1 | <sect1 id="postlfs-config-bootdisk">
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2 | <?dbhtml filename="bootdisk.html" dir="postlfs"?>
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3 | <title>Creating a custom bootdisk</title>
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4 |
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5 | <sect2>
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6 | <title>How to create a decent bootdisk</title>
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7 | <para>The intent here is to create a "rescue bootdisk" that will load
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8 | enough 'linux' to enable you to do rescue operations. What is presented here
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9 | is enough to do file manipulation, mounting and unmounting, and other tasks.
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10 | This, however, is not the limit. The minimal disk is described here, and you
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11 | can add anything you can fit on the floppy.</para>
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12 | </sect2>
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13 |
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14 | <sect2>
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15 | <title>Boot Disk/Rescue Disk</title>
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16 |
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17 | <para>First we will create a loopback file on which we build the root file
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18 | system for our rescue disk image. This is commonly known as the initial
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19 | ramdisk, or initrd for short, and it is automatically loaded by the boot
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20 | process if all setup is done correctly.</para>
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21 |
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22 | <para>Next we'll make a file system on the loopback file and use
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23 | <command>mount</command> to mount the loopback file as a regular disk, allowing
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24 | us to read and write files there. The following commands will build us a 4 MB
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25 | image.</para>
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26 |
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27 | <screen><userinput><command>dd if=/dev/zero of=/tmp/rfloppy bs=1k count=4096 &&
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28 | mke2fs -m 0 -N 2000 /tmp/rfloppy &&
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29 | mount -o loop /tmp/rfloppy /mnt/loop1 &&
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30 | rmdir /mnt/loop1/lost+found/</command></userinput></screen>
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31 |
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32 | <para>Now that we have a file mounted and usable, let's prepare it to be
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33 | filled with useful material. Since this is only a rescue floppy we'll
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34 | only need to set up the minimum directories.</para>
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35 |
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36 | <screen><userinput><command>mkdir /mnt/loop1/{dev,proc,etc,sbin,bin,lib,mnt,usr,var}</command></userinput></screen>
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37 |
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38 | <para>Next, we will set up the device files. I use devfs on my system, so
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39 | the following command works well, as I only have the devices I use
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40 | anyway. If you used <command>MAKEDEV</command> to create your devices, you'll
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41 | want to trim the <filename>/mnt/loop1/dev</filename> directory to reclaim the
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42 | inode space wasted by the devices you don't use in the <filename>dev</filename>
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43 | directory.</para>
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44 |
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45 | <screen><userinput><command>cp -dpR /dev/* /mnt/loop1/dev</command></userinput></screen>
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46 |
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47 | <para>Now to tend to the <filename>/etc</filename> directory. To start, all we
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48 | will do is use the passwd and group file that worked for our static chroot
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49 | environment when we built <acronym>LFS</acronym>. We'll also copy the startup scripts over and a
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50 | few other files that serve well as starting points.</para>
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51 |
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52 | <screen><userinput><command>cp -ax /etc/rc* /mnt/loop1/etc
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53 | cp -ax /etc/fstab /mnt/loop1/etc
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54 | echo "root:x:0:0:root:/root:/bin/bash" > /mnt/loop1/etc/passwd
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55 | cat > /mnt/loop1/etc/group << "EOF"</command>
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56 | root:x:0:
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57 | bin:x:1:
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58 | sys:x:2:
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59 | kmem:x:3:
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60 | tty:x:4:
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61 | tape:x:5:
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62 | daemon:x:6:
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63 | floppy:x:7:
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64 | disk:x:8:
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65 | lp:x:9:
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66 | dialout:x:10:
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67 | audio:x:11:
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68 | <command>EOF</command></userinput></screen>
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69 |
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70 | <para>To prevent automatic mounting of hard drive partitions, make sure to add
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71 | the noauto option in their fstab entry. Also, add the following entries to the
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72 | <filename>/mnt/loop1/etc/fstab</filename> to assist with mounting our
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73 | floppy and the ram image</para>
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74 |
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75 | <screen><userinput>/dev/ram0 / ext2 defaults
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76 | /dev/fd0 / ext2 defaults</userinput></screen>
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77 |
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78 | <para>Next, we will install <ulink
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79 | url="http://www.busybox.net/downloads/busybox-0.60.4.tar.bz2">busybox</ulink>
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80 | onto the image. Busybox incorporates many of the unix functions into a single
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81 | small executable file.</para>
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82 |
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83 | <screen><userinput><command>make &&
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84 | make PREFIX=/mnt/loop1 install &&
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85 | cp -ax /var/utmp /mnt/loop1/var &&
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86 | mkdir /mnt/loop1/var/log</command></userinput></screen>
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87 |
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88 | <para>Also, keeping in mind your space limitations, copy any other binaries and
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89 | libraries you need to the image. Use the <userinput>ldd</userinput> command to
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90 | see which libraries you will need to copy over for any executables.</para>
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91 |
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92 | <para>Now, since I use devfs to create devices on the fly and free up precious
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93 | inodes on the floppy, we'll also install devfsd to facilitate the
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94 | devices that busybox expects to find.</para>
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95 |
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96 | <screen><userinput><command>mv GNUmakefile Makefile &&
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97 | make &&
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98 | make PREFIX=/mnt/loop1 install &&
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99 | cp /lib/libc.so.6 /lib/ld-linux.so.2 /lib/libdl.so.2 /tmp &&
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100 | strip --strip-deb /tmp/ld-linux.so.2 /tmp/libc.so.6 /tmp/libdl.so.2 &&
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101 | mv /tmp/ld-linux.so.2 /tmp/libc.so.6 /tmp/libdl.so.2 /mnt/loop1/lib/</command></userinput></screen>
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102 |
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103 | <para>We will also need to set up an rc script to handle the devfsd startup.
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104 | Put this in <filename>/mnt/loop1/etc/init.d/rcS</filename>.</para>
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105 |
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106 | <screen><userinput>#!/bin/sh
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107 | mount -t devfs devfs /dev
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108 | /sbin/devfsd /dev</userinput></screen>
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109 |
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110 | <para>Next create your compressed root filesystem. We use -9 with gzip to
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111 | make the smallest possible compressed image.</para>
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112 |
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113 | <screen><userinput><command>umount /mnt/loop1 && dd if=/tmp/rfloppy bs=1k | gzip -v9 > rootfs.gz</command></userinput></screen>
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114 |
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115 | <para><userinput><command>ls -l rootfs.gz</command></userinput> to make
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116 | sure it will fit on the diskette.</para>
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117 |
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118 | <para>Make a custom kernel that is optimized for size. Include only those
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119 | features you will need to rescue your system. no sense in building in support
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120 | for things like xfree86 dri, etc, as most rescues are performed from the
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121 | command prompt.</para>
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122 |
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123 | <screen><userinput><command>dd if=rescueimg of=/dev/floppy/0 bs=1k</command>
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124 | 429+1 records in
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125 | 429+1 records out
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126 | <command>rdev /dev/floppy/0 /dev/floppy/0
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127 | rdev -R /dev/floppy/0 0</command></userinput></screen>
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128 |
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129 | <para>In this example the rescueimage (KERNEL) was 429+1 blocks in size.
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130 | We will remember this for the next command. We now write the root file
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131 | system right after the kernel on the floppy by doing 16384+429+1=
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132 | 16814.</para>
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133 |
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134 | <screen><userinput><command>rdev -r /dev/floppy/0 16814
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135 | dd if=rootfs.gz of=/dev/floppy/0 bs=1k seek=430</command></userinput></screen>
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136 |
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137 | <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>
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138 | </sect2>
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139 | </sect1>
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