[Dnsmasq-discuss] dnsmasq and multicast (224.0.0.12)

David Dombrowsky david.dombrowsky at redlion.net
Fri May 11 14:23:42 BST 2012


On 05/10/2012 12:39 PM, Simon Kelley wrote:
> On 09/05/12 21:00, David Dombrowsky wrote:
>>
>> Please forgive the newbie question.
>>
>> Is there anything in dnsmasq which implements listening on a
>> multicast address?
> 
> No.
>> The IANA address allocation does say that
>> 224.0.0.12 is a standard DHCP server address, on which I assume DHCP
>> servers can listen.
>>
> That's a new one for me: do you have a reference to any standard for
> that?

http://www.iana.org/assignments/multicast-addresses/multicast-addresses.xml

> DHCP for IPv6 uses multicast, but it's possible there because hosts
> can assign themselves a link-local address before attempting DHCP.
> In the IPv4 world that's not possible and various nasty hacks are
> used to bootstrap things.
> 
>> First: is this even a responsibility of this application layer of
>> the stack, or is it supposed to be implemented in kernel or network
>> configuration?
>>
> 
> I don't know. I imagine a standard document would tell us.

The interwebs don't really seem to help reveal how this multicast
addressed is used and how it relates to DHCP servers.  It seems to
be a mystery.

>> Second: the end goal is to have multiple dnsmasq-based DHCP servers
>> listening on not-well-known IP addresses on the network.  Also, the
>> servers should only receive requests from a proprietary relay agent,
>> which won't know the IP address of the DHCP servers.  I think
>> multicast is the way to solve this, but I'm not positive.  Any ideas?
>>
>>
> 
> It's well out of the usual use-case. Why do you want multiple
> servers. for availability? If so, be aware that dnsmasq doesn't
> implement the DHCP failover protocol, and the ISC server may be a
> better choice.

We need two servers to avoid the single point of failure problem.
The servers really don't need any failover protocol, because all the
addresses are statically assigned anyway.

-- 
David Dombrowsky, Software Engineer
Sixnet, a Red Lion business | www.sixnet.com
+1 (518) 877‐5173 x161 office
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