[Dnsmasq-discuss] Multiple interfaces responding to DHCP requests

Geert Stappers stappers at stappers.nl
Tue May 18 19:57:02 UTC 2021


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.
-- 
Silence is hard to parse



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