[Bug 1867375] Re: systemd-networkd: Setting UseRoutes to False results in DHCP default gateway not being installed

[Dan Streetman]

Upstream this is implemented by two separate PRs:
https://github.com/systemd/systemd/pull/14983
https://github.com/systemd/systemd/pull/15136

Pulling only the first would unfortunately break backwards compatibility
for Bionic users who currently expect UseRoutes=False to also ignore the
DHCP gateway (just as it broke upstream users when added).

However, also adding the second without any changes would still break backwards compatibility; it only adds a new option (UseGateway) that defaults to true, but current behavior in Bionic ties these options together, i.e. in Bionic for backwards compatibility, we would need to:
1) update the behavior of UseRoutes to only use/ignore DHCP-provided routes, separate from the DHCP-provided gateway
2) add UseGateway option to use/ignore the DHCP-provided gateway
3) add logic so that UseGateway defaults (i.e. if not set) to the setting of UseRoutes

Since this could potentially leave a system without networking if done
wrong, I'm not going to include it in my current round of system
patches; I'll look at it again next cycle (unless someone else wants to
handle it).

** Changed in: systemd (Ubuntu)
       Status: New => Fix Released

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

Title:
  systemd-networkd: Setting UseRoutes to False results in DHCP default
  gateway not being installed

Status in systemd package in Ubuntu:
  Fix Released
Status in systemd source package in Bionic:
  New
Status in systemd source package in Eoan:
  New

Bug 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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