[Dnsmasq-discuss] Overlapping/duplicate subnets

/dev/rob0 rob0 at gmx.co.uk
Wed Feb 22 15:07:20 GMT 2017

On Wed, Feb 22, 2017 at 02:55:30PM +0100, S. M. Hossein Hamidi wrote:
> I am new to dnsmasq and my mind is busy with a DHCP scenario that I 
> would like to consult with you before getting my hands dirty.
> Suppose that we have several subnets which are behind a NAT device 
> which additionally acts as a DHCP relay agent. I know that using 
> DHCP relay agent information option, I can distinguish between 
> different subnets. However, the additional constraint is to use 
> duplicate or overlapping subnets instead of distinct subnets.

That is a very strange constraint.  RFC 1918 is quite large enough; 
it's not necessary to share your netblocks.  And how will your NAT 
device distinguish one segment with a shared netblock from another 
segment using the same netblock?

Sounds like broken IP networking to me.  Compliant IP stacks do 
routing based on IP address blocks.

> Since, the traffic comes from each segment goes through NAT, it 
> wouldn't be any problem to use duplicate IP addresses, knowing
> that each segment has its own autonomy.

But again, how does the NAT device do this?

> So, may I know if can I exploit tagging feature of dnsmasq so
> that it can handle separate potentially overlapping leases for
> each subnet?

I'm pretty sure this would not be possible in dnsmasq nor in ISC 
dhcpd.  Either one could do a single netblock shared among many 
distinct segments, but each would only be able to give out any one 
address to only one client.

But wait and see what Simon says. :)
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