DHCRelay PXE Master Server
Alex Burger
Canada


Overview

One of the limitations of most PXE servers is that there can only be one PXE server per subnet.  For example, if a Microsoft RIS server is configured on a subnet, it is not possible to also have an Altiris PXE server on the same subnet.  The reason one may want to have both on the same network is so that RIS can be used to image Windows desktops while Altiris is used to deploy and manage thin clients.  A simple solution is to put the thin clients on a separate subnet / VLAN, but this can become an administrative headache in a large network.  Some PXE servers allow for a 'Master PXE' to be configured which can redirect requests to another PXE server, but this is not supported by Microsoft RIS.

To solve this problem, I have created a patch for the ISC DHCP Relay server which allows you to specify a PXE server to relay a client's request to based on the client's MAC address.  The MAC addresses and PXE server IP addresses are specified in a text file.  If a match is not found, then the server specified on the dhcrelay command line is used.

Below is a sample network design to allow the use of both a RIS server for deploying Windows and an Altiris server for managing thin clients.  In the sample network, there are much fewer thin clients than regular desktops, so the text file only contains the thin client MAC addresses and a default server is specified on the dhcrelay command line.
Create an /etc/dhcrelay.hosts file that has on each line the MAC address of the client and the IP address of the PXE server it should be forwarded to.  For example:

00:11:22:33:44:55    192.168.3.10    # Thin Client 1
00:11:22:33:44:56    192.168.3.10    # Thin Client 2
00:11:22:33:44:57    192.168.3.10    # Thin Client 3
00:11:22:33:44:58    192.168.3.10    # Thin Client 4

Launch dhcrelay using:

dhcrelay -ch /etc/dhcrelay.hosts 192.168.2.10

The above command will tell dhcrelay to load the /etc/dhcrelay.hosts file and forward requests from clients that are not listed in the dhcrelay.hosts file to 192.168.2.10.

For troubleshooting, start dhcrelay in debug mode and prevent it from forking to the background using:

dhcrelay -D -d -ch /etc/dhcrelay.hosts 192.168.2.10

The patch is available here.

Future enhancements may be to use a binary search tree to speed up MAC address searches, and to allow wildcards.   I am currently using it with 100 MAC addresses and there is certainly no performance issue but it would be more efficient with a binary search tree.

Please email me with any suggestions or comments.

This page was last updated on July 11th, 2008.


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