I've just spent a little while diagnosing a DHCP issue which really
confused me. I had just set up a new Linux box on which I am intending
to run several VMs using KVM/Qemu. The box runs Debian 10.
I configured a network bridge on it as normal, and ported over an
existing VM image from a different machine. All worked as expected.
Then I tried to provision a new VM and it didn't seem to be able to talk
to the network properly. It worked if I used the in-built NAT network
interface, but not if I connected it to br0. I therefore installed the
OS using the NAT connection, and then switched it back to br0 (so I
would have decent tools at my disposal to work out why).
It turned out the new VM was picking up exactly the IPv6 address which I
expected, but a totally surprising IPv4 address. At first I thought it
was getting it from the KVM/Qemu installation (as it would for the NAT
configuration) but then I realised that it was from a similar but
different IPv4 address range.
It turned out that the router to which the physical box was connected
had an unintended DHCP server running on it, and that was allocating the
rogue IP address. What puzzles me now is why this had not caused issues
in the past. It must have been there for years.
Both the physical box and the first VM had fixed IP addresses allocated
via DHCP on the site's two intended DHCP servers, and both those worked
correctly. Only the new VM had not got a fixed one, and thus seemed to
end up with one allocated by the router.
Is there something in a DHCPOFFER to make fixed addresses more
attractive to the client? All the DHCPDISCOVERs must have passed
through the router with the rogue DHCP server, and the real DHCP servers
were further away so the rogue response must have come back first and
yet the first two ignored the rogue responses and went for the (correct)
fixed addresses, whilst the new VM went for the one from the router.
I suppose it would be logical for there to be something like that in the
DHCPOFFER, but I can't find a reference to it.
TIA,
John
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