[Dnsmasq-discuss] DHCP for Multiple Subnets

Robert Sweetnam lists at fsck.eu
Wed May 29 12:14:59 BST 2013

----- Original Message -----
From: "Simon Kelley" <simon at thekelleys.org.uk>
To: "Robert Sweetnam" <lists at fsck.eu>
Cc: dnsmasq-discuss at lists.thekelleys.org.uk
Sent: Wednesday, 29 May, 2013 11:13:06 AM
Subject: Re: [Dnsmasq-discuss] DHCP for Multiple Subnets

On 29/05/13 10:57, Robert Sweetnam wrote:

> Are your subnets all in the same broadcast-domain (ie the piece of
> virtual "wire" attached to eth0) or are they different physical networks
> connected via routers to the eth0 "wire"?
> Cheers,
> Simon.
> Hi Simon,
> They are the latter. That is they are different physical networks connected via routers.

In that case you need something called a "DHCP-helper" or "DHCP-relay" 
which runs on a machine which does have an interface on the physical 
network. Normally it runs on the router, but it doesn't have to. The 
DHCP-helper receives the "where am I" broadcast from the client, tags it 
with the address of the interface in which it was received (which solves 
you original problem) and sends it on to the DHCP server for address 

Most dedicated routers include this function: you just have to configure 
the address of the machine running dnsmasq. If your router can't do it, 
but you have a machine on each network running Linux (either the router 
or another machine) then


will do the trick.

You don't need any extra configuration in dnsmasq, beyond providing 
suitable dhcp-ranges.



Thanks for the very quick response Simon.

I will give that a try.

Many thanks again,

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