[Bug 1833193] Re: systemd-networkd fails to apply static IPv4 when the static IP is the same as previously configured by DHCP

[Lars Erik Pedersen]

Repro instructions:
- (Enable systemd-networkd debug logging)
- Start a server with its interface configured with DHCP
- In the netplan config, configure the interface wtih static addressing, using the exact same address as it already has gotten from DHCP
- Either wait for the initial DHCP lease time to expire, and observe that the interface loses its address; OR observe in the debug log that networkd applied the address with a lease time, even though you told netplan to use static addressing

Networkd config with static addressing:
cat /run/systemd/network/10-netplan-ens3.network 
[Match]
MACAddress=<redacted>

[Network]
LinkLocalAddressing=ipv6
Address=10.212.132.107/24
Gateway=10.212.132.1
DNS=8.8.8.8
DNS=8.8.4.4
Domains=<redacted>

Networkd config with DHCP:
cat /run/systemd/network/10-netplan-ens3.network
[Match]
MACAddress=<redacted>
[Network]
DHCP=ipv4
LinkLocalAddressing=ipv6

[DHCP]
RouteMetric=100
UseMTU=true

Debug log output from systemd-networkd when netplan tells it to apply static addressing when the interface already has an address from DHCP:
Sep 30 10:36:15 nettest1 systemd[1]: Stopped Network Service.
Sep 30 10:36:16 nettest1 systemd[1]: Starting Network Service...
<snip>
Sep 30 10:36:16 nettest1 systemd-networkd[6086]: ens3: Adding address: 10.212.132.107/24 (valid for 11h 52min 55s)

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https://bugs.launchpad.net/bugs/1833193

Title:
  systemd-networkd fails to apply static IPv4 when the static IP is the
  same as previously configured by DHCP

Status in systemd package in Ubuntu:
  New

Bug description:
  In bionic, running systemd 237-3ubuntu10.22 and netplan.io
  0.97-0ubuntu1~18.04.1, systemd-networkd fails to configure an
  interface with a static IPv4 address if the statically confiugred
  address is the same as the interface already has gotten from DHCP.

  This will cause the interface to loose its IP address when the DHCP
  lease exires, even though you've told netplan to configure it as
  static.

  I expect systemd-networkd to actually configure an IP address as
  static, regardless of what address the interface has before from DHCP.

  # lsb_release  -rd
  Description:	Ubuntu 18.04.2 LTS
  Release:	18.04

  # apt-cache policy systemd
  systemd:
    Installed: 237-3ubuntu10.22

  # apt-cache policy netplan.io
  netplan.io:
    Installed: 0.97-0ubuntu1~18.04.1

  A paste of systemd-networkd's debug log when I run "netplan apply" and
  the interface already has the static IP configured from DHCP.

  It seems like upon a restart, systemd-networkd will allways add
  whatever IP config it had before the service stopped, and then apply
  changes (if any). Since my new config has the same IP as it already
  had, it does nothing even though the new config has static
  configuration.

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