PXE only Config [OT]

Simon Hobson dhcp1 at thehobsons.co.uk
Tue Dec 4 15:08:21 UTC 2007

Bruce Hudson wrote:

>     I am saying that what you want to do will require source code changes.
>If you look at the "discover" code in the server, it is all about finding
>an existing lease or allocating one. In short, it is all about address
>management. Filling in the other parameters is almost an after-thought. I
>see no way to avoid it. Playing with "dhcp-parameter-request-list" might
>be a way to force your production DHCP server to supply PXE parameters
>that the client did not ask for but the "Client IP Address" is part of the
>basic header from the original BOOTP protocol. It is not a parameter so it
>cannot be controlled this way.
>     This is according to the DHCP RFC (2131) which states "designated DHCP
>server hosts allocate network addresses and deliver configuration parameters
>to dynamically configured hosts". In short, there was probably a reason why
>the people that designed PXE thought they could get away with using the
>presence of an IP address in the reply to distinguish their servers, which
>uses the DHCP protocol, from "real" DHCP servers.
>    In a perfect world, what PXE is trying to accomplish is what DHCPINFORM
>was designed to solve -- provide a way for a client that has an IP address
>to broadcast a second request to ask for further configuration. The PXE
>spec mentions DHCPINFORM in reference to the "boot server" dialogs. They
>should have used it earlier. (Not that that helps you. We are stuck with
>the PXE we have.)

I've been thinking that perhaps the way to get what is required (from 
the ISC server) might be to strip out the address management stuff 
and leave in what the dhcp-discover flow does. Then of course, all 
the other stuff will need hacking to respond only to PXE requests and 
ignore the rest.

Sounds like a lot of work to me - but then at least the source is 
available unlike any of the commercial offerings.

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