[Dnsmasq-discuss] Multiple interfaces responding to DHCP requests

Nick Howitt nick at howitts.co.uk
Wed May 19 20:15:26 UTC 2021



On 18/05/2021 21:15, Nick Howitt wrote:
> 
> 
> 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!
> 
I've heard back and, yes, it was a network bridge caused by a phone with 
a bridge port plugged into both LAN's. It has all been sorted now.

Thanks for the help, Nick.



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