Rowland Penny <rpenny241155@???> writes:

[...]

> Finally, in desperation, I ran 'dmesg | grep eth0' and found my problem:
>

> root@server:~# dmesg | grep eth0
> [    0.921998] r8169 0000:02:00.0 eth0: RTL8168b/8111b at 0xffffc90000006000, 00:1d:60:fc:29:e6, XID 18000000 IRQ 41
> [    0.922001] r8169 0000:02:00.0 eth0: jumbo features [frames: 4080 bytes, tx checksumming: ko]
> [    7.620169] systemd-udevd[362]: renamed network interface eth0 to eth1

>
> Why, oh why, did systemd-udevd rename eth0 to eth1 ????????

Short answer:

Because the MAC address is different from the one formerly recorded for
eth0. It should be possible to create an empty file

/etc/udev/rules.d/75-persistent-net-generator.rules

to stop this.

NB: This means you get unpredictable interface names if more than one
driver needs to be loaded dynamically. That's not even particularly
difficult to avoid but one must first overcome the mindset that some
graduate student with a laptop must be a demigod because
$important_company is paying him to put his ill-conceived theories into
practice.