isolinuxcfg

isolinuxcfg is the conventional name for the configuration file used by the ISOLINUX bootloader to control booting from ISO images. The file is typically named isolinux.cfg and resides in the isolinux/ directory within an ISO image. It is a plain text file read by the bootloader at startup and defines a boot menu and the parameters passed to kernels or initramfs images.

Global directives at the top of isolinux.cfg control overall behavior. DEFAULT sets which boot label boots automatically if no user input is received; TIMEOUT specifies the delay before the default selection; PROMPT enables or disables user interaction; UI selects a user interface file (for example a menu system); MENU TITLE provides a header line displayed in the boot menu.

Boot options are defined as labeled blocks. Each option starts with a LABEL name, and may include a MENU LABEL that appears in the menu. A label typically includes KERNEL pointing to the kernel image within the ISO, and optionally INITRD for the initramfs, plus an APPEND line supplying kernel parameters. Some options may use additional directives to chain load other configurations or booters.

The isolinux.cfg file is central to customizing boot behavior, enabling selections such as installation, live operation, diagnostic modes, or recovery tools. It is part of the Syslinux family of bootloaders and is widely used in Linux distributions’ ISO images. Editing isolinux.cfg requires care to preserve syntax and path references; changes should be validated by testing boot from the ISO.

See also: ISOLINUX, Syslinux, isolinux.bin, bootloader configuration.