I think this will do it:

- Separate DHCP server, e.g. a typical home router.
- dnsmasq in proxyDHCP mode, i.e. something like:
  dhcp-range=set:proxy,192.168.0.0,proxy,255.255.255.0

The full config we're using is generated from this template:
https://github.com/ltsp/ltsp/blob/main/ltsp/server/dnsmasq/ltsp-dnsmasq.conf

When a proxyDHCP client tries to boot, dnsmasq ...hangs.
It doesn't even reply to TFTP or DNS requests anymore.
systemctl stop dnsmasq or killall dnsmasq don't kill it, it needs kill -9 to 
stop and restart.

The same issue was reported (to me not to launchpad) due to the 22.04 and 26.04 
dnsmasq updates.
And this morning I was able to reproduce it by installing dnsmasq 2.93 from 
stonking, into my resolute box.


So I'm thinking that this was actually broken upstream, and then Ubuntu 
backported the specific security fix that causes the regression.

I'm hoping to find time in the following days to bisect and pinpoint it,
but if someone can work on it now, it should be very easy to reproduce.

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https://bugs.launchpad.net/bugs/2166430

Title:
  dnsmasq 2.91-0ubuntu0.24.04.1 update broke ProxyDHCP mode; please stop
  the phased update

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