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Networking

What is PXE?

PXE (Preboot eXecution Environment) is a standard that lets a computer boot from a server over the network through its network card, without a local drive.

What is PXE?

PXE, usually pronounced pixie, is network boot. Support for it lives in the network interface card's firmware or in the UEFI network stack, so it runs before any operating system loads. The client gets an IP address, downloads a small boot program from a server, and runs it. That program then loads an installer, an imaging tool, a recovery environment, or a full operating system for diskless and thin clients.

Organizations use PXE to install or re-image many computers from one central server, which is far faster than walking a USB drive from desk to desk.

How a PXE boot works

  1. The firmware reaches a network boot entry, either because it is first in the boot order or because a technician picked it from the one-time boot menu. It may be labeled PXE, IPv4 or IPv6 network boot, or with the adapter's name.
  2. The PXE client broadcasts a DHCP Discover that identifies it as a PXE client.
  3. A DHCP server answers with an IP address plus boot details: the boot server's address (DHCP option 66) and the boot file name (option 67). In some setups a separate PXE service supplies the boot details instead.
  4. The client downloads the boot file from the boot server using TFTP (Trivial File Transfer Protocol, UDP port 69).
  5. The boot program runs and loads the next stage, such as Windows PE and a deployment tool, which then copies the full operating system image to the local drive.

What PXE needs

  • A PXE-capable NIC. Nearly all onboard Ethernet adapters support it. PXE is normally done over wired Ethernet.
  • Firmware settings. Enable the network stack or PXE option in BIOS/UEFI. The name varies by vendor, for example Network Stack, PXE Boot, or LAN Boot ROM. UEFI firmware may list separate IPv4 and IPv6 PXE entries.
  • A boot entry. Put network boot in the boot order or choose it from the one-time boot menu.
  • Server side. DHCP plus a boot server running TFTP. Windows Deployment Services (WDS) on Windows Server is a long-standing example; other deployment and imaging products include their own PXE service.
  • Routing. DHCP Discover is a broadcast, and routers do not forward broadcasts. If clients and the DHCP server are on different subnets, the router needs a DHCP relay (IP helper) configured.
  • Matching boot files. UEFI and legacy BIOS clients need different boot files, so the server must offer the right one for each.

Troubleshooting PXE

  • PXE-E61: Media test failure, check cable: the NIC sees no network link. Check the cable, the switch port, and the link lights.
  • PXE-E53: No boot filename received: the client got an address but no boot file information. Check the DHCP options or the PXE server.
  • No DHCP offer on another subnet: DHCP relay is missing on the router.
  • A home or office PC shows a PXE or network boot message before an error: the firmware fell through to network boot because the internal drive was not found or is ranked lower in the boot order. Check the drive and the boot order before suspecting the network.
  • Long pause at every startup: network boot is above the internal drive and the PC waits for a server that never answers. Move the internal drive up.

Wake-on-LAN is often paired with PXE so a deployment server can power machines on overnight and image them over the network.

PXE on the CompTIA A+ exam

PXE appears on the Core 1 acronym list as Preboot eXecution Environment. Core 1 objective 3.5 lists boot options under BIOS/UEFI settings, and network boot is one of those options. Core 2 operating system installation topics list network boot as an installation method. Remember the chain: PXE needs a PXE-capable NIC enabled in firmware, a DHCP server, and a TFTP boot server. If a question asks how to deploy an OS to many computers that have no optical drive and no USB media, the answer is PXE network boot.

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Related terms

boot order

Boot order is the priority list in BIOS/UEFI setup that decides which device a computer tries to start from first, such as an SSD, USB drive or network.

DHCP

DHCP (Dynamic Host Configuration Protocol) automatically assigns devices an IP address, subnet mask, default gateway and DNS servers. UDP ports 67/68.

NIC

A network interface card (NIC) is a hardware component that connects a computer or other device to a computer network, enabling it to communicate with other devices on that network.

OS image

An OS image is a complete, bootable copy of an operating system that can be deployed and installed on a computer or virtual machine.

TFTP

TFTP (Trivial File Transfer Protocol) is a simple file transfer protocol used to transfer files over a network, often for network device configuration and software updates.

wake-on-LAN

Wake-on-LAN (WoL) is a networking standard that allows a computer to be powered on or awakened from a low-power state by sending a specially crafted network message called a magic packet.

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