extlinux

Extlinux is a lightweight boot loader that is part of the Syslinux project, designed to boot Linux kernels from ext2/3/4 file systems on BIOS-based systems. It is typically used to boot Linux from USB drives, hard disks, or other bootable media where a simple, compact boot loader is desirable. Extlinux works by loading a Linux kernel and an initial ramdisk (initrd) from the file system and passing a kernel command line to configure the boot.

Configuration is driven by a file named extlinux.conf, usually located under /boot/extlinux/ or a similar directory. The file defines boot entries through labeled sections that specify the kernel image, the initrd image, and any append options such as root device, filesystem options, and kernel parameters. A typical entry points the loader to the kernel and initrd, then provides arguments like root=, ro, and quiet. Extlinux can present a simple menu, support timeouts, and offer multiple boot options.

Extlinux is part of the broader Syslinux family, which also includes ISOLINUX for optical media and the boot code used by other Syslinux variants. While extlinux focuses on BIOS-based booting from ext2/3/4 partitions, the Syslinux project as a whole can support other file systems and configurations, and there are other bootloaders in the ecosystem tailored for UEFI or more complex multi-boot setups.

Limitations include lack of native support for UEFI; extlinux is intended for BIOS environments. It is well-suited for lightweight, straightforward boot configurations, embedded systems, and live USBs where a small, easily configurable boot loader is preferred.

Example usage typically involves placing vmlinuz and initrd images on the boot partition and configuring extlinux.conf with KERNEL and INITRD paths and APPEND options.