[Dnsmasq-discuss] Ideas for handing out "per blade" information

Simon Kelley simon at thekelleys.org.uk
Thu Oct 22 13:13:16 BST 2009

Paul Smith wrote:
> Hi all.
> I have an environment with lots of blades (on the order of 150-200 or
> so), all getting IP information from a central server using dnsmasq as
> the DHCP server.
> What I need to do is provide some information to each blade when it
> registers, and (some of) this information will be specific to a given
> blade (said another way, it will be different for each blade).  I was
> hoping to use the DHCP response to supply this information, rather than
> invent some new protocol of my own, create a daemon, etc. etc.
> On the server I actually already know the MAC addresses that the blades
> will register with, so I was going to use that as a key to look up the
> per-blade information when the DHCP request is received.
> Also, ideally the information would be obtained by running a script with
> the MAC address as the argument, since these addresses can change during
> the runtime of the system (someone can pull a blade and put a new one,
> with a different MAC, in, for example, or they could even swap two
> blades which means the information I want give them will change, since
> some of it is position-dependent) and I'd prefer to grab these
> dynamically rather than having some kind of timeout etc.  But, if that's
> not possible I can work with it.
> I've been looking through the dnsmasq man page and there are a lot of
> options, but I'm not sure how to use them to do what I want.
> The only mapping it appears that I can do based on MAC is dhcp-host
> (maybe creating a file and giving it with dhcp-hostsfile).  But the only
> way I can set options, that I can see, is using dhcp-option which is
> based on network-id.  There is no way to set the network-id based on the
> MAC etc. that I can see.  There doesn't seem to be any way to give
> specific options for a specific client.
> Soooo... anyone have any ideas for me?
> Thanks!

network-ids are sadly misnamed: they have grown into much more general 
tags now. You can certainly set a net-id tag in a dhcp-host line, and 
then use that to select the options to be sent to a particular host.

There's no way to change this information completely dynamically. You 
can however put all the dhcp-host information into a file and point at 
that file with --dhcp-hostsfile, similarly with options and 
dhcp-optsfile. Unlike a general configuration, these files _are_ re-read 
when dnsmasq gets SIGHUP.



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