Nailed it!

Specifically, you were spot on about the /etc/default/ifplugd file and 
its HOTPLUG_INTERFACES line. Changing that to HOTPLUG_INTERFACES="eth0 
eth1" meant that it just worked out of the box.

In fact, I suspect that eth0 could be omitted because it's part of the 
hardware (eth1 is a USB dongle so hotplugging may be useful). I'd prefer 
a way to specifically /exclude/ wg* interfaces rather then include the 
known ones but that may be too complex.

I hope this discussion and its solution will be a useful part of the 
WireGuard documentation. It'll hopefully stop others becoming confused 
in the future.

Many thanks for your help in resolving this! I'm very much enjoying 
playing with WireGuard. I heard about it during your talk at FOSDEM by 
the way.

Regards,

Jim.


On 11/09/17 01:52, Jason A. Donenfeld wrote:
> I figured it out. ifplugd clears all addresses when bringing up an 
> interface:
>
> void interface_up(int fd, char *iface) {
> ...
>         ((struct sockaddr_in *)(&ifr.ifr_addr))->sin_addr.s_addr = 
> INADDR_ANY;
>         if (ioctl(fd, SIOCSIFADDR, &ifr) < 0) {
> ...
> }
>
> Since wg-quick sets the address before bringing it up, this running in 
> between is problematic. One workaround in wg-quick would be for me to 
> reverse the order of setting the IP and bringing it up, but this 
> introduces other races I'm not very fond of introducing. So, rather, 
> we should address the larger question: why on earth is ifplugd being 
> started when wg0 is added?
>
> Sep 10 23:57:51 janus ifplugd(wg0)[14109]: ifplugd 0.28 initializing.
> Sep 10 23:57:51 janus ifplugd(wg0)[14109]: Using interface 
> wg0/00:00:00:00:00:00
> Sep 10 23:57:51 janus ifplugd(wg0)[14109]: Using detection mode: 
> IFF_RUNNING
>
> What causes it to be launched here? Digging a bit deeper, it looks 
> like ifplugd is being launched by a udev rule which calls a Debian 
> file called ifplugd.agent. It is in here that unholy hotplugging occurs.
>
> I don't actually have a Debian system running to fish around and see, 
> but my guess is that you have a file /etc/default/ifplugd that has in 
> it HOTPLUG_INTERFACES=all. If you change this to 
> HOTPLUG_INTERFACES="wlan0 eth0" or something more restrictive, things 
> might work better. Just a guess.