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© Rakhesh Sasidharan

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Having both DHCP server and PXE server as IP Helpers

I always thought you don’t specify a PXE server in the list of IP Helpers. Rather, you specify DHCP servers, and the DHCP servers have options 66 (boot server) and 67 (boot file name) which specify the PXE server name. Turns out I was wrong. I recently saw that at work we have both a DHCP and PXE server defined as IP helpers, and the DHCP server didn’t have any options 66 & 67 specified, yet things worked fine. How was that possible?

Turns out that is because you can specify both DHCP servers and PXE servers as IP helpers (and if you don’t have IP helpers, you can have both DHCP servers and PXE servers on the same LAN as clients … listening to requests and replying). There won’t be a clash. This is because the PXE client (which is the component on the client machine that does PXE booting) will take the IP address from the DHCP server and the PXE details from the PXE server. This article cleared things for me, here’s some excerpts:

PXE does not come with a dedicated boot protocol. It is simply DHCP packets extended with additional DHCP options. It’s formerly known as the bootstrap protocol. If a PXE-enabled network card sends out an DHCP discover package, it will add DHCP option 60, which includes the string “PXEClient:Arch:xxxxx:UNDI:yyyzzz”. Then it waits for DHCP offers.

It will only respond if it gets a DHCP offer including option 60 which means: I am PXE capable and able to send out boot server and boot file information.

The DHCP offer can be splitted into two independent packages, coming from different servers. The DHCP server can send out the DHCP offer containing the clients IP address and the PXE server can send out the DHCP offer containing the option 60.

Check out that article for some WireShark captures and traffic flows.

The PXEClient option is useful when you want to specify separate policies for BIOS and UEFI clients. Check out this article for more info on that.

Having both DHCP server and PXE server as IP Helpers by rakhesh is licensed under a Creative Commons Attribution 4.0 International License.