[Dnsmasq-discuss] Multiple interfaces responding to DHCP requests

Nick Howitt nick at howitts.co.uk
Tue May 18 20:15:41 UTC 2021



On 18/05/2021 20:57, Geert Stappers wrote:
> 
> On Fri, May 14, 2021 at 08:30:42AM +0100, Nick Howitt wrote:
>> 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.
>>
> 
> And how did go?
> 
> 
> 
> Groeten
> Geert Stappers
> 
> P.S.
> FWIW for the two SSH sessions would I use one session.
> In that session `tmux` and the screen split horizontal.
> 
He was not going to get to the site until today to check. I will let you 
know as soon as I find out. I am intrigued as well. tmux is a new one 
for me!



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