James Saxon's List: Ops Box Making

• PXE (Pre eXecution Environment), affectionately pronounced Pixie (as in fairy dust), is a method of having an end computer (client) boot using only its network card. This method of booting was created way back in 1999

• PXE booting is one of the many ways you can boot a computer (or embedded device, or just about anything, possibly a toaster even) entirely from the network without any form of storage on the destination computer (RAM aside).
• The PXE client has to receive an additional DHCP option that 99% of the routers you've seen do not support.

• PXE Boot Process

   

  1. PXE client boots up, starts up PXE boot ROM. 

  2. The PXE boot ROM sends a DHCP request. 

  3. The responding DHCP request (should) contain an additional DHCP option, the "filename" options. (This is why the average hardware router won't work. You can't specify the needed options.) 

  4. The PXE client attempts to download the file specified over TFTP. (If not specified, the client tries to connect to the computer that gave it the DHCP lease. If the "next-server" param is specified, it will instead attempt to download the file off of that server. ) 

  5. If the PXE client downloads the file, it then executes this file and from there PXE is out of the picture. That's the entire PXE process. 

  6. If any of the above steps fail, the computer should continue on with the boot process.

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• 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.

• The PXE protocol is approximately a combination of DHCP and TFTP, albeit with subtle modifications to both. DHCP is used to locate the appropriate boot server or servers, with TFTP used to download the initial bootstrap program and additional files.
• The PXE options identify the firmware as capable of PXE, but they will be ignored by standard DHCP servers. If the firmware receives DHCPOFFERs from such servers, it may configure itself by requesting one of the offered configurations.
• To contact a PXE Boot Server the booting system must have an IP address (perhaps from a DHCP server).
• A PXE Boot Server receiving an extended DHCPREQUEST configured for the requested type and client architecture responds with an extended DHCPACK
• After receiving the requested extended DHCPACK, the Network Bootstrap Program is uploaded into RAM
• It has access to the APIs of the PXE firmware extension (Pre-boot, UDP, TFTP, Universal Network Device Interface (UNDI)).
• it can be integrated completely into standard DHCP services

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