Preboot Execution Environment

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For a high-level overview of network booting, see Network booting.

The Preboot eXecution Environment (PXE, also known as Pre-Execution Environment; sometimes pronounced "pixie") is an environment to boot computers using a network interface independently of data storage devices (like hard disks) or installed operating systems.

Overview[edit]

PXE was introduced as part of the Wired for Management framework by Intel and is described in the specification (version 2.1) published by Intel and SystemSoft on September 20, 1999.[1] It makes use of several User Datagram Protocol (UDP) based network protocols like Dynamic Host Configuration Protocol (DHCP) and Trivial File Transfer Protocol (TFTP). It also uses concepts like globally unique identifier (GUID), universally unique identifier (UUID) and Universal Network Device Interface. The client (the computer to be bootstrapped via PXE) side of the PXE equation is implemented either as a Network Interface Card (NIC) BIOS extension or as UEFI code implementing a basic network card driver and the PXE standardized application programming interfaces (APIs).

The firmware on the PXE booting client initially asks (by DHCP) for the required network configuration and network booting parameters. After parsing the PXE enabled DHCP server answer, the client will be able to set its own network IP address, IP Mask, etc, and to point to the network located booting resources, based on the received TFTP Server IP address and the name of the network bootstrap program (NBP). The Client transfers the NBP into its own random-access memory (RAM) using TFTP, possibly verifies it, and finally boots from it. NBPs are just the first link in the boot chain process and they generally request via TFTP complementary files in order to get running a minimalistic OS executive (i.e. WindowsPE, or a basic Linux kernel+initrd). When the small OS executive is alive it loads its own fully capable network drivers and the rest of transfers for booting or installing a full OS are performed not by TFTP but at this point using more powerful protocols like HTTP, CIFS, NFS, etc.

Environment details[edit]

The PXE environment relies on a combination of DHCP and TFTP infrastructure protocols. DHCP is used to provide the appropriate client network parameters and specifically the location (IP address) of the TFTP server hosting, ready for download, the initial bootstrap program (NBP) and complementary files.

To initiate a PXE bootstrap session the DHCP component of the PXE firmware broadcasts a DHCPDISCOVER packet containing PXE-specific options to port 67/UDP (DHCP server port). The PXE options identify the request as a PXE transaction. Standard DHCP servers (non PXE enabled) will be able to answer regular networking information but no the PXE specific parameters. A PXE client will not be able to boot if only receives answers from non PXE enabled DHCP servers.

ProxyDHCP[edit]

If a PXE redirection service (ProxyDHCP) receives a PXE DHCPDISCOVER, it replies with a proxyDHCP DHCPOFFER to the client's port 68/UDP (DHCP client port). A proxyDHCP offer does not provide IP information. This particular characteristic allows us to split in two the PXE DHCP needs; a regular DHCP server providing only network configuration parameters while a proxyDHCP provides only the PXE specific information. The client in this situation continues the negotiation process independently with each server until it gathers all the required booting parameters. A proxyDHCP remains silent when receiving a non PXE DHCPDISCOVER inquire.

A proxyDHCP DHCPOFFER contains mainly:

  • a PXE Discovery Control field to recommend multicasting, broadcasting, or unicasting to contact PXE boot servers
  • a list of IP addresses of each available PXE Boot Server Type
  • a PXE Boot Menu with each entry representing a PXE Boot Server Type
  • a PXE Boot Prompt telling the user to press a certain key to see the boot menu
  • a timeout to launch the first boot menu entry if it expires

The proxyDHCP service may also run on the same host as the standard DHCP service. Since two services cannot use the same port 67/UDP, the proxyDHCP runs on port 4011/UDP and expects the DHCPDISCOVER packets from PXE Clients to be DHCPREQUESTs.[1]:18 The standard DHCP service has to send a special combination of PXE options in its DHCPOFFER, so the PXE client knows to look for a proxyDHCP on the same host, port 4011/UDP.

Today modern PXE implementations (2014) do not count on PXE embedded Boot Menu capabilities; instead they implement the Boot Manager/Boot Loader strategy. A Boot Manager is an NBP able to retrieve by TFTP a configuration file containing its menu information and displays it on the booting client screen. When the user selects a boot option the Boot Manager chain loads (TFTP transfer an run) the corresponding Boot Loader with its required parameters. A Boot Loader is an OS dependent piece of code able to continue with the boot/install process.

Boot server contact[edit]

To contact a PXE Boot Server the booting system must have an IP address (perhaps from a DHCP server).

It multicasts or unicasts a DHCPREQUEST packet extended with PXE-specific options (extended DHCPREQUEST) to port 4011/UDP or broadcasts it to port 67/UDP. This packet contains the PXE Boot Server type and the PXE Boot Layer, allowing multiple boot server types to run from one daemon. The extended DHCPREQUEST may be a DHCPINFORM.[1]:16

A PXE Boot Server receiving an extended DHCPREQUEST configured for the requested type and client architecture responds with an extended DHCPACK including:

  • the complete file path to download the NBP via TFTP.
  • PXE Boot Server type and PXE Boot Layer it answered
  • the multicast TFTP configuration, if MTFTP as described in the PXE specification should be used.

The booting system accepts information from only one extended DHCPOFFER.

A 2.1 version PXE Boot Server supports "Boot Integrity Services"[2] allowing the Client to verify downloaded NBPs using a checksum file which is downloaded from the same boot server as the NBP.

To get the file path of this credentials file another exchange of extended DHCPREQUEST and extended DHCPACK is required.

Network bootstrap program[edit]

After receiving the requested extended DHCPACK, the Network Bootstrap Program is uploaded into RAM and after it is verified or if verification is not required, the NBP will be executed. It has access to the APIs of the PXE firmware extension (Pre-boot, UDP, TFTP, Universal Network Device Interface (UNDI)). Its functions or tasks are not described in the PXE specification.

Integration[edit]

The PXE Client/Server Protocol was designed so:

  • it can be used in the same network as an existing DHCP environment without interference
  • it can be integrated completely into standard DHCP services
  • it can be easily extended at the most important points without a call for papers
  • every service (DHCP, Proxy DHCP, Boot Server) can be implemented standalone or in any combination of them.

The design goal of utilizing existing DHCP and TFTP servers cannot be achieved in a strictly conforming implementation. Some aspects of the PXE protocol require that the DHCP and TFTP servers be modified and communicate. One specific example is using multicast, where DHCP packets provide the multicast group information rather than an opening RFC-2090 multicast TFTP exchange. The impact of this is minimal as the most common PXE client implementation (written by Intel and provided at no cost as a linkable IA32 binary module) interoperates with a combination of isolated DHCP and unicast TFTP servers.

Availability[edit]

PXE was designed to be applicable to many system architectures. The 2.1 version of the specification assigns architecture identifiers to six system types, including IA-64 and DEC Alpha. However, the specification only completely covers IA-32. Intel included PXE in the EFI for IA-64, creating a de facto standard with the implementation.

Additionally, the PXE firmware extension was designed as an Option ROM for the IA-32 BIOS, so a personal computer (PC) can be made PXE-capable by installing a network interface controller (NIC) that provides a PXE Option ROM. This procedure also applies to the newer AMD64 processor standard for PC.

Besides proprietary PXE boot images, alternative open source implementations are also available, including gPXE, iPXE and PXELINUX.[3][4][5] They can be either chainloaded from proprietary PXE implementations, or burned into EPROMs of network adapters as a complete replacement.[6][7]

See also[edit]

References[edit]

  1. ^ a b c "Preboot Execution Environment (PXE) Specification - Version 2.1" (PDF). Intel Corporation. 1999-09-20. Retrieved 2014-02-08. 
  2. ^ "Boot Integrity Services Application Programming Interface" (PDF). Retrieved 2009-02-18. 
  3. ^ "about — Etherboot/gPXE Wiki". 
  4. ^ "iPXE: open source network boot firmware". ipxe.org. Retrieved 2013-10-01. 
  5. ^ "PXELINUX: A bootloader for Linux using the PXE network booting protocol". uni-hamburg.de. 2008. Retrieved 2013-12-25. 
  6. ^ "Chainloading iPXE". ipxe.org. Retrieved 2013-10-01. 
  7. ^ "Burning iPXE into ROM". ipxe.org. Retrieved 2013-10-01. 

External links[edit]