[Bug 2034595] [NEW] If the loopback interface is managed via Netplan with network-manager, netplan apply will take it down and NM will not bring it back

[Danilo Egea Gondolfo]

Public bug reported:

NM is not bringing the loopback interface back automatically after
netplan apply flushes its addresses. It's not clear if it's a bug in NM
or intended behavior.

It's bad because we have services listening on the loopback interface,
such as resolved.

How to reproduce:

1) Install network-manager in Mantic and let it manage at least one of your interfaces. You'll see it's managing 'lo' automatically
2) Try to change the 'lo' connection, something like nmcli con mod lo mtu 1500 NM will emit a YAML (an nm-device) for 'lo'
3)Run netplan apply
4)You'll see that the 'lo' connection is down and the 'lo' interface has no IPs

You can bring it back with nmcli con up lo

See https://github.com/canonical/netplan/pull/408

** Affects: netplan.io (Ubuntu)
     Importance: Undecided
         Status: New

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Title:
  If the loopback interface is managed via Netplan with network-manager,
  netplan apply will take it down and NM will not bring it back

Status in netplan.io package in Ubuntu:
  New

Bug description:
  NM is not bringing the loopback interface back automatically after
  netplan apply flushes its addresses. It's not clear if it's a bug in
  NM or intended behavior.

  It's bad because we have services listening on the loopback interface,
  such as resolved.

  How to reproduce:

  1) Install network-manager in Mantic and let it manage at least one of your interfaces. You'll see it's managing 'lo' automatically
  2) Try to change the 'lo' connection, something like nmcli con mod lo mtu 1500 NM will emit a YAML (an nm-device) for 'lo'
  3)Run netplan apply
  4)You'll see that the 'lo' connection is down and the 'lo' interface has no IPs

  You can bring it back with nmcli con up lo

  See https://github.com/canonical/netplan/pull/408

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