PXE
acronymStands for: Preboot Execution Environment
The firmware convention for booting a machine over the network: a DHCP exchange, then a TFTP download of a boot program.
RFC 5970 describes the aim: serve every boot and configuration file from one central server when there are many machines to maintain. A PXE boot is a DHCP exchange followed by a download, so a failed one is read in the capture: the architecture the client announced, the file it was offered and the server named for it.
The client identifies itself in its DHCP messages. RFC 4578 (2006), which recorded Intel's options for the IETF, defines option 93, client system architecture (16-bit types), option 94, network interface identifier (type 1, the Universal Network Device Interface, and its version), and option 97, client machine identifier (a 16-octet GUID), each of which MUST be present in every DHCP and PXE packet a compliant client or server sends. The UEFI specification adds option 60, a case-sensitive class identifier beginning PXEClient that embeds the architecture and interface version.
The reply names the file and the server: RFC 2131 gives the DHCP header a 128-octet file field for the boot file name and siaddr for the next server in the bootstrap. The client fetches that network bootstrap program by TFTP and runs it, and the architecture decides which file fits: RFC 4578 lists type 0 as Intel x86PC, and the UEFI specification calls 64-bit UEFI type 0x07, a value RFC 4578's own table printed against EFI BC.
RFC 5970 (2010) carried the idea to DHCPv6: the boot file becomes a URL in option 59, options 61 and 62 mirror 93 and 94, and HTTP is strongly recommended, superseding for IPv6 the old advice to use TFTP. UEFI HTTP Boot, in the specification's words a higher-performance replacement for TFTP-based PXE, announces type 0x0F or 0x10 and the class HTTPClient. RFC 5970 warns that a rogue DHCPv6 server could hand out the URL of a malicious image, and recommends HTTPS or signed boot images.
Also known as: preboot execution environment, pxe boot, network boot, netboot, pxeclient, nbp
Sources
- RFC 4578, Dynamic Host Configuration Protocol (DHCP) Options for the Intel Preboot eXecution Environment (PXE), November 2006: sections 1, 2.1 to 2.3 and 5 (read 2026-10-19)
- RFC 2131, Dynamic Host Configuration Protocol, March 1997: section 2 (read 2026-10-19)
- RFC 5970, DHCPv6 Options for Network Boot, September 2010: sections 1, 3, 5 and 7 (read 2026-10-19)
- UEFI Specification 2.10, chapter 24, Network Protocols: SNP, PXE, BIS and HTTP Boot, 2022: Table 24.1 and sections 24.7.1, 24.7.2 and 24.7.4 (read 2026-10-19)