PXE only Config [OT]

Bruce Hudson Bruce.Hudson at Dal.Ca
Tue Dec 4 13:53:39 UTC 2007

> Thanks for the clarification.  The technical name is a DHCP proxy.  So are
> you saying that ISC DHCP cannot be configured as a Proxy DHCP or that it
> is just difficult.  It seems that the main difference is that the proxy
> can reply with a null client IP address field (from the spec," If this is
> a Proxy DHCP Service, then the client IP address field is null (").

    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 apologize for the confusing term "PXE server". 

    While the term perhaps confuses things between the redirection and the
boot services, it might be less confusing that "Proxy DHCP" if if stops
people thinking that the latter is actually a DHCP server. What is even
more confusing is that a "DHCP Proxy" (as opposed to "proxy DHCP") is
something completely different.
Bruce A. Hudson				| Bruce.Hudson at Dal.CA
UCIS, Networks and Systems		|
Dalhousie University			|
Halifax, Nova Scotia, Canada		| (902) 494-3405

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