Can ISC DHCP be configured to forward/route to another DHCP Server?

Simon Hobson dhcp1 at
Thu Dec 11 20:23:59 UTC 2008

<gordon.marler at> wrote:
>Looking for conceptual guidance on a problem that has come up due to the
>merging of two companies, each with different views on how to handle
>DHCP (and DNS).
>The goal is to have a newly built ISC DHCP server that gets the first
>opportunity to respond to any and all DHCP queries.  We would like to
>put IP Helper entries for this (and only this) DHCP server in all
>routers in our infrastructure.  The DHCP will be configured to only
>respond to clients that are already in DNS, and which have a known MAC
>address/IP address/hostname mapping in the DHCP configuration (inserted
>via OMAPI).  Seems an ideal case for "ignore unknown-clients".  This
>DHCP server is solely intended to support Solaris/Linux OS installation
>boots.  The configurability and availability of a usable API in ISC DHCP
>is the driving factor for using it in this case.
>We must, however, fall back to a pre-existing Microsoft AD/DHCP/DNS
>server configuration in case the first ISC server cannot (and should
>not) respond to a particular DHCP client.  This would include all
>desktops and other clients that would be doing purely Dynamic DHCP/DNS
>Said another way, we're really looking to configure ISC DHCP to
>intercept ALL DHCP traffic, responding directly only to traffic it's
>interested in (static DHCP, no DDNS), and forwarding/routing the
>remaining DHCP requests to the Microsoft DHCP server.
>Any guidance on how this might be approached would be appreciated.

It can't be done.

There is no facility to "answer or forward" requests. The server 
either answers or it doesn't.

There are some bits you can do, sort of.

You can configure the ISC server to answer only certain clients and 
ignore others. This can be done on any criteria you can write a 
selection expression for. But, AFAIK there is no equivalent facility 
on the Windows server and so you WILL get conflicts.

I believe you can serve your Windows (and other clients) with the ISC 
server, so you might consider just completely switching. It is likely 
to be a lot less pain than trying to run two different servers.

Simon Hobson

Visit for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

More information about the dhcp-users mailing list