Version notes

Graham Chapman wrote the original Bootdisk-HOWTO and he supported it through version 3.1. Tom Fawcett started as co-author around the time kernel v2 was introduced. He is the document's current maintainer.

This information is intended for Linux on the Intel platform. Much of this information may be applicable to Linux on other processors, but we have no first-hand experience or information about this. If you have experience with bootdisks on other platforms, please contact us.

Yet to do

Any volunteers?

1. Describe (or link to another document that describes) how to create other bootable disk-like things, such as CDROMs, ZIP disks and LS110 disks.

2. Describe how to deal with the huge libc.so shared libraries. The options are basically to get older, smaller libraries or to cut down existing libraries.

3. Re-analyze distribution bootdisks and update the "How the Pros do it" section.

4. Delete section that describes how to upgrade existing distribution bootdisks. This is usually more trouble than it's worth.

5. Rewrite/streamline the Troubleshooting section.

Feedback and credits

I welcome any feedback, good or bad, on the content of this document. I/we have done our best to ensure that the instructions and information herein are accurate and reliable. Please let me know if you find errors or omissions. When writing, please indicate the version number of the document you're referencing.

We thank the many people who assisted with corrections and suggestions. Their contributions have made it far better than we could ever have done alone.

Send comments, corrections and questions to the author at the email address above. I don't mind trying to answer questions, but if you have a specific question about why your bootdisk doesn't work, please read the section called Troubleshooting, or The Agony of Defeat first.

Distribution policy

Copyright © 1995,1996,1997,1998,1999,2000 by Tom Fawcett and Graham Chapman. This document may be distributed under the terms set forth in the Linux Documentation Project License. Please contact the authors if you are unable to get the license.

This is free documentation. It is distributed in the hope that it will be useful, but without any warranty; without even the implied warranty of merchantability or fitness for a particular purpose.

The boot process

All PC systems start the boot process by executing code in ROM (specifically, the BIOS) to load the sector from sector 0, cylinder 0 of the boot drive. The boot drive is usually the first floppy drive (designated A: in DOS and /dev/fd0 in Linux). The BIOS then tries to execute this sector. On most bootable disks, sector 0, cylinder 0 contains either:

• code from a boot loader such as LILO, which locates the kernel, loads it and executes it to start the boot proper.

• the start of an operating system kernel, such as Linux.

If a Linux kernel has been raw-copied to a diskette, the first sector of the disk will be the first sector of the Linux kernel itself. This first sector will continue the boot process by loading the rest of the kernel from the boot device.

Once the kernel is completely loaded, it goes through some basic device initialization. It then tries to load and mount a root filesystem from some device. A root filesystem is simply a filesystem that is mounted as ``/''. The kernel has to be told where to look for the root filesystem; if it cannot find a loadable image there, it halts.

In some boot situations ‐ often when booting from a diskette ‐ the root filesystem is loaded into a ramdisk, which is RAM accessed by the system as if it were a disk. There are two reasons why the system loads to ramdisk. First, RAM is several orders of magnitude faster than a floppy disk, so system operation is fast; and second, the kernel can load a compressed filesystem from the floppy and uncompress it onto the ramdisk, allowing many more files to be squeezed onto the diskette.

Once the root filesystem is loaded and mounted, you see a message like:

VFS: Mounted root (ext2 filesystem) readonly.

At this point the system finds the init program on the root filesystem (in /bin or /sbin) and executes it. init reads its configuration file /etc/inittab, looks for a line designated sysinit, and executes the named script. The sysinit script is usually something like /etc/rc or /etc/init.d/boot. This script is a set of shell commands that set up basic system services, such as:

• Running fsck on all the disks,

• Loading necessary kernel modules,

• Starting swapping,

• Initializing the network,

• Mounting disks mentioned in fstab.

This script often invokes various other scripts to do modular initialization. For example, in the common SysVinit structure, the directory /etc/rc.d/ contains a complex structure of subdirectories whose files specify how to enable and shut down most system services. However, on a bootdisk the sysinit script is often very simple.

When the sysinit script finishes control returns to init, which then enters the default runlevel, specified in inittab with the initdefault keyword. The runlevel line usually specifies a program like getty, which is responsible for handling commununications through the console and ttys. It is the getty program which prints the familiar ``login:'' prompt. The getty program in turn invokes the login program to handle login validation and to set up user sessions.

Disk types

Having reviewed the basic boot process, we can now define various kinds of disks involved. We classify disks into four types. The discussion here and throughout this document uses the term ``disk'' to refer to floppy diskettes unless otherwise specified, though most of the discussion could apply equally well to hard disks.

In general, when we talk about ``building a bootdisk'' we mean creating both the boot (kernel) and root (files) portions. They may be either together (a single boot/root disk) or separate (boot + root disks). The most flexible approach for rescue diskettes is probably to use separate boot and root diskettes, and one or more utility diskettes to handle the overflow.

Overview

A root filesystem must contain everything needed to support a full Linux system. To be able to do this, the disk must include the minimum requirements for a Linux system:

• The basic file system structure,

• Minimum set of directories: /dev, /proc, /bin, /etc, /lib, /usr, /tmp,

• Basic set of utilities: sh, ls, cp, mv, etc.,

• Minimum set of config files: rc, inittab, fstab, etc.,

• Devices: /dev/hd*, /dev/tty*, /dev/fd0, etc.,

• Runtime library to provide basic functions used by utilities.

Of course, any system only becomes useful when you can run something on it, and a root diskette usually only becomes useful when you can do something like:

• Check a file system on another drive, for example to check your root file system on your hard drive, you need to be able to boot Linux from another drive, as you can with a root diskette system. Then you can run fsck on your original root drive while it is not mounted.

• Restore all or part of your original root drive from backup using archive and compression utilities such as cpio, tar, gzip and ftape.

We will describe how to build a compressed filesystem, so called because it is compressed on disk and, when booted, is uncompressed onto a ramdisk. With a compressed filesystem you can fit many files (approximately six megabytes) onto a standard 1440K diskette. Because the filesystem is much larger than a diskette, it cannot be built on the diskette. We have to build it elsewhere, compress it, then copy it to the diskette.

Creating the filesystem

In order to build such a root filesystem, you need a spare device that is large enough to hold all the files before compression. You will need a device capable of holding about four megabytes. There are several choices:

• Use a ramdisk (DEVICE = /dev/ram0). In this case, memory is used to simulate a disk drive. The ramdisk must be large enough to hold a filesystem of the appropriate size. If you use LILO, check your configuration file (/etc/lilo.conf) for a line like RAMDISK = nnn which determines the maximum RAM that can be allocated to a ramdisk. The default is 4096K, which should be sufficient. You should probably not try to use such a ramdisk on a machine with less than 8MB of RAM. Check to make sure you have a device like /dev/ram0, /dev/ram or /dev/ramdisk. If not, create /dev/ram0 with mknod (major number 1, minor 0).

• If you have an unused hard disk partition that is large enough (several megabytes), this is acceptable.

• Use a loopback device, which allows a disk file to be treated as a device. Using a loopback device you can create a three megabyte file on your hard disk and build the filesystem on it.

  Type man losetup for instructions on using loopback devices. If you don't have losetup, you can get it along with compatible versions of mount and unmount from the util-linux package in the directory ftp://ftp.win.tue.nl/pub/linux/utils/util-linux/.

  If you do not have a loop device (/dev/loop0, /dev/loop1, etc.) on your system, you will have to create one with ``mknod /dev/loop0 b 7 0''. Once you've installed these special mount and umount binaries, create a temporary file on a hard disk with enough capacity (eg, /tmp/fsfile). You can use a command like:

  dd if=/dev/zero of=/tmp/fsfile bs=1k count=nnn

  to create an nnn-block file.

  Use the file name in place of DEVICE below. When you issue a mount command you must include the option -o loop to tell mount to use a loopback device. For example:

  mount -o loop -t ext2 /tmp/fsfile /mnt

  will mount /tmp/fsfile via a loopback device at the mount point /mnt. A df will confirm this.

After you've chosen one of these options, prepare the DEVICE with:

dd if=/dev/zero of=DEVICE bs=1k count=4096

This command zeroes out the device.

Next, create the filesystem. The Linux kernel recognizes two file system types for root disks to be automatically copied to ramdisk. These are minix and ext2, of which ext2 is preferred. If using ext2, you may find it useful to use the -i option to specify more inodes than the default; -i 2000 is suggested so that you don't run out of inodes. Alternatively, you can save on inodes by removing lots of unnecessary /dev files. mke2fs will by default create 360 inodes on a 1.44Mb diskette. I find that 120 inodes is ample on my current rescue root diskette, but if you include all the devices in the /dev directory then you will easily exceed 360. Using a compressed root filesystem allows a larger filesystem, and hence more inodes by default, but you may still need to either reduce the number of files or increase the number of inodes.

So the command you use will look like:

mke2fs -m 0 -i 2000 DEVICE

(If you're using a loopback device, the disk file you're using should be supplied in place of this DEVICE.)

The mke2fs command will automatically detect the space available and configure itself accordingly. The ``-m 0'' parameter prevents it from reserving space for root, and hence provides more usable space on the disk.

Next, mount the device:

mount -t ext2 DEVICE /mnt

(You must create a mount point /mnt if it does not already exist.) In the remaining sections, all destination directory names are assumed to be relative to /mnt.

Populating the filesystem

Here is a reasonable minimum set of directories for your root filesystem [1]:

• /dev -- Devices, required to perform I/O

• /proc -- Directory stub required by the proc filesystem

• /etc -- System configuration files

• /sbin -- Critical system binaries

• /bin -- Essential binaries considered part of the system

• /lib -- Shared libraries to provide run-time support

• /mnt -- A mount point for maintenance on other disks

• /usr -- Additional utilities and applications

Three of these directories will be empty on the root filesystem, so they only need to be created with mkdir. The /proc directory is basically a stub under which the proc filesystem is placed. The directories /mnt and /usr are only mount points for use after the boot/root system is running. Hence again, these directories only need to be created.

The remaining four directories are described in the following sections.

        ls -ltru

Providing for PAM and NSS

Your system may require dynamically loaded libraries that are not visible to ldd. If you don't provide for these, you may have trouble logging in or using your bootdisk.

Modules

If you have a modular kernel, you must consider which modules you may want to load from your bootdisk after booting. You might want to include ftape and zftape modules if your backup tapes are on floppy tape, modules for SCSI devices if you have them, and possibly modules for PPP or SLIP support if you want to access the net in an emergency.

These modules may be placed in /lib/modules. You should also include insmod, rmmod and lsmod. Depending on whether you want to load modules automatically, you might also include modprobe, depmod and swapout. If you use kerneld, include it along with /etc/conf.modules.

However, the main advantage to using modules is that you can move non-critical modules to a utility disk and load them when needed, thus using less space on your root disk. If you may have to deal with many different devices, this approach is preferable to building one huge kernel with many drivers built in.

Some final details

Some system programs, such as login, complain if the file /var/run/utmp and the directory /var/log do not exist. So:

        mkdir -p /mnt/var/{log,run{
        touch /mnt/var/run/utmp

Finally, after you have set up all the libraries you need, run ldconfig to remake /etc/ld.so.cache on the root filesystem. The cache tells the loader where to find the libraries. To remake ld.so.cache, issue the following commands:

chdir /mnt; chroot /mnt /sbin/ldconfig

The chroot is necessary because ldconfig always remakes the cache for the root filesystem.

Wrapping it up

Once you have finished constructing the root filesystem, unmount it, copy it to a file and compress it:

umount /mnt dd if=DEVICE bs=1k | gzip -v9 > rootfs.gz

When this finishes you will have a file rootfs.gz. This is your compressed root filesystem. You should check its size to make sure it will fit on a diskette; if it doesn't you'll have to go back and remove some files. Some suggestions for reducing root filesystem size appear in the section called Reducing root filesystem size.

Transferring the kernel with LILO

The first thing you must do is create a small configuration file for LILO. It should look like this:

boot =/dev/fd0 install =/boot/boot.b map =/boot/map read-write backup =/dev/null compact image = KERNEL label = Bootdisk root =/dev/fd0

For an explanation of these parameters, see LILO's user documentation. You will probably also want to add an append=... line to this file from your hard disk's /etc/lilo.conf file.

Save this file as bdlilo.conf.

You now have to create a small filesystem, which we shall call a kernel filesystem, to distinguish it from the root filesystem.

First, figure out how large the filesystem should be. Take the size of your kernel in blocks (the size shown by ``ls -l KERNEL'' divided by 1024 and rounded up) and add 50. Fifty blocks is approximately the space needed for inodes plus other files. You can calculate this number exactly if you want to, or just use 50. If you're creating a two-disk set, you may as well overestimate the space since the first disk is only used for the kernel anyway. Call this number KERNEL_BLOCKS.

Put a floppy diskette in the drive (for simplicity we'll assume /dev/fd0) and create an ext2 kernel filesystem on it:

mke2fs -i 8192 -m 0 /dev/fd0 KERNEL_BLOCKS

The ``-i 8192'' specifies that we want one inode per 8192 bytes. Next, mount the filesystem, remove the lost+found directory, and create dev and boot directories for LILO:

mount /dev/fd0 /mnt rm -rf /mnt/lost+found mkdir /mnt/{boot,dev}

Next, create devices /dev/null and /dev/fd0. Instead of looking up the device numbers, you can just copy them from your hard disk using -R:

cp -R /dev/{null,fd0} /mnt/dev

LILO needs a copy of its boot loader, boot.b, which you can take from your hard disk. It is usually kept in the /boot directory.

cp /boot/boot.b /mnt/boot

Finally, copy in the LILO configuration file you created in the last section, along with your kernel. Both can be put in the root directory:

cp bdlilo.conf KERNEL /mnt

Everything LILO needs is now on the kernel filesystem, so you are ready to run it. LILO's -r flag is used for installing the boot loader on some other root:

lilo -v -C bdlilo.conf -r /mnt

LILO should run without error, after which the kernel filesystem should look something like this:

total 361 1 –rw–r––r–– 1 root root 176 Jan 10 07:22 bdlilo.conf 1 drwxr–xr–x 2 root root 1024 Jan 10 07:23 boot/ 1 drwxr–xr–x 2 root root 1024 Jan 10 07:22 dev/ 358 –rw–r––r–– 1 root root 362707 Jan 10 07:23 vmlinuz boot: total 8 4 –rw–r––r–– 1 root root 3708 Jan 10 07:22 boot.b 4 –rw––––––– 1 root root 3584 Jan 10 07:23 map dev: total 0 0 brw–r––––– 1 root root 2, 0 Jan 10 07:22 fd0 0 crw–r––r–– 1 root root 1, 3 Jan 10 07:22 null

Do not worry if the file sizes are slightly different from yours.

Now leave the diskette in the drive and go to the section called Setting the ramdisk word.

Transferring the kernel without LILO

If you are not using LILO, transfer the kernel to the bootdisk with dd:

% dd if=KERNEL of=/dev/fd0 bs=1k 353+1 records in 353+1 records out

In this example, dd wrote 353 complete records + 1 partial record, so the kernel occupies the first 354 blocks of the diskette. Call this number KERNEL_BLOCKS and remember it for use in the next section.

Finally, set the root device to be the diskette itself, then set the root to be loaded read/write:

rdev /dev/fd0 /dev/fd0 rdev -R /dev/fd0 0

Be careful to use a capital -R in the second rdev command.

Setting the ramdisk word

Inside the kernel image is the ramdisk word that specifies where the root filesystem is to be found, along with other options. The word can be accessed and set via the rdev command, and its contents are interpreted as follows:

If bit 15 is set, on boot-up you will be prompted to place a new floppy diskette in the drive. This is necessary for a two-disk boot set.

There are two cases, depending on whether you are building a single boot/root diskette or a double ``boot+root'' diskette set.

1. If you are building a single disk, the compressed root filesystem will be placed right after the kernel, so the offset will be the first free block (which should be the same as KERNEL_BLOCKS). Bit 14 will be set to 1, and bit 15 will be zero. For example, say you're building a single disk and the root filesystem will begin at block 253 (decimal). The ramdisk word value should be 253 (decimal) with bit 14 set to 1 and bit 15 set to 0. To calculate the value you can simply add the decimal values. 253 + (2^14) = 253 + 16384 = 16637. If you don't quite understand where this number comes from, plug it into a scientific calculator and convert it to binary,

2. If you are building a two-disk set, the root filesystem will begin at block zero of the second disk, so the offset will be zero. Bit 14 will be set to 1 and bit 15 will be 1. The decimal value will be 2^14 + 2^15 = 49152 in this case.

After carefully calculating the value for the ramdisk word, set it with rdev -r. Be sure to use the decimal value. If you used LILO, the argument to rdev here should be the mounted kernel path, e.g. /mnt/vmlinuz; if you copied the kernel with dd, instead use the floppy device name (e.g., /dev/fd0).

rdev -r KERNEL_OR_FLOPPY_DRIVE VALUE

If you used LILO, unmount the diskette now.

Transferring the root filesystem

The last step is to transfer the root filesystem.

• If the root filesystem will be placed on the same disk as the kernel, transfer it using dd with the seek option, which specifies how many blocks to skip:

  dd if=rootfs.gz of=/dev/fd0 bs=1k seek=KERNEL_BLOCKS

• If the root filesystem will be placed on a second disk, remove the first diskette, put the second diskette in the drive, then transfer the root filesystem to it:

  dd if=rootfs.gz of=/dev/fd0 bs=1k

Congratulations, you are done!

Reducing root filesystem size

Sometimes a root filesystem is too large to fit on a diskette even after compression. Here are some ways to reduce the filesystem size:

1. Increase the diskette density. By default, floppy diskettes are formatted at 1440K, but higher density formats are available. fdformat will format disks for the following sizes: 1600, 1680, 1722, 1743, 1760, 1840, and 1920. Most 1440K drives will support 1722K, and this is what I always use for bootdisks. See the fdformat man page and /usr/src/linux/Documentation/devices.txt.

2. Replace your shell. Some of the popular shells for Linux, such as bash and tcsh, are large and require many libraries. Light-weight alternatives exist, such as ash, lsh, kiss and smash, which are much smaller and require few (or no) libraries. Most of these replacement shells are available from http://metalab.unc.edu/pub/Linux/system/shells/. Make sure any shell you use is capable of running commands in all the rc files you include on your bootdisk.

3. Strip libraries and binaries. Many libraries and binaries are distributed with debugging information. Running file on these files will tell you ``not stripped'' if so. When copying binaries to your root filesystem, it is good practice to use:

   objcopy --strip-all FROM TO

   Important: When copying libraries, be sure to use strip-debug instead of strip-all.

4. If you deleted or moved files much when you were creating the root filesystem, create it again. See the NOTE ABOVE on the importance of not having dirty blocks in the filesystem.

5. Move non-critical files to a utility disk. If some of your binaries are not needed immediately to boot or login, you can move them to a utility disk. See the section called Building a utility disk for details. You may also consider moving modules to a utility disk as well.

Non-ramdisk root filesystems

the section called Building a root filesystem gave instructions for building a compressed root filesystem which is loaded to ramdisk when the system boots. This method has many advantages so it is commonly used. However, some systems with little memory cannot afford the RAM needed for this, and they must use root filesystems mounted directly from the diskette.

Such filesystems are actually easier to build than compressed root filesystems because they can be built on a diskette rather than on some other device, and they do not have to be compressed. We will outline the procedure as it differs from the instructions above. If you choose to do this, keep in mind that you will have much less space available.

1. Calculate how much space you will have available for root files. If you are building a single boot/root disk, you must fit all blocks for the kernel plus all blocks for the root filesystem on the one disk.

2. Using mke2fs, create a root filesystem on a diskette of the appropriate size.

3. Populate the filesystem as described above.

4. When done, unmount the filesystem and transfer it to a disk file but do not compress it.

5. Transfer the kernel to a floppy diskette, as described above. When calculating the ramdisk word, set bit 14 to zero, to indicate that the root filesystem is not to be loaded to ramdisk. Run the rdev's as described.

6. Transfer the root filesystem as before.

There are several shortcuts you can take. If you are building a two-disk set, you can build the complete root filesystem directly on the second disk and you need not transfer it to a hard disk file and then back. Also, if you are building a single boot/root disk and using LILO, you can build a single filesystem on the entire disk, containing the kernel, LILO files and root files, and simply run LILO as the last step.

Building a utility disk

Building a utility disk is relatively easy -- simply create a filesystem on a formatted disk and copy files to it. To use it with a bootdisk, mount it manually after the system is booted.

In the instructions above, we mentioned that the utility disk could be mounted as /usr. In this case, binaries could be placed into a /bin directory on your utility disk, so that placing /usr/bin in your path will access them. Additional libraries needed by the binaries are placed in /lib on the utility disk.

There are several important points to keep in mind when designing a utility disk:

1. Do not place critical system binaries or libraries onto the utility disk, since it will not be mountable until after the system has booted.

2. You cannot access a floppy diskette and a floppy tape drive simultaneously. This means that if you have a floppy tape drive, you will not be able to access it while your utility disk is mounted.

3. Access to files on the utility disk will be slow.

Appendix Appendix D shows a sample of files on a utility disk. Here are some ideas for files you may find useful: programs for examining and manipulating disks (format, fdisk) and filesystems (mke2fs, fsck, debugfs, isofs.o), a lightweight text editor (elvis, jove), compression and archive utilities (gzip, bzip, tar, cpio, afio), tape utilities (mt, ftmt, tob, taper), communications utilities (ppp.o, slip.o, minicom) and utilities for devices (setserial, mknod).