Thanks for the explanation, Dan! I was off down a wrong path, I
appreciate the correction.
I've just downloaded the Azure image from cloud-images.u.c and it
includes this in `/etc/netplan/90-hotplug-azure.yaml`:
# This netplan yaml is delivered in Azure cloud images to support
# attaching and detaching nics after the instance first boot.
# Cloud-init otherwise handles initial boot network configuration in
# /etc/netplan/50-cloud-init.yaml
network:
version: 2
ethernets:
ephemeral:
dhcp4: true
match:
driver: hv_netvsc
name: '!eth0'
optional: true
hotpluggedeth0:
dhcp4: true
match:
driver: hv_netvsc
name: 'eth0'
This file is not present in a booted system, because cloud-init removes
it during boot:
2020-11-09 18:12:09,306 - handlers.py[DEBUG]: start:
azure-ds/maybe_remove_ubuntu_network_config_scripts:
maybe_remove_ubuntu_network_config_scripts
2020-11-09 18:12:09,307 - DataSourceAzure.py[INFO]: Removing Ubuntu extended
network scripts because cloud-init updates Azure network configuration on the
following event: System boot.
2020-11-09 18:12:09,307 - util.py[DEBUG]: Attempting to remove
/etc/netplan/90-hotplug-azure.yaml
2020-11-09 18:12:09,307 - handlers.py[DEBUG]: finish:
azure-ds/maybe_remove_ubuntu_network_config_scripts: SUCCESS:
maybe_remove_ubuntu_network_config_scripts
It does this before the regular cloud-init network configuration is
written, or `netplan generate` is called:
2020-11-09 18:12:09,465 - util.py[DEBUG]: Writing to
/etc/netplan/50-cloud-init.yaml - wb: [644] 603 bytes
2020-11-09 18:12:09,466 - subp.py[DEBUG]: Running command ['netplan',
'generate'] with allowed return codes [0] (shell=False, capture=True)
cloud-init also runs a couple of udevadm commands right after `netplan
generate`:
2020-11-09 18:12:09,813 - subp.py[DEBUG]: Running command ['udevadm',
'test-builtin', 'net_setup_link', '/sys/class/net/eth0'] with allowed return
codes [0] (shell=False, capture=True)
2020-11-09 18:12:09,828 - subp.py[DEBUG]: Running command ['udevadm',
'test-builtin', 'net_setup_link', '/sys/class/net/lo'] with allowed return
codes [0] (shell=False, capture=True)
This all happens before systemd-networkd starts:
Nov 09 18:12:09.956027 focal-1604945439 systemd[1]: Starting Network
Service...
So: I'm not really sure what's going on here. I've tried restoring `90
-hotplug-azure.yaml` and removing `50-cloud-init.yaml`; that doesn't
cause the issue to reproduce on a subsequent boot.
One thing worth noting, that could lead to unexpected state: cloud-init
performs a DHCP on this interface (in order to be able to fetch the
network configuration it is going to apply). It does this in a sandbox
(i.e. it doesn't use system configuration for it), but potentially that
could mean that there's (kernel?) state for that interface which
{udev,network}d interpret in a way that leads to this issue?
** Changed in: cloud-init (Ubuntu)
Status: Incomplete => New
** Changed in: systemd (Ubuntu)
Status: New => Incomplete
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https://bugs.launchpad.net/bugs/1902960
Title:
Upgrade from 245.4-4ubuntu3.3 to 245.4-4ubuntu3.2 appears to break DNS
resolution in some cases
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