This is a project designed to document the creation of bootable (El Torito) CD's, particularly multi-boot CD's. I am not an expert, however I have put together a number of bootable CD's. A lot of people have asked about how to do this, so this project is intended as a write-up of the steps necessary to build one.
This documentation has been written based on my system (RedHat Linux 9), but the concepts and programs are pretty general, and should work in most cases. Here are the packages (and versions) that I'm working with:
mkbootdisk (1.5.1-1) - Creates a boot floppy disk for booting a system. syslinux (2.00-4) - A simple kernel loader which boots from a FAT filesystem. mkisofs (2.0-11.9.1) - Creates an image of an ISO9660 filesystem. dd (4.5.3)
Just a Boot CD
If all that you're trying to do is to create a bootable CD (rather than a boot floppy) that can boot your system, you only need to use the --iso option to mkbootdisk, like this:
mkbootdisk --device /tmp/cdproject.iso --iso <kernel_version>
If you've already got the floppy disk or disk image, you can use the -b option to mkisofs to tell it to use your image as the "El Torito" boot image:
$ mkdir /tmp/cdproject $ dd if=/dev/fd0 of=/tmp/cdproject/boot.img $ mkisofs -o /tmp/cdproject.iso -b boot.img /tmp/cdproject
Note: For simplicity, you can use `uname -r' to insert the current kernel version.
To boot multiple images from a CD, I use isolinux. It's a boot loader that runs off bootable CD's, and it's part of the syslinux package in most distributions.
isolinux requires that several files be installed onto the CD, including a configuration file. This will be demonstrated in the next several sections.
A Simple Example
We'll start by first building a small disk with just memtest86 and the contents of a system boot floppy.
Create a directory to serve as the root directory of your file tree on the CD:
$ mkdir /tmp/cdproject
Create a subdirectory called isolinux:
$ mkdir /tmp/cdproject/isolinux
Copy isolinux.bin and memdisk into the isolinux directory (under RedHat 9, these files are in /usr/lib/syslinux/):
$ cp <path_to>/isolinux.bin /tmp/cdproject/isolinux $ cp <path_to>/memisk /tmp/cdproject/isolinux
Copy in the files and images that you'll want to boot and use:
# Build the system boot image $ mkbootdisk --device /tmp/cdproject/isolinux/linux.img <kernel_version> # or $ dd if=/dev/fd0 of=/tmp/cdproject/isolinux/linux.img # Get a working image of memtest86 $ cd /tmp $ wget http://www.memtest86.com/memt30.zip $ unzip memt30.zip $ mv memt86/memtest.bin /tmp/cdproject/isolinux/memtest $ rm -r memt86 memt30.zip
Create a config file in isolinux:
$ <favorite_flamewar_inspiring_editor> /tmp/cdproject/isolinux/isolinux.cfg default 0 display bootmsg.txt prompt 1 label 0 localboot 0x80 label memtest kernel memtest label linux kernel memdisk append initrd=linux.img
Create a boot message file:
$ <favorite_flamewar_inspiring_editor> /tmp/cdproject/isolinux/bootmsg.txt 0) Boot from first harddisk (0x80) memtest) Start memtest86 linux) Linux Bootdisk
Create the iso image from everything:
$ mkisofs -o /tmp/cdproject.iso -b isolinux/isolinux.bin \ -c isolinux/boot.cat -no-emul-boot -boot-load-size 4 \ -boot-info-table /tmp/cdproject
You should now have an ISO image in /tmp/cdproject.iso
Booting with various methods
isolinux will boot a number of different types of systems. Here is an example of the file command on one CD I've created (the "x86 boot sector" entries are bootable):
$ file * bootmsg.txt: data dban040.img: x86 boot sector, system SYSLINUX, FAT (12 bit) delldiag.img: x86 boot sector isolinux.bin: data isolinux.cfg: ASCII text master.img: x86 boot sector memdisk: x86 boot sector, extended partition table memtest: x86 boot sector ntpasswd.img: x86 boot sector, system SYSLINUX, FAT (12 bit) pqmagic8.img: x86 boot sector slave.img: x86 boot sector splash.lss: data tomsrtbt.img: x86 boot sectorFloppy Images
To create a disk image of a floppy (1.44M or 2.88M floppy image, for example), just do a dd of the entire image, and include the image file in the isolinux.cfg file:
label <bootlabel> kernel memdisk append initrd=image.img
As an example, here is how we would add tomsrtbt to our CD:
# Download tomsrtbt ElTorito image $ cd /tmp/cdproject/isolinux $ wget http://www.tux.org/pub/distributions/tinylinux/tomsrtbt/tomsrtbt-2.0.103.ElTorito.288.img.bz2 $ bunzip2 tomsrtbt-2.0.103.ElTorito.288.img.bz2 # syslinux/isolinux only supports "plain" ISO 9660 filenames (8.3 format, and not RockRidge or Joliet) $ mv tomsrtbt-2.0.103.ElTorito.288.img tomsrtbt.img # add to isolinux.cfg file label tomsrtbt kernel memdisk append initrd=tomsrtbt.imgHard Disk Images
isolinux can also be used to boot from a "hard disk image". While more complex, this certainly opens up a lot of other options. In the above example, the "delldiag.img" image is actually a 10 MB disk image. Here is an example of how a disk image like this could be created:
$ dd if=/dev/zero of=disk.img bs=1M seek=10 count=0 # Make a filesystem (we'll just make a DOS boot system as an example) $ mkdosfs /tmp/disk.img # Insert a boot sector into the disk image (copy the 446 byte # boot code from a booting system--floppy, for example) # For further information on its location, see this link. $ dd if=bootsect.img of=disk.img bs=1 count=446 seek=62 skip=62 conv=notrunc # Mount the disk image and copy in all the necessary data $ mount -oloop /tmp/disk.img /mnt/tmp # Copy in the data and unmount $ umount /mnt/tmpNow, this disk image can be included in the isolinux.cfg file like this:
label <disk_label> kernel memdisk append initrd=disk.img floppy c=10 h=64 s=32Linux kernel, initrd, and rootfs
To create a Linux boot disk that contains a root filesystem, you first create a kernel that can load the initrd (initial ramdisk). Then, you create the initrd and a root filesystem. This process is a lot more complicated than most of the other boot systems. For more information, visit Paul's Boot CD.Etherboot/PXE???
CD ISO images
As far as I know, there exist no emulators that will allow booting from an ISO image (as a file on the disc). There are actually quite a few reasons that this isn't very practical. First, the boot code would need to understand the ISO 9660 filesystem and where to find the El Torito boot code. Next it would somehow need to tell the boot code that its boundaries are really the boundaries of the ISO image located on the CD. This is where problems are likely to occur. Most ISO images are designed to be burned directly to a CD, not as a file on a CD. As a result, it is likely that the programs contained on the CD will not notice the difference, will not mount the ISO image loopback, and will not be able to find the files it is expecting to find.
It's not impossible to do, though. A carefully crafted initrd could be designed to detect the environment it is in, and work around the problem. This would involve mounting the CD as a filesystem and checking to see if it contains the expected files. If they are found, continue on as normal. If instead, it finds an ISO image file that it believes to be itself, this file is then loopback mounted somewhere else on the filesystem. This method is probably much more complex than most people will find practical.Booting other Devices
To have isolinux boot a local hard drive or other device, use the following targets:
|Next boot device||localboot -1|
|First floppy drive||localboot 0x00|
|Primary hard drive||localboot 0x80|
There are a lot of extra options that can be used with isolinux, including color, multiple pages of instructions (as in RedHat's CD's when they boot--try hitting F7 some time, and see what they've put in), and pictures. Most of these options are detailed in the
There are a number of projects that have built rescue or toolkit CD's. Here is a list of some of them.
- Rescue disk images and tools for creating rescue disks
- flea - by Ed Schaller
- Timo's Rescue CD Set
- Gentoo Live CD
- Crash Recovery Kit for Linux
- Bart's way to create bootable CD-Roms (for Windows/Dos)
- Linux Boot Loaders Compared
- Information on Making a Bootable Linux CD with a Hard Disk Image
- Paul's Boot CD
- Ranish Partition Manager
- Creating a Windows XP recovery console
- geteltorito - Extract the El Torito boot image from a CD image
- El Torito information
- Timo's Rescue CD Set
- Partition BootSector. Master Boot Record. Fat32. Fat16
- Linux-Magazine article - Multiboot CD's with BootScriptor