[Bug 1867375] Re: systemd-networkd: UseRoutes behavior change with introduction of UseGateway param

[Launchpad Bug Tracker]

** Merge proposal linked:
   https://code.launchpad.net/~ddstreet/ubuntu/+source/systemd/+git/systemd/+merge/382806

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https://bugs.launchpad.net/bugs/1867375

Title:
  systemd-networkd: UseRoutes behavior change with introduction of
  UseGateway param

Status in netplan.io package in Ubuntu:
  New
Status in systemd package in Ubuntu:
  Fix Released
Status in netplan.io source package in Bionic:
  Invalid
Status in systemd source package in Bionic:
  Confirmed
Status in netplan.io source package in Eoan:
  Invalid
Status in systemd source package in Eoan:
  Confirmed
Status in netplan.io source package in Focal:
  New
Status in systemd source package in Focal:
  Fix Released

Bug description:
  [impact]

  the networkd UseRoutes parameter allowed ignoring all routes provided
  by a dhcp4 server, including the route via the dhcp4-provided gateway.
  This was the behavior of networkd until recently, and in Focal the
  UseRoutes parameter does *not* prevent networkd from adding the route
  via the dhcp4-provided gateway.  This is now controlled with a new
  parameter, UseGateway.

  The systemd in Focal unfortunately has part of the upstream code; it
  no longer ignores the gateway route when UseRoutes=false is specified,
  but also it does not include the UseGateway parameter.

  Before Focal, networkd did not allow ignoring dhcpv4 routes and the
  gateway separately; the UseRoutes= parameter either used, or ignored,
  both.  This is why upstream added the UseGateway= paramter, which
  should be backported to allow control of ignoring the dhcpv4 routes
  and gateway separately.

  [test case]

  In Focal, bug 1872589 has a good test case netplan config, but a very
  quick test can be done with the networkd config:

  [Match]
  Name=eth0

  [Network]
  DHCP=ipv4
  LinkLocalAddressing=ipv6

  [DHCP]
  RouteMetric=100
  UseMTU=true
  UseRoutes=false

  this results in the system incorrectly setting a route via the default
  gateway:

  root at lp1867375-f:~# ip r
  default via 10.202.51.1 dev eth0 proto dhcp src 10.202.51.86 metric 100
  10.202.51.0/24 dev eth0 proto kernel scope link src 10.202.51.86
  10.202.51.1 dev eth0 proto dhcp scope link src 10.202.51.86 metric 100

  Before Focal, the above test case shows "correct" behavior, but there
  is no way to ignore the dhcpv4 routes but use the dhcpv4 gateway.
  This pre-Focal, the test case would be to use the UseGateway=
  parameter, e.g.:

  [Match]
  Name=eth0

  [Network]
  DHCP=ipv4
  LinkLocalAddressing=ipv6

  [DHCP]
  RouteMetric=100
  UseMTU=true
  UseRoutes=false
  UseGateway=true

  which *should* result in the default gateway being configured:

  root at lp1867375-f:~# ip r
  default via 10.202.51.1 dev eth0 proto dhcp src 10.202.51.86 metric 100
  10.202.51.0/24 dev eth0 proto kernel scope link src 10.202.51.86
  10.202.51.1 dev eth0 proto dhcp scope link src 10.202.51.86 metric 100

  [regression potential]

  Any regression would likely involve incorrect setting, or ignoring, of
  dhcpv4-provided routes, and/or incorrect setting, or ignoring, of the
  dhcpv4-provided gateway.

  [scope]

  One possible direction is this upstream PR
  https://github.com/systemd/systemd/pull/15443

  If accepted, that (and previous commits adding UseGateway= parameter)
  would need to be applied to Debian and all Ubuntu releases; Focal,
  Eoan, Bionic, and Xenial.

  [other info]

  to properly support separation of 'routes' and 'gateways' received via
  dhcp4, SRU releases should also support the UseGateway parameter,
  which is what this bug was originally opened requesting.

  [original description]

  requesting backport of PR https://github.com/systemd/systemd/pull/14983 which fixes the issue described in
  https://github.com/systemd/systemd/issues/14982 to 18.04 (bionic)

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