PXE booting with 3.2.x: DHCP-Server-IP-Address

[Hermann.Lauer at uni-heidelberg.de]

Dear all,

when debugging why PXE booting didn't work after switching from ISC-dhcp to
freeradius I noticed, that the "next-server" in ISC is in freeradius taken
from "DHCP-Server-IP-Address". This field "siaddr" in the DHCP Packets (not in the
option area) ist used by the clients.

A comment in raddb/sites-available/dhcp says:

	#  The source IP for unicast packets is chosen from the first
	#  one of the following items which returns a valid IP
	#  address:
	#
	#	src_ipaddr
	#	ipaddr
	#	reply:DHCP-Server-IP-Address
	#	reply:DHCP-DHCP-Server-Identifier

So I never thought that I need to use "DHCP-Server-IP-Address" for the
TFTP server (which is not the same as the dhcp server at our site).

The first usefull hint was https://github.com/nagios-plugins/nagios-plugins/blob/master/plugins-root/check_dhcp.c#L958:

 /*
   * RFC 2131 (2.) says: "DHCP clarifies the interpretation of the
   * 'siaddr' field as the address of the server to use in the next step
   * of the client's bootstrap process.  A DHCP server may return its own
   * address in the 'siaddr' field, if the server is prepared to supply
   * the next bootstrap service (e.g., delivery of an operating system
   * executable image).  A DHCP server always returns its own address in
   * the 'server identifier' option."

So the setting of "siaddr" in freeradius for pxe booting to a different server
seems to be not possible, if you need to set the DHCP-Server-IP-Address also dynamically
in freeradius processing.

Sorry if I misunderstand things - I'm not fluent in the RFC and PXE stuff in BIOS.

Thanks a lot,
 greetings
   Hermann

P.S: A quick check of v4 master seems to have a syntax change only in the raddb comment

-- 
Administration/Zentrale Dienste, Interdiziplinaeres 
Zentrum fuer wissenschaftliches Rechnen der Universitaet Heidelberg
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Email: Hermann.Lauer at iwr.uni-heidelberg.de