Creating a custom bootdisk

How to create a decent bootdisk

The intent here is to create a "rescue bootdisk" that will load enough 'linux' to enable you to do rescue operations. With what is presented here you will be able to do file manipulation, mounting and unmounting, and other tasks. This however is not the limit. The minimal disk is described here, and you can add anything you can fit on the floppy.

Boot disk/Rescue Disk

First we will create a loopback file to build our rescue disk image on, next we'll make a file system on the image file, then we'll use 'mount' to mount the file as a regular disk, allowing us to read and write files from the loopback file. The following commands will build us a 4 MB image.

dd if=/dev/zero of=/tmp/rfloppy bs=1k count=4096 &&
mke2fs -m 0 -N 2000 /tmp/rfloppy &&
mount -o loop /tmp/rfloppy /mnt/loop1 &&
rmdir /mnt/loop1/lost+found/

Now that we have a file mounted and usable, let's prepare it to be filled with useful material. Since this is only a rescue floppy we'll only need to set up the minimum directories.

mkdir /mnt/loop1/{dev,proc,etc,sbin,bin,lib,mnt,usr,var}

Next, we will set up the device files. I use devfs on my system, so the following command works well, as I only have the devices I use anyway. If you used MAKEDEV to create your devices, you'll want to trim the /mnt/loop1/dev directory to reclaim the inode space wasted by all of the devices in the dev directory you don't use.

cp -dpR /dev/* /mnt/loop1/dev

Now to tend to the /etc directory. To start, all we will do is use the passwd and group file that worked for our static chroot environment when we built LFS. We'll also copy the startup scripts over and a few other files that serve well as starting points.

cp -ax /etc/rc* /mnt/loop1/etc
cp -ax /etc/fstab /mnt/loop1/etc
echo "root:x:0:0:root:/root:/bin/bash" > /mnt/loop1/etc/passwd
cat > /mnt/loop1/etc/group << "EOF"

To prevent automatic mounting of hard drive partitions, make sure to add the noauto option in their fstab entry. Also, add the following entries to the /mnt/loop1/etc/fstab to assist with mounting our floppy and the ram image

/dev/ram0       /               ext2    defaults
/dev/fd0        /               ext2    defaults

Next, we will install busybox onto the image. Busybox incorporates many of the unix functions into a single small executable file.

make &&
make PREFIX=/mnt/loop1 install &&
cp -ax /var/utmp /mnt/loop1/var &&
mkdir /mnt/loop1/var/log

Also, keeping in mind your space limitations, copy any other binaries and libraries you need to the image. Use the ldd command to see which libraries you will need to copy over for any executables.

Now, since I use devfs to create devices on the fly and free up precious inodes on the floppy, we'll also install devfsd to facilitate the devices that busybox expects to find.

mv GNUmakefile Makefile &&
make &&
make PREFIX=/mnt/loop1 install &&
cp /lib/ /lib/ /lib/ /tmp &/&
strip --strip-deb /tmp/ /tmp/ /tmp/ &&
mv /tmp/ /tmp/ /tmp/ /mnt/loop1/lib/

We will also need to set up an rc script to handle the devfsd startup. Put this in /mnt/loop1/etc/init.d/rcS.

mount -t devfs devfs /dev
/sbin/devfsd /dev

Next create your compressed root filesystem. We use -9 with gzip to make the smallest possible compressed image.

umount /mnt/loop1 && dd if=/tmp/rfloppy bs=1k | gzip -v9 > rootfs.gz
ls -l rootfs.gz to make sure it will fit on the diskette.

Make a custom kernel that is optimized for size. Include only those features you will need to rescue your system. no sense in building in support for things like xfree86 dri, etc, as most rescues are performed from the command prompt.

dd if=rescueimg of=/dev/floppy/0 bs=1k
        429+1 records in
        429+1 records out
rdev /dev/floppy/0 /dev/floppy/0
rdev -R /dev/floppy/0 0

In this example the rescueimage(KERNEL) was 429+1 blocks in size. We will remember this for the next command. We now write the root file system right after the kernel on the floppy by doing 16384+429+1= 16814.

rdev -r /dev/floppy/0 16814
dd if=rootfs.gz of=/dev/floppy/0 bs=1k seek=430

In this command we use seek to find the end of the kernel (429+1) and write the root file system to the floppy.