[Bug 1836695] Re: systemd fails to setup static routes at boot when using DHCP

[Dan Streetman]

verified on bionic

ubuntu at lp1836695-b:~$ dpkg -l systemd|grep ii
ii  systemd        237-3ubuntu10.39 amd64        system and service manager
ubuntu at lp1836695-b:~$ ip r
10.0.0.0/8 via 192.168.122.1 dev ens3 proto static 
192.168.122.0/24 dev ens3 proto kernel scope link src 192.168.122.35 


** Tags removed: verification-needed verification-needed-bionic
** Tags added: verification-done verification-done-bionic

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Title:
  systemd fails to setup static routes at boot when using DHCP

Status in netplan:
  Invalid
Status in systemd package in Ubuntu:
  Fix Released
Status in systemd source package in Bionic:
  Fix Committed
Status in systemd source package in Disco:
  Won't Fix
Status in systemd source package in Eoan:
  Fix Released

Bug description:
  [impact]

  a systemd-networkd configuration that uses ipv4 dhcp but ignores the
  dhcp-provided route, and instead sets up a static route, and also does
  not include a static ipv4 address, fails to actually create the static
  route.

  This is due to networkd attempting to set up the static route before
  the dhcp ipv4 address is assigned, and the kernel does not allow
  creation of the route before setting up corresponding ipv4 address.

  this results in a network that does have the dhcp-provided ipv4
  address, but is missing both its dhcp-provided route (because networkd
  is configured to ignore it) and the static route (because networkd was
  not able to create it).

  [test case]

  (remove or unconfigure netplan, so it will not conflict with this
  manual networkd configuration)

  create a networkd config file, e.g.:

  $ cat /etc/systemd/network/10-eth0.network 
  [Match]
  Name=eth0

  [Network]
  DHCP=ipv4

  [Route]
  Destination=10.0.0.0/8
  Gateway=10.202.51.1

  [DHCP]
  UseRoutes=false

  
  then *reboot* the system, and check that the static route was not applied:

  $ ip r
  10.202.51.0/24 dev eth0 proto kernel scope link src 10.202.51.254 

  
  note that because networkd does not remove ipv4 addresses that it manages (including dhcpv4 addresses), restarting networkd after the initial boot
  will correctly create the static route, e.g.:

  ubuntu at lp1836695-b:~$ ip r
  10.202.51.0/24 dev eth0 proto kernel scope link src 10.202.51.254 
  ubuntu at lp1836695-b:~$ sudo systemctl restart systemd-networkd
  ubuntu at lp1836695-b:~$ ip r
  10.0.0.0/8 via 10.202.51.1 dev eth0 proto static 
  10.202.51.0/24 dev eth0 proto kernel scope link src 10.202.51.254 

  [regression potential]

  adjusting how networkd works always carries the risk of breaking
  networking.

  TBD detailed regression potential after analyzing fix.

  [other info]

  original description:

  --

  
  Consider the following setup:

  network:
    version: 2
    renderer: networkd
    ethernets:
      ens4:
        dhcp-identifier: mac
        dhcp4: yes
        dhcp4-overrides:
          use-dns: no
          use-ntp: no
          send-hostname: no
          use-hostname: no
          use-routes: no
        routes:
        - to: 10.0.0.0/8
          via: 10.50.0.1
        optional: true

  Thus I only need to get the IP address by DHCP, then add some static
  routes. This setup doesn't work. Apparently `routes` keyword only
  works when using static addresses.

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