Getting ISC DHCP to respond with pxe information, but not with lease

Jeff Anderson jefferya at programmerq.net
Mon Dec 3 22:16:20 MST 2007


Michael L Torrie wrote:
> Mike Lovell wrote:
>   
>> IIRC, this isn't really a function of the DHCP server but of the 
>> switches and routers in the middle. From my limited knowledge from the 
>> Harold B Lee Library system, Altirus has a regular DHCP server in it 
>> serving up requests. The switches and routers in the middle actually get 
>> a lease from the campus DHCP server and tag on the extra information 
>> from a lease from the Altirus server. I can't remember what the function 
>> in the switches was. My recommendation to check and see if this is the 
>> case would be Dan Wells in OIT or Jonathan Mecham in the Library. I 
>> might be completely off here though. I didn't really build that system.
>>     
>
> Interesting.  I was asking originally because Dan Wells asked me about
> it.  OIT is trying to set up a pxeboot system for campus that works in
> conjunction with their existing dhcp server.
>
> He indicated that altirus worked by responding to the dhcp request, but
> without a lease.  The host would receive 2 dhcp responses, one with a
> lease, and one with pxe info, and figure out what to do.  At least
> that's what he claimed.  Your explanation sounds more logical, though.
>
>   
From my humble understanding of the dhcp protocol, when a client puts
out a request, as many dhcp servers that want to can respond. The client
chooses (usually the quickest) one and then sends out a packet saying
which server it chose.

My guess is that when a NIC is looking for a dhcp server when it is
trying to pxe boot, if it got a lease, it'd want pxe info with it. I can
completely understand a behaviour of a NIC to put a lease from one
source, and a set of pxe info from another. If it is in the rfc spec,
I'd be willing to put it into an infrastructure. If it isn't in the rfc,
it is possible that some NICs might be able to do that, but it would
probably depend on the NIC. I like the router re-writing responses. That
seems the cleanest to me.

I'd still like to see a dual dhcp setup environment. That sounds
interesting.

After looking up PXE Specs, it seems that it would have to be a router
re-writing the dhcp packets on the fly. PXE information is just
additional parameters in a regular dhcp lease. The client must choose
one, and only one of the responses it gets. It is free to choose from
any of them, but it is not allowed to mix options. That would seem to
include the pxe options.
http://www.pix.net/software/pxeboot/archive/pxespec.pdf (pdf page 25,
doc page number is 23)

Jeff Anderson
> Michael
>
>   
>> Mike Lovell
>>
>> /*
>> PLUG: http://plug.org, #utah on irc.freenode.net
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>>
>>     
>
>
> /*
> PLUG: http://plug.org, #utah on irc.freenode.net
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>
>   


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