[Bug 2071363] [NEW] netplan apply triggers udev add events for every network interfdace

[Alberto Contreras]

Public bug reported:

netplan.io version 1.0-3ubuntu1, on Ubuntu 24.10, triggers udev add
events for every already present network interface:

```
$ lxc launch ubuntu-daily:o oo

# open `udevadm monitor` on another window within the container

o: # netplan apply


o: # udevadm monitor 
monitor will print the received events for:
UDEV - the event which udev sends out after rule processing
KERNEL - the kernel uevent

KERNEL[23616.687850] add      /devices/virtual/net/eth0 (net)
KERNEL[23616.687890] add      /devices/virtual/net/lo (net)
UDEV  [23616.691806] add      /devices/virtual/net/lo (net)
UDEV  [23616.697810] add      /devices/virtual/net/eth0 (net)
```

This is wrong by its own, but, with cloud-init on aws images is even
worse because:

cloud-init installs the following udev rules to configure hotplugged
nics:

```
# cat /etc/udev/rules.d/90-cloud-init-hook-hotplug.rules 
# Installed by cloud-init due to network hotplug userdata
ACTION!="add|remove", GOTO="cloudinit_end"

ENV{ID_NET_DRIVER}=="vif|ena|ixgbevf", GOTO="cloudinit_hook"
GOTO="cloudinit_end"

LABEL="cloudinit_hook"
SUBSYSTEM=="net", RUN+="/usr/lib/cloud-init/hook-hotplug"
LABEL="cloudinit_end"

```

which gets triggered by the udev add events, and hook-hotplug instructs
cloud-init to configure the new nic, and during that reconfiguration
`netplan apply` is called by cloud-init, which starts an infinite loop.

** Affects: cloud-init (Ubuntu)
     Importance: Undecided
         Status: Invalid

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

** Affects: cloud-init (Ubuntu Oracular)
     Importance: Undecided
         Status: Invalid

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

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

** Also affects: cloud-init (Ubuntu Oracular)
   Importance: Undecided
       Status: New

** Also affects: netplan.io (Ubuntu Oracular)
   Importance: Undecided
       Status: New

-- 
You received this bug notification because you are a member of Ubuntu
Foundations Bugs, which is subscribed to netplan.io in Ubuntu.
Matching subscriptions: foundations-bugs
https://bugs.launchpad.net/bugs/2071363

Title:
  netplan apply triggers udev add events for every network interfdace

Status in cloud-init package in Ubuntu:
  Invalid
Status in netplan.io package in Ubuntu:
  New
Status in cloud-init source package in Oracular:
  Invalid
Status in netplan.io source package in Oracular:
  New

Bug description:
  netplan.io version 1.0-3ubuntu1, on Ubuntu 24.10, triggers udev add
  events for every already present network interface:

  ```
  $ lxc launch ubuntu-daily:o oo

  # open `udevadm monitor` on another window within the container

  o: # netplan apply

  
  o: # udevadm monitor 
  monitor will print the received events for:
  UDEV - the event which udev sends out after rule processing
  KERNEL - the kernel uevent

  KERNEL[23616.687850] add      /devices/virtual/net/eth0 (net)
  KERNEL[23616.687890] add      /devices/virtual/net/lo (net)
  UDEV  [23616.691806] add      /devices/virtual/net/lo (net)
  UDEV  [23616.697810] add      /devices/virtual/net/eth0 (net)
  ```

  This is wrong by its own, but, with cloud-init on aws images is even
  worse because:

  cloud-init installs the following udev rules to configure hotplugged
  nics:

  ```
  # cat /etc/udev/rules.d/90-cloud-init-hook-hotplug.rules 
  # Installed by cloud-init due to network hotplug userdata
  ACTION!="add|remove", GOTO="cloudinit_end"

  ENV{ID_NET_DRIVER}=="vif|ena|ixgbevf", GOTO="cloudinit_hook"
  GOTO="cloudinit_end"

  LABEL="cloudinit_hook"
  SUBSYSTEM=="net", RUN+="/usr/lib/cloud-init/hook-hotplug"
  LABEL="cloudinit_end"

  ```

  which gets triggered by the udev add events, and hook-hotplug
  instructs cloud-init to configure the new nic, and during that
  reconfiguration `netplan apply` is called by cloud-init, which starts
  an infinite loop.

To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/cloud-init/+bug/2071363/+subscriptions