[Bug 2092945] Re: upgrade from 22 to 24 broke static ip

[Nick Rosbrook]

I believe this is because in systemd v250 (which is in between 22.04's
systemd v249, and 24.04's systemd v255), a network interface naming
scheme was added for Xen netfront interfaces[1].

So, basically, udev is using a different name for the interface now.

One way to work around this in the future would be to set your own
interface name using the netplan config. By default, udev's policy is to
keep interface names that were set by the admin.

[1] https://www.freedesktop.org/software/systemd/man/latest/systemd.net-
naming-scheme.html#v250

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

** Changed in: systemd (Ubuntu)
   Importance: Undecided => Low

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https://bugs.launchpad.net/bugs/2092945

Title:
  upgrade from 22 to 24 broke static ip

Status in systemd package in Ubuntu:
  Confirmed

Bug description:
  In the past week, I upgraded three machines using do-release-upgrade from Ubuntu server 22 to 24.
  All had static IPs configured:
  #1 with /etc/network/interfaces (oldschool!) (hostname=ocelot)
  #2 /etc/netplan/00-installer-config.yaml (as originally created by subiquity) (hostname=toolbox)
  #3 /etc/netplan/90-config.yaml (hostname=couch)

  All were simple (physical) configurations: just one ethernet interface, one network. (plus docker in #2-3, plus some ppp's in #3). #3 also had multiple IPs bound to eth0 (to enable routing by source ip addr)
  #1-2 are running as a vm under the same xen hypervisor (ubuntu 22.04.3), #3 runs bare-metal (dell xeon)

  In cases #1-2, the config file was NOT updated from 'eth0' to 'enX0' and the machine did not come back on the network. Luckily, I had access to the console, or I'd be screwed. 
  In case #3, 'eth0' is still the configured interface.

  So... I guess it's xen related, then? Note the hypervisor (host os)
  did not change, only the guest vm was upgraded from 22 to 24.

  #1 hvm
  ```
  $ cat ocelot.hvm
  builder = "hvm"
  name = "ocelot-hvm"
  memory = "1024"
  vcpus = 1
  vif = ['']
  #disk = ['phy:/dev/lvg/ocelot-root,hda,w','file:/root/dmg/ubuntu-16.04.3-server-amd64.iso,hdc:cdrom,r'] # lol, long upgrade history here!
  disk = ['phy:/dev/lvg/ocelot-root,hda,w']
  vnc = 1
  boot="dc"
  ```

  #2 hvm
  ```
  $ cat toolbox.hvm 
  builder = "hvm"
  name = "toolbox-hvm"
  memory = "3072"
  vcpus = 1
  vif = ['']
  #disk = ['phy:/dev/lvg/toolbox-root,hda,w','file:/root/dmg/ubuntu-20.04.4-live-server-amd64.iso,hdc:cdrom,r']
  disk = ['phy:/dev/lvg/toolbox-root,hda,w']
  vnc = 1
  boot="dc"
  ```

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