7ubuntu3.2) ...
[  143.680250] cloud-init[765]: Preparing to unpack 
.../systemd_242-7ubuntu3.3_amd64.deb ...
[  143.785319] cloud-init[765]: Unpacking systemd (242-7ubuntu3.3) over 
(242-7ubuntu3.2) ...
[  144.362457] cloud-init[765]: Preparing to unpack 
.../libsystemd0_242-7ubuntu3.3_amd64.deb ...
[  144.366022] cloud-init[765]: Unpacking libsystemd0:amd64 (242-7ubuntu3.3) 
over (242-7ubuntu3.2) ...
[  144.423950] cloud-init[765]: Setting up libsystemd0:amd64 (242-7ubuntu3.3) 
...
[  149.340746] cloud-init[765]: Setting up udev (242-7ubuntu3.3) ...
[  152.700118] cloud-init[765]: Setting up systemd (242-7ubuntu3.3) ...
[  154.478301] cloud-init[765]: Setting up systemd-sysv (242-7ubuntu3.3) ...
[  154.481899] cloud-init[765]: Setting up libnss-systemd:amd64 
(242-7ubuntu3.3) ...
[  154.485400] cloud-init[765]: Setting up libpam-systemd:amd64 
(242-7ubuntu3.3) ...
[  198.012728] cloud-init[765]: systemd is already the newest version 
(242-7ubuntu3.3).
(neipa) ipv6_mtu % cat 
output/EoanTestNetworkMtu/collect/etc_netplan/50-cloud-init.yaml
# This file is generated from information provided by the datasource.  Changes
# to it will not persist across an instance reboot.  To disable cloud-init's
# network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
network:
    version: 2
    ethernets:
        interface0:
            addresses:
            - 192.168.1.2/24
            - 2001:4800:78ff:1b:be76:4eff:fe06:1000/64
            ipv6-mtu: 1501
            match:
                macaddress: '52:54:00:12:34:00'
            mtu: 1601
            set-name: interface0
        interface1:
            addresses:
            - 192.168.2.2/24
            - 2001:4800:78ff:1b:be76:4eff:fe06:2000/64
            ipv6-mtu: 1501
            match:
                macaddress: '52:54:00:12:34:02'
            mtu: 1501
            set-name: interface1
        interface2:
            dhcp4: true
            match:
                macaddress: '52:54:00:12:34:04'
            set-name: interface2
        interface4:
            addresses:
            - 2001:4800:78ff:1b:be76:4eff:fe06:5000/64
            - 192.168.5.2/24
            ipv6-mtu: 8900
            match:
                macaddress: '52:54:00:12:34:08'
            mtu: 9000
            set-name: interface4
        interface5:
            addresses:
            - 2001:4800:78ff:1b:be76:4eff:fe06:6000/64
            - 192.168.6.2/24
            ipv6-mtu: 1480
            match:
                macaddress: 52:54:00:12:34:0c
            mtu: 1480
            set-name: interface5
(neipa) ipv6_mtu % grep . output/EoanTestNetworkMtu/collect/interface0_*
output/EoanTestNetworkMtu/collect/interface0_dev_mtu:1601
output/EoanTestNetworkMtu/collect/interface0_ipv6_mtu:1501
(neipa) ipv6_mtu % grep . output/EoanTestNetworkMtu/collect/interface1_*
output/EoanTestNetworkMtu/collect/interface1_dev_mtu:1501
output/EoanTestNetworkMtu/collect/interface1_ipv6_mtu:1501
(neipa) ipv6_mtu % grep . output/EoanTestNetworkMtu/collect/interface4_*
output/EoanTestNetworkMtu/collect/interface4_dev_mtu:9000
output/EoanTestNetworkMtu/collect/interface4_ipv6_mtu:8900
(neipa) ipv6_mtu % grep . output/EoanTestNetworkMtu/collect/interface5_*
output/EoanTestNetworkMtu/collect/interface5_dev_mtu:1480
output/EoanTestNetworkMtu/collect/interface5_ipv6_mtu:1480

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https://bugs.launchpad.net/bugs/1671951

Title:
  networkd should allow configuring IPV6 MTU

Status in systemd:
  Unknown
Status in cloud-init package in Ubuntu:
  Confirmed
Status in netplan.io package in Ubuntu:
  Fix Released
Status in systemd package in Ubuntu:
  Fix Released
Status in cloud-init source package in Bionic:
  Confirmed
Status in netplan.io source package in Bionic:
  Fix Released
Status in systemd source package in Bionic:
  Fix Committed
Status in cloud-init source package in Disco:
  New
Status in netplan.io source package in Disco:
  Fix Released
Status in systemd source package in Disco:
  Won't Fix
Status in cloud-init source package in Eoan:
  New
Status in netplan.io source package in Eoan:
  Fix Released
Status in systemd source package in Eoan:
  Fix Committed
Status in cloud-init source package in Focal:
  Confirmed
Status in netplan.io source package in Focal:
  Fix Released
Status in systemd source package in Focal:
  Fix Released

Bug description:
  = netplan.io =

  [Impact]

   * IPv6 traffic failing to send/receive due to incompatible/low MTU
  setting. Specifically, IPv6 traffic may have higher MTU requirements
  than IPv4 traffic and thus may need to be overridden and/or set to a
  higher value than IPv6 traffic.

  [Test Case]
   * Apply a netplan configuration that specifices ipv6-mtu:

  network:
    version: 2
    ethernets:
      eth0:
        dhcp4: true
        dhcp6: true
        ipv6-mtu: 6000

   * Check that MTU bytes, is at least IPv6MTUBytes on the interface:

  $ sysctl net.ipv6.conf.eth0.mtu
  net.ipv6.conf.eth0.mtu = 6000

  [Regression Potential]

   * This is a future compatible backport of an additional keyword not
  used by default. It may result in MTU change to a higher value, which
  should not cause loss of connectivity.

  [Other Info]

   * Original bug report below

  = end of netplan.io =

  = systemd =

  [Impact]

   * IPv6 traffic failing to send/receive due to incompatible/low MTU
  setting. Specifically, IPv6 traffic may have higher MTU requirements
  than IPv4 traffic and thus may need to be overridden and/or set to a
  higher value than IPv6 traffic.

  [Test Case]

   * Use IPv6MTUBytes= setting in a .network unit
   * Restart systemd-network
   * Check that there no error messages / warnings about not-recognizing this 
option
   * Check that MTU bytes, is at least IPv6MTUBytes on the interface

  [Regression Potential]

   * This is a future compatible backport of an additional keyword not
  used by default. It may result in MTU change to a higher value, which
  should not cause loss of connectivity.

  [Other Info]

   * Original bug report below

  = end of systemd =

  1) Zesty
  2) systemd-232-19
  3) I need to configure the IPV6 MTU for tunneling by adding an 
IPv6MTUBytes=1480 value in the .network file for an interface with an IPV6 
static address in the [Network] section
  4) networkd does not parse or read the value and does not apply this 
configuration to the interface.

  Upstream has discussed this issue here:

  https://github.com/systemd/systemd/pull/1533

  But it's been closed in favor of only setting via RA.

  However, we know of multiple use-case which are currently supported in
  ifdupdown where we want to retain control over IPV6 MTU values outside
  of PMTU Discovery configurations.

  Some context from those discussions

  >> Client systems that route their ipv6 packets to a 6in4 router also
  >> have to have their ipv6 mtu lowered.  They could lower their link mtu,
  >> so their ipv6 packets are small enough, but that reduces performance
  >> of their ipv4 network.

          Yes.  Anything that creates a PMTUD black hole can result in
  situations where the higher header overhead of IPv6 will cause IPv4 to
  pass but IPv6 traffic to be dropped.

          One example here is egress from an ipsec tunnel wherein the next
  hop MTU is too low for IPv6 datagrams to pass.  Another is VM ->
  whatever -> host bridge -> tunnel ingress.  If the datagram cannot enter
  the tunnel due to size, it is dropped, and an ICMP response uses the
  tunnel address as a source, which may not be routable back to the
  origin.  This one is an issue with IPv4 as well, and is one case where
  manually setting the IPv6 MTU lower than the (also manually set) device
  MTU is of benefit.

          In essence, any of these sort of cases that require an explicit
  setting of the device MTU will likely require a setting of the IPv6 mtu
  as well to account for its larger header overhead.

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