[Bug 1812760] Re: networkd: [Route] PreferredSource not working in *.network files

[Launchpad Bug Tracker]

** Merge proposal linked:
   https://code.launchpad.net/~daxtens/ubuntu/+source/systemd/+git/systemd/+merge/362056

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https://bugs.launchpad.net/bugs/1812760

Title:
  networkd: [Route] PreferredSource not working in *.network files

Status in systemd package in Ubuntu:
  In Progress

Bug description:
  [Impact]

  Users cannot create IPv6 routes that specify PreferredSource. This
  means that users cannot specify a number of valid IPv6 routes that are
  useful in some circumstances. These routes can be created with the
  'ip' tool, just not with systemd.

  This was reported upstream in systemd issue #5882 is fixed by pulling
  in the changes in systemd PR #11375 -
  https://github.com/systemd/systemd/pull/11375

  [Test Case]

  Start a Bionic or Cosmic VM.

  Add the following netplan yaml (adjust for ethernet card and MAC):

  network:
      version: 2
      ethernets:
          ens3:
              dhcp4: true
              match:
                  macaddress: 52:54:00:e2:c2:d7
              set-name: ens3
              addresses: ["fd8f:1d7d:b141::2/64", "fd8f:1d7d:b141::200/64"]
              routes:
                - to: "a::/16"
                  via: "fd8f:1d7d:b141::1"
                  from: "fd8f:1d7d:b141::2"
                - to: "fd8f:1d7d:b141::/64"
                  scope: link
                  from: "fd8f:1d7d:b141::2"
                  metric: 255

  Run netplan apply or reboot. Wait ~10s.

  Currently, ip -6 route will not include a route to "a::/16", and will
  not include the route to "fd8f:1d7d:b141::/64" that has
  "fd8f:1d7d:b141::2" as the source address - both those addresses will
  be missing.

  Correct behaviour is for ip -6 route to report the following:

  ubuntu at b-np:~$ ip -6 route
  a::/16 via fd8f:1d7d:b141::1 dev ens3 proto static src fd8f:1d7d:b141::2 metric 1024 pref medium
  fd8f:1d7d:b141::/64 dev ens3 proto static src fd8f:1d7d:b141::2 metric 255 pref medium
  fd8f:1d7d:b141::/64 dev ens3 proto kernel metric 256 pref medium
  fe80::/64 dev ens3 proto kernel metric 256 pref medium

  
  [Regression Potential] 

  This changes the state machine in systemd which configures the links.
  It passes systemd's internal tests, and has been approved by systemd
  maintainers, but it remains possible that the changes will break the
  configuration of obscure network setups.

  The backport requires pulling in two further commits that also change
  behaviour: currently systemd deletes all addresses and routes that
  were attached to an interface. With this change, it will only delete
  those that are not specified in the configuration files. I do not know
  how this could cause issues - it moves from a surprising behaviour to
  a less surprising behaviour, but it's worth pointing out.

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