source: postlfs/config/bootdisk.xml@ 84cf3abc

<sect1 id="postlfs-config-bootdisk">
<?dbhtml filename="bootdisk.html" dir="postlfs"?>
<title>Creating a custom bootdisk</title>

<para>How to create a decent bootdisk</para>
<para>The intent here is to create a "rescue bootdisk" that will load
enough 'linux' to enable you to do rescue operations.  What is presented here 
is enough 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.</para>

<para>Boot Disk/Rescue Disk</para>

<para>First we will create a loopback file on which we build the root file
system for our rescue disk image.  This is commonly known as the initial
ramdisk, or initrd for short, and it is automatically loaded by the boot
process if all setup is done correctly.</para>

<para>Next we'll make a file system on the loopback file and use
<command>mount</command> to mount the loopback file as a regular disk, allowing 
us to read and write files there. The following commands will build us a 4 MB 
image.</para>

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

<para>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.</para>

<screen><userinput><command>mkdir /mnt/loop1/{dev,proc,etc,sbin,bin,lib,mnt,usr,var}</command></userinput></screen>

<para>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 <command>MAKEDEV</command> to create your devices, you'll 
want to trim the <filename>/mnt/loop1/dev</filename> directory to reclaim the 
inode space wasted by the devices you don't use in the <filename>dev</filename> 
directory.</para>

<screen><userinput><command>cp -dpR /dev/* /mnt/loop1/dev</command></userinput></screen>

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

<screen><userinput><command>cp -ax /etc/rc* /mnt/loop1/etc
cp -ax /etc/fstab /mnt/loop1/etc
echo "root:x:0:0:root:/root:/bin/bash" &gt; /mnt/loop1/etc/passwd
cat &gt; /mnt/loop1/etc/group &lt;&lt; "EOF"</command>
root:x:0:
bin:x:1:
sys:x:2:
kmem:x:3:
tty:x:4:
tape:x:5:
daemon:x:6:
floppy:x:7:
disk:x:8:
lp:x:9:
dialout:x:10:
audio:x:11:
<command>EOF</command></userinput></screen>

<para>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 
<filename>/mnt/loop1/etc/fstab</filename> to assist with mounting our 
floppy and the ram image</para>

<screen><userinput>/dev/ram0       /               ext2    defaults
/dev/fd0        /               ext2    defaults</userinput></screen>

<para>Next, we will install <ulink
url="http://www.busybox.net/downloads/busybox-0.60.4.tar.bz2">busybox</ulink>
onto the image.  Busybox incorporates many of the unix functions into a single 
small executable file.</para>

<screen><userinput><command>make &amp;&amp;
make PREFIX=/mnt/loop1 install &amp;&amp;
cp -ax /var/utmp /mnt/loop1/var &amp;&amp;
mkdir /mnt/loop1/var/log</command></userinput></screen>

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

<para>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.</para>

<screen><userinput><command>mv GNUmakefile Makefile &amp;&amp;
make &amp;&amp;
make PREFIX=/mnt/loop1 install &amp;&amp;
cp /lib/libc.so.6 /lib/ld-linux.so.2 /lib/libdl.so.2 /tmp &amp;&amp;
strip --strip-deb /tmp/ld-linux.so.2 /tmp/libc.so.6 /tmp/libdl.so.2 &amp;&amp;
mv /tmp/ld-linux.so.2 /tmp/libc.so.6 /tmp/libdl.so.2 /mnt/loop1/lib/</command></userinput></screen>

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

<screen><userinput>#!/bin/sh
mount -t devfs devfs /dev
/sbin/devfsd /dev</userinput></screen>

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

<screen><userinput><command>umount /mnt/loop1 &amp;&amp; dd if=/tmp/rfloppy bs=1k | gzip -v9 > rootfs.gz</command></userinput></screen>  

<para><userinput><command>ls -l rootfs.gz</command></userinput> to make
sure it will fit on the diskette.</para>

<para>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.</para>

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

<para>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.</para>

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

<para>In this command we use seek to find the end of the kernel  (429+1) and write the root file system to the floppy.</para>
</sect1>