[Dnsmasq-discuss] Multiple interfaces responding to DHCP requests

[Nick Howitt]

On 13/05/2021 22:30, Geert Stappers via Dnsmasq-discuss wrote:
> 
> On Thu, May 13, 2021 at 09:31:21PM +0100, Nick Howitt wrote:
>> On 13/05/2021 19:26, Geert Stappers via Dnsmasq-discuss wrote:
>>> On Thu, May 13, 2021 at 01:05:07PM +0100, Nick Howitt wrote:
>>>> I am trying to help someone who has a set up with three LAN's, all on
>>>> different subnets and all acting as DHCP servers. He is getting an odd
>>>> result that when a device on the enp2s0 LAN requests an IP, both enp2s0 and
>>>> enp3s0 respond with IP's. I've never seen this before and my own server does
>>>> not act this way.
>>>>
>>>>       ...
>>>>
>>>>   From the dnsmasq log:
>>>> May  8 11:23:39 dnsmasq-dhcp[7226]: DHCPDISCOVER(enp2s0) de:ad:c0:de:ca:fe
>>>> May  8 11:23:39 dnsmasq-dhcp[7226]: DHCPOFFER(enp2s0) 192.168.1.214 de:ad:c0:de:ca:fe
>>>> May  8 11:23:42 dnsmasq-dhcp[7226]: DHCPDISCOVER(enp3s0) de:ad:c0:de:ca:fe
>>>> May  8 11:23:42 dnsmasq-dhcp[7226]: DHCPOFFER(enp3s0) 192.168.168.215 de:ad:c0:de:ca:fe
>>>>
>>>>       ...
>>>>
>>>> I have a similar dual LAN set up and it works as expected with each
>>>> LAN only responding with its own LAN DHCP settings.
>>>
>>> That means the "problem" is outside dnsmasq.
>>>
>>>
>>>> Thanks,
>>>>
>>>
>>> Feel welcome to report back.
>>
>> OK, but if the problem is outside dnsmasq, why does dnsmasq log two
>> DHCPDISCOVERs and DHCPOFFERs? Could he have a networking problem with the
>> two LAN's bridged somewhere?
> 
> Yes
> 
> 
>> Perhaps I can ask him to unplug the cable to
>> enp3s0 and see if the problem goes away.
> 
> It is surely an usefull trouble shoot check.
> 
or perhaps less intrusive is two ssh sessions running:

tcpdump -nni enp2s0 '(ether host de:ad:c0:de:ca:fe and port 67)'

and:

tcpdump -nni enp3s0 '(ether host de:ad:c0:de:ca:fe and port 67)'

Then try the nmap test. If both sessions respond, then there is a bridge.