[Bug 1793555] [NEW] networkd tears down bridge ip address when the last device is pulled out from the bridge

[Vladimir Pouzanov]

Public bug reported:

In Ubuntu 18.04.1 LTS (systemd 237) pulling out the last device out of
the bridge removes the bridge ip address.

given the following config:

# cat /etc/systemd/network/vmbr0.netdev
[NetDev]
Name=vmbr0
Kind=bridge

[Bridge]
HelloTimeSec=0
ForwardDelaySec=0
STP=no

# cat /etc/systemd/network/vmbr0.network
[Match]
Name=vmbr0

[Network]
Address=10.10.0.1/16
ConfigureWithoutCarrier=yes
DHCP=no
IPForward=yes
IPv6AcceptRA=no
LinkLocalAddressing=no

networkd would bring up vmbr0 with 10.10.0.1/16 on system boot despite
it not having any devices, allowing services to bind to 10.10.0.1.

However, if you add a device and then remove it (e.g. by starting and
then stopping a virtual machine connected to the bridge), networkd would
tear down the interface:

3: vmbr0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default qlen 1000
    link/ether 16:19:4c:7f:e8:c4 brd ff:ff:ff:ff:ff:ff

thus making any service listening on 10.10.0.1 inaccessible.

previously on Xenial the bridge would stay intact.

** Affects: systemd (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1793555

Title:
  networkd tears down bridge ip address when the last device is pulled
  out from the bridge

Status in systemd package in Ubuntu:
  New

Bug description:
  In Ubuntu 18.04.1 LTS (systemd 237) pulling out the last device out of
  the bridge removes the bridge ip address.

  given the following config:

  # cat /etc/systemd/network/vmbr0.netdev
  [NetDev]
  Name=vmbr0
  Kind=bridge

  [Bridge]
  HelloTimeSec=0
  ForwardDelaySec=0
  STP=no

  # cat /etc/systemd/network/vmbr0.network
  [Match]
  Name=vmbr0

  [Network]
  Address=10.10.0.1/16
  ConfigureWithoutCarrier=yes
  DHCP=no
  IPForward=yes
  IPv6AcceptRA=no
  LinkLocalAddressing=no

  networkd would bring up vmbr0 with 10.10.0.1/16 on system boot despite
  it not having any devices, allowing services to bind to 10.10.0.1.

  However, if you add a device and then remove it (e.g. by starting and
  then stopping a virtual machine connected to the bridge), networkd
  would tear down the interface:

  3: vmbr0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default qlen 1000
      link/ether 16:19:4c:7f:e8:c4 brd ff:ff:ff:ff:ff:ff

  thus making any service listening on 10.10.0.1 inaccessible.

  previously on Xenial the bridge would stay intact.

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