[Bug 2004478] Re: systemd-networkd's dhcp4 client ignores local subnet routes

[Launchpad Bug Tracker]

** Merge proposal linked:
   https://code.launchpad.net/~enr0n/ubuntu/+source/systemd/+git/systemd/+merge/438554

** Merge proposal linked:
   https://code.launchpad.net/~enr0n/ubuntu/+source/systemd/+git/systemd/+merge/438555

-- 
You received this bug notification because you are a member of Ubuntu
Foundations Bugs, which is subscribed to systemd in Ubuntu.
https://bugs.launchpad.net/bugs/2004478

Title:
  systemd-networkd's dhcp4 client ignores local subnet routes

Status in systemd package in Ubuntu:
  Triaged
Status in systemd source package in Jammy:
  Triaged
Status in systemd source package in Kinetic:
  Triaged

Bug description:
  [Impact]

  If a DHCP server pushes down a local subnet route with a null gateway,
  the systemd-networkd DHCP client does not correctly install the route.
  Instead, the route is ignored.

  [Test Plan]

  Taken from
  https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/2004478/comments/2.

  * Start a Jammy LXD container:

  $ lxc launch ubuntu-daily:jammy jammy
  $ lxc exec jammy bash

  * Create a veth pair:

  $ ip link add veth0 up type veth peer name veth1
  $ ip addr add 172.20.0.1/24 dev veth0
  $ cat > /etc/netplan/60-veth1.yaml <<EOF
  network:
    version: 2
    ethernets:
      veth1:
        dhcp4: true
  EOF

  * Start dnsmasq in the background with the following routes specified:

  $ dnsmasq -kq -C /dev/null -z -i veth0 -I lo -F
  172.20.0.10,172.20.0.150 -O option:classless-static-
  route,169.254.0.0/24,0.0.0.0,169.254.1.0/24,172.20.0.2 &

  * Apply the netplan config:

  $ netplan generate && netplan apply

  * Check the routing table (may take a few seconds to update):

  $ ip route
  [...]

  * On an affected system, the route for 169.254.0.0/24 is missing.

  [Where problems could occur]

  This patch is in the systemd-networkd DHCP client. It adds logic to
  install routes with a link scope whenever the DHCP server pushes local
  subnet routes with a null gateway. If any problems occurred, it would
  related to systemd-networkd's handling of such routes pushed from a
  DHCP server.

  [Original Description]

  RFC3442 specifies option 121 (Classless Static Routes) that allow a
  DHCP server to push arbitrary routes to a client. It has a Local
  Subnet Routes section expliciting the behavior of routes with a null
  (0.0.0.0) gateway.

  Such routes are to be installed on the interface with a Link scope, to
  mark them as directly available on the link without any gateway.

  Networkd currently drops those routes, which is against the RFC, as
  Linux has proper support for such routes.

  This has been observed as broken on Ubuntu Jammy 22.04 LTS, but
  working in previous LTS (Focall 20.04 and Bionic 18.04 tested).

  1. Ubuntu release: 22.04.1 LTS
  2. Systemd release: 249.11-0ubuntu3.6
  3. Expected behavior: routes from DHCP option 121 with a gateway set to 0.0.0.0 to be installed as "scope link"
  4. Observed behavior: routes are silently dropped by systemd-networkd's dhcp client (actually logged with a debug level, effectively silently with the default configuration)

  I wrote a fix that has been accepted in upstream systemd:
  https://github.com/systemd/systemd/pull/26234, which applies cleanly
  to the current package's sources obtained through apt-get source.

  As this is a fix for something that is an actual regression, can it be
  backported to Ubuntu 22.04's systemd tree?

  Thanks.

To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/2004478/+subscriptions