[syslinux] Question about pxelinux and gpxelinux

[Jeffrey Hutzelman]

--On Tuesday, January 06, 2009 12:19:14 AM +0800 Sean Lv 
<jie.lv.mvp at gmail.com> wrote:


> 	2, From rfc5071, pxelinux should request non-standard DHCP option
> from DHCP server, option 208-211. In my environment, I am using a
> Microsoft DHCP (Windows 2008). And I tried to set option 67 (boot file
> name) to publish pxelinux.0 & gpxelinux.0. I used wildpacket(just like
> Wireshark) to capture the packets btw PXE client and DHCP server. I could
> not see PXE client request option 208/209/210/211, although I am sure I
> configured these syslinux-DHCP option on my DHCP Server.  Why? I searched
> the google and official web site, all examples are using a linux-DHCP
> Server. I am not familiar with linux, and I am not sure if my issue is
> related to my Micrsoft-DHCP Server.

Hrm; RFC5071 doesn't seem to mention this problem, but it should, and the 
documentation for PXELINUX (PXELINUX.DOC) does mention it.  The issue is 
that PXELINUX doesn't send a DHCP request at all; that's done by the PXE 
stack, before PXELINUX has even been loaded.  Naturally, the PXE stack has 
no reason to know that you want it to request PXELINUX-specific options, 
and so does not do so.

To make this problem go away, you need to either...
(1) Force your DHCP server to send the options in question even though they
    do not appear in the client's parameter request list, or
(2) Tell your DHCP server to provide these options as encapsulated vendor
    options, which changes both how they are encoded and when they are
    sent, such that they will be included if the client requests the
    vendor-encapsulated-options option (which they must, since PXE itself
    also uses vendor options).

The required configuration will differ considerably from one DHCP server to 
the next; pxelinux.doc describes how to do these things for the ISC DHCP 
server, but you will need to find some other source of information on how 
to configure the Microsoft server.

-- Jeffrey T. Hutzelman (N3NHS) <jhutz+ at cmu.edu>
   Carnegie Mellon University - Pittsburgh, PA