PXE (Preboot eXecution Environment) is a widely used standard for booting computers over a network. It allows a computer to start up and load its operating system from a server located on the network instead of from local storage devices like hard drives or solid-state drives. Here's a detailed overview of how PXE works:
1. Client Configuration: To use PXE, the client computer's BIOS or UEFI firmware must support network booting. Typically, this involves configuring the boot order in the BIOS/UEFI settings to prioritize the network boot option.
2. PXE Client Initialization: When the computer is powered on or restarted, it performs a series of initialization steps. During this process, if PXE boot is enabled and prioritized, the computer sends out a special DHCP (Dynamic Host Configuration Protocol) request called a PXE DHCP request to the network.
3. DHCP Server Response: The DHCP server on the network receives the PXE DHCP request from the client. In response, the DHCP server provides the client with an IP address, subnet mask, default gateway, DNS server information, and most importantly, the location of the PXE boot server (also known as the TFTP server).
4. TFTP Server Interaction: The client uses the information provided by the DHCP server to connect to the PXE boot server, which typically runs a TFTP (Trivial File Transfer Protocol) service. TFTP is a lightweight protocol used for transferring files over a network.
5. PXE Bootloader Transfer: The client downloads a small program called the PXE bootloader or PXE bootstrap loader from the TFTP server. This bootloader is responsible for further network communication and loading the operating system kernel or boot image.
6. Boot Image Retrieval: Once the PXE bootloader is loaded, it retrieves the boot image specified in its configuration from the TFTP server. This boot image usually contains the kernel of the operating system to be booted, along with any initial RAM disk (initrd) or initial RAM filesystem (initramfs) required for the boot process.
7. Operating System Boot: After successfully downloading the boot image, the PXE bootloader hands off control to the operating system kernel. From this point onward, the boot process proceeds as if the operating system were booted from a local storage device.
8. Network Installation or Boot Environment: Depending on the configuration, the operating system may be set up to perform a network installation, where the installation files are fetched from a network repository, or to boot into a network-based environment such as a diskless workstation setup or a system rescue environment.
In summary, PXE allows computers to boot over a network by leveraging DHCP to obtain network configuration information and TFTP to retrieve the necessary boot files. This makes it a valuable tool for network administrators, system integrators, and anyone needing to deploy or manage a large number of computers efficiently.