[Dnsmasq-discuss] dnsmasq responding to DHCPINFORM only

Nabeel Jafferali nabeel at jazinga.com
Thu Dec 29 16:30:31 GMT 2011


Hello.

We've recently noticed a problem with our setup, that was based on the
information in this (old) thread.

Essentially, we used Simon's instructions (below) to configure dnsmasq to
not reply to DHCP requests for leases, but to only respond to DHCP INFORM
requests. We also had option 66 parameters configued for a specific MAC
address range. This worked great in our scenario, where we had devices
getting IP/DNS/etc. from the primary DHCP server, and subsequently getting
DHCP option 66 info from the dnsmasq instance replying to a DHCP INFORM
request.

However, we've noticed some Windows PCs also request DHCP INFORM, and it
appears dnsmasq replies to those requests and provides DNS server
information - those PCs then start to use the DNS servers supplied by
dnsmasq instead of the DNS servers supplied by the primary DHCP server.

A few questions:

1. How can I cause dnsmasq to only reply to DHCP INFORM requests from a
certain MAC range? I already have the MAC range configured, and have
dnsmasq configured to provide specific option 66 information to that MAC
range only.
2. If not possible, how can I cause DNS server information to not be
included in the reply to DHCP INFORM?
3. If both not possible, where do I specify the specific DNS servers which
dnsmasq uses in the aforementioned reply? Perhaps I can just replicate the
information that the primary DHCP server sends.

Any help would be sincerely appreciated.

--
Nabeel Jafferali
Director - Product Mgmt.
Jazinga Inc.
416.548.4755 x103



On Wed, Aug 18, 2010 at 5:30 PM, Nabeel Jafferali <nabeel at jazinga.com>wrote:

> Thank you Simon. That worked perfectly, I have the client devices getting
> their boot parameters from dnsmasq but still getting their IP addresses
> from
> the primary DHCP server (a Linksys router, in my test case).
>
> --
> Nabeel Jafferali
> Director - Product Mgmt.
> Jazinga Inc.
> 416.548.4755 x103
>
>
> -----Original Message-----
> From: dnsmasq-discuss-bounces at lists.thekelleys.org.uk
> [mailto:dnsmasq-discuss-bounces at lists.thekelleys.org.uk] On Behalf Of
> Simon
> Kelley
> Sent: August-18-10 5:19 PM
> To: dnsmasq discussion list
> Subject: Re: [Dnsmasq-discuss] dnsmasq responding to DHCPINFORM only
>
> Nabeel Jafferali wrote:
> > Is it possible to have dnsmasq run alongside an existing DHCP server on
> the
> > network, but only respond to DHCP INFORM requests with the appropriate
> > option #. Essentially, I want to have my existing DHCP server hand out
> IPs,
> > but when the client device doesn't receive option 66 information, it
> send
> a
> > DHCP INFORM request, which I want dnsmasq to reply to.
> >
> >
> > I tried to set up dnsmasq.conf to listen on the appropriate interface,
> did
> > not set it as authoritative, did not include any IP address range, but
> did
> > specify dhcp-mac and dhcp-option entries.
> >
> >
> >
>
> If you enable dnsmasq with a dhcp-range that looks something like
>
> dhcp-range=192.168.0.1,static
>
> then it will not hand out leases, but it will reply to DHCPINFORM.
>
> BUT, most clients, once they have a DHCP lease, will unicast DHCPINFORM
> requests directly to the DHCP server which supplied the lease, so in
> those circumstances dnsmasq would never see the request and therefore
> not reply.
>
> Simon.
>
>
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