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Revision as of 10:20, 17 June 2014 by Tom-elliott.7271 (talk | contribs) (What are the differences between the different PXE files?)
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What is PXE?

Preboot Execution Environment (PXE)

The PXE protocol is approximately a combination of DHCP and TFTP, although 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.

To initiate a PXE bootstrap session the PXE firmware broadcasts a DHCPDISCOVER packet extended with PXE-specific options (extended DHCPDISCOVER) to port 67/UDP (DHCP server port). 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.

What is iPXE?

Formerly gPXE project, iPXE is an open source PXE implementation and bootloader. It can be used to enable computers without built-in PXE support to boot from the network, or to extend an existing PXE implementation with support for additional protocols. While traditional PXE clients use TFTP to transfer data, iPXE adds the ability to retrieve data through other protocols like HTTP, iSCSI, ATA over Ethernet (AoE), and Fibre Channel over Ethernet (FCoE), and can work with Wi-Fi rather than requiring a wired connection.

  • YES, (technically) wireless imaging is supported but largely untested due to the absence in most BIOS setups.

What are the differences between the different PXE files?


  • ipxe has drivers native to the ipxe project. Those drivers are handled from the iPXE developers.
  • undionly uses the "undi" stack made by the manufacturer of the NIC.


  • .pxe is an image designed to be chain loaded, unloading both the underlying PXE and UNDI code sections. This is ultimately the goal, but there's not enough information to allow this to actually work flawlessly every time. It uses, purely, the drivers from the iPXE information. One of the benefits is the codebase for the drivers are handled by the iPXE developers. So, in theory and given enough time, all NICs could potentially be supported.
  • .kpxe unloads just the pxe stack and is the normal file we want in use as it seems to be the best between pxe/chaining I can find without flashing roms.
  • .kkpxe keeps both undi and pxe stacks in place. kkpxe works best for buggy hardware. Only recommended if you're having weird issues with the undionly.kpxe
  • .kkkpxe is only used to generate the ipxelinux.0 file. This is only used in conjunction with the syslinux project. When gpxe was the developed software of this type the file was called gpxelinux.0 which can usually be built with modern syslinux.

Undi and iPXE Stack

The UNDI driver is a generic driver that works on network cards that have a vendor UNDI ROM. The ROM contains driver code that is supposed to conform to the PXE/UNDI specification. iPXE can load the UNDI driver and use it instead of a native driver.

Depending on the iPXE image type, UNDI support works as follows:

  • undionly.kpxe is loaded from a vendor PXE stack and uses UNDI on the network card that it was booted from.
  • All-driver (ipxe) or undi images can load the UNDI for PCI network cards. The network boot ROM must be enabled in the BIOS in order for the UNDI ROM to be visible to iPXE. Note that only the first network card is supported with UNDI since multiple instances of UNDI is unreliable and cannot be supported.

Why write native drivers if UNDI works with every network card?

  • iPXE is an open source PXE stack and provides UNDI services. iPXE cannot be used as an option ROM without a native driver.
  • UNDI is slow because iPXE must switch CPU modes when calling it.
  • UNDI ROMs can be buggy or violate the PXE specification. Native drivers are known to work with iPXE and can be fixed if there is a bug since they are part of the iPXE codebase.
  • Enabling the network boot ROM in the BIOS is not always possible or desirable.