1 | <?xml version="1.0" encoding="ISO-8859-1"?>
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2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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3 | "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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4 | <!ENTITY % general-entities SYSTEM "../general.ent">
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5 | %general-entities;
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6 | ]>
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7 |
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8 | <sect1 id="ch-config-symlinks">
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9 | <?dbhtml filename="symlinks.html"?>
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10 |
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11 | <title>Managing Devices</title>
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12 |
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13 | <sect2 revision="sysv">
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14 |
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15 | <title>Network Devices</title>
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16 |
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17 | <para>Udev, by default, names network devices according to Firmware/BIOS
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18 | data or physical characteristics like the bus, slot, or MAC address. The
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19 | purpose of this naming convention is to ensure that network devices are
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20 | named consistently and not based on the time the network card was
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21 | discovered. For example, on a computer having two network cards made by
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22 | Intel and Realtek, the network card manufactured by Intel may become eth0
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23 | and the Realtek card becomes eth1. In some cases, after a reboot the cards
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24 | could get renumbered the other way around.</para>
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25 |
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26 | <para>In the new naming scheme, typical network device names would then
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27 | be something like enp5s0 or wlp3s0. If this naming convention is not
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28 | desired, the traditional naming scheme or a custom scheme can be
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29 | implemented.</para>
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30 |
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31 | <sect3>
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32 | <title>Disabling Persistent Naming on the Kernel Command Line</title>
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33 |
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34 | <para>The traditional naming scheme using eth0, eth1, etc can be
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35 | restored by adding <userinput>net.ifnames=0</userinput> on the
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36 | kernel command line. This is most appropriate for those systems
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37 | that have only one ethernet device of the same type. Laptops
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38 | often have multiple ethernet connections that are named eth0 and
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39 | wlan0 and are also candidates for this method. The command line
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40 | is passed in the GRUB configuration file.
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41 | See <xref linkend="grub-cfg"/>.</para>
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42 | </sect3>
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43 |
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44 | <sect3>
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45 | <title>Creating Custom Udev Rules</title>
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46 |
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47 | <para>The naming scheme can be customized by creating custom udev
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48 | rules. A script has been included that generates the initial rules.
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49 | Generate these rules by running:</para>
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50 |
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51 | <screen role="install"><userinput>bash /usr/lib/udev/init-net-rules.sh</userinput></screen>
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52 |
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53 | <para> Now, inspect the
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54 | <filename>/etc/udev/rules.d/70-persistent-net.rules</filename> file, to
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55 | find out which name was assigned to which network device:</para>
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56 |
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57 | <screen role="nodump"><userinput>cat /etc/udev/rules.d/70-persistent-net.rules</userinput></screen>
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58 |
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59 | <note><para>In some cases such as when MAC addresses have been assigned to
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60 | a network card manually or in a virtual environment such as Qemu or Xen,
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61 | the network rules file may not have been generated because addresses
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62 | are not consistently assigned. In these cases, this method cannot
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63 | be used.</para></note>
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64 |
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65 | <para>The file begins with a comment block followed by two lines for each
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66 | NIC. The first line for each NIC is a commented description showing its
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67 | hardware IDs (e.g. its PCI vendor and device IDs, if it's a PCI card),
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68 | along with its driver in parentheses, if the driver can be found. Neither
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69 | the hardware ID nor the driver is used to determine which name to give an
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70 | interface; this information is only for reference. The second line is the
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71 | udev rule that matches this NIC and actually assigns it a name.</para>
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72 |
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73 | <para>All udev rules are made up of several keys, separated by commas and
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74 | optional whitespace. This rule's keys and an explanation of each of them
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75 | are as follows:</para>
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76 |
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77 | <itemizedlist>
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78 | <listitem>
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79 | <para><literal>SUBSYSTEM=="net"</literal> - This tells udev to ignore
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80 | devices that are not network cards.</para>
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81 | </listitem>
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82 | <listitem>
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83 | <para><literal>ACTION=="add"</literal> - This tells udev to ignore this
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84 | rule for a uevent that isn't an add ("remove" and "change" uevents also
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85 | happen, but don't need to rename network interfaces).</para>
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86 | </listitem>
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87 | <listitem>
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88 | <para><literal>DRIVERS=="?*"</literal> - This exists so that udev will
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89 | ignore VLAN or bridge sub-interfaces (because these sub-interfaces do
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90 | not have drivers). These sub-interfaces are skipped because the name
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91 | that would be assigned would collide with their parent devices.</para>
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92 | </listitem>
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93 | <listitem>
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94 | <para><literal>ATTR{address}</literal> - The value of this key is the
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95 | NIC's MAC address.</para>
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96 | </listitem>
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97 | <listitem>
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98 | <para><literal>ATTR{type}=="1"</literal> - This ensures the rule only
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99 | matches the primary interface in the case of certain wireless drivers
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100 | which create multiple virtual interfaces. The secondary interfaces are
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101 | skipped for the same reason that VLAN and bridge sub-interfaces are
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102 | skipped: there would be a name collision otherwise.</para>
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103 | </listitem>
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104 | <listitem>
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105 | <para><literal>NAME</literal> - The value of this key is the name that
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106 | udev will assign to this interface.</para>
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107 | </listitem>
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108 | </itemizedlist>
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109 |
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110 | <para>The value of <literal>NAME</literal> is the important part. Make sure
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111 | you know which name has been assigned to each of your network cards before
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112 | proceeding, and be sure to use that <literal>NAME</literal> value when
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113 | creating your configuration files below.</para>
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114 |
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115 | </sect3>
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116 |
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117 | </sect2>
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118 |
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119 | <sect2 revision="sysv">
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120 |
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121 | <title>CD-ROM symlinks</title>
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122 |
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123 | <para>Some software that you may want to install later (e.g., various
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124 | media players) expect the <filename class="symlink">/dev/cdrom</filename>
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125 | and <filename class="symlink">/dev/dvd</filename> symlinks to exist, and
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126 | to point to a CD-ROM or DVD-ROM device. Also, it may be convenient to put
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127 | references to those symlinks into <filename>/etc/fstab</filename>. Udev
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128 | comes with a script that will generate rules files to create these symlinks
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129 | for you, depending on the capabilities of each device, but you need to
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130 | decide which of two modes of operation you wish to have the script use.</para>
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131 |
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132 | <para>First, the script can operate in <quote>by-path</quote> mode (used by
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133 | default for USB and FireWire devices), where the rules it creates depend on
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134 | the physical path to the CD or DVD device. Second, it can operate in
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135 | <quote>by-id</quote> mode (default for IDE and SCSI devices), where the
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136 | rules it creates depend on identification strings stored on the CD or DVD
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137 | device itself. The path is determined by udev's <command>path_id</command>
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138 | script, and the identification strings are read from the hardware by its
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139 | <command>ata_id</command> or <command>scsi_id</command> programs, depending
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140 | on which type of device you have.</para>
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141 |
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142 | <para>There are advantages to each approach; the correct approach to use
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143 | will depend on what kinds of device changes may happen. If you expect the
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144 | physical path to the device (that is, the ports and/or slots that it plugs
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145 | into) to change, for example because you plan on moving the drive to a
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146 | different IDE port or a different USB connector, then you should use the
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147 | <quote>by-id</quote> mode. On the other hand, if you expect the device's
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148 | identification to change, for example because it may die, and you would
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149 | replace it with a different device with the same capabilities and which
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150 | is plugged into the same connectors, then you should use the
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151 | <quote>by-path</quote> mode.</para>
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152 |
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153 | <para>If either type of change is possible with your drive, then choose a
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154 | mode based on the type of change you expect to happen more often.</para>
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155 |
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156 | <!-- If you use by-id mode, the symlinks will survive even the transition
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157 | to libata for IDE drives, but that is not for the book. -->
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158 |
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159 | <important><para>External devices (for example, a USB-connected CD drive)
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160 | should not use by-path persistence, because each time the device is plugged
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161 | into a new external port, its physical path will change. All
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162 | externally-connected devices will have this problem if you write udev rules
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163 | to recognize them by their physical path; the problem is not limited to CD
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164 | and DVD drives.</para></important>
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165 |
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166 | <para>If you wish to see the values that the udev scripts will use, then
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167 | for the appropriate CD-ROM device, find the corresponding directory under
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168 | <filename class="directory">/sys</filename> (e.g., this can be
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169 | <filename class="directory">/sys/block/hdd</filename>) and
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170 | run a command similar to the following:</para>
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171 |
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172 | <screen role="nodump"><userinput>udevadm test /sys/block/hdd</userinput></screen>
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173 |
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174 | <para>Look at the lines containing the output of various *_id programs.
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175 | The <quote>by-id</quote> mode will use the ID_SERIAL value if it exists and
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176 | is not empty, otherwise it will use a combination of ID_MODEL and
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177 | ID_REVISION. The <quote>by-path</quote> mode will use the ID_PATH value.</para>
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178 |
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179 | <para>If the default mode is not suitable for your situation, then the
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180 | following modification can be made to the
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181 | <filename>/etc/udev/rules.d/83-cdrom-symlinks.rules</filename> file,
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182 | as follows (where <replaceable>mode</replaceable> is one of
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183 | <quote>by-id</quote> or <quote>by-path</quote>):</para>
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184 |
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185 | <screen role="nodump"><userinput>sed -e 's/"write_cd_rules"/"write_cd_rules <replaceable>mode</replaceable>"/' \
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186 | -i /etc/udev/rules.d/83-cdrom-symlinks.rules</userinput></screen>
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187 |
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188 | <para>Note that it is not necessary to create the rules files or symlinks
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189 | at this time because you have bind-mounted the host's
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190 | <filename class="directory">/dev</filename> directory into the LFS system
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191 | and we assume the symlinks exist on the host. The rules and symlinks will
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192 | be created the first time you boot your LFS system.</para>
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193 |
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194 | <para>However, if you have multiple CD-ROM devices, then the symlinks
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195 | generated at that time may point to different devices than they point to on
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196 | your host because devices are not discovered in a predictable order. The
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197 | assignments created when you first boot the LFS system will be stable, so
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198 | this is only an issue if you need the symlinks on both systems to point to
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199 | the same device. If you need that, then inspect (and possibly edit) the
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200 | generated <filename>/etc/udev/rules.d/70-persistent-cd.rules</filename>
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201 | file after booting, to make sure the assigned symlinks match what you need.</para>
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202 |
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203 | </sect2>
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204 |
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205 | <sect2>
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206 |
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207 | <title>Dealing with duplicate devices</title>
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208 |
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209 | <para>As explained in <xref linkend="ch-config-udev"/>, the order in
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210 | which devices with the same function appear in
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211 | <filename class="directory">/dev</filename> is essentially random.
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212 | E.g., if you have a USB web camera and a TV tuner, sometimes
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213 | <filename>/dev/video0</filename> refers to the camera and
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214 | <filename>/dev/video1</filename> refers to the tuner, and sometimes
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215 | after a reboot the order changes.
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216 | For all classes of hardware except sound cards and network cards, this is
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217 | fixable by creating udev rules for custom persistent symlinks.
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218 | The case of network cards is covered separately in
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219 | <xref linkend="ch-config-network"/>, and sound card configuration can
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220 | be found in <ulink url="&blfs-book;postlfs/devices.html">BLFS</ulink>.</para>
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221 |
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222 | <para>For each of your devices that is likely to have this problem
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223 | (even if the problem doesn't exist in your current Linux distribution),
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224 | find the corresponding directory under
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225 | <filename class="directory">/sys/class</filename> or
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226 | <filename class="directory">/sys/block</filename>.
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227 | For video devices, this may be
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228 | <filename
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229 | class="directory">/sys/class/video4linux/video<replaceable>X</replaceable></filename>.
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230 | Figure out the attributes that identify the device uniquely (usually,
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231 | vendor and product IDs and/or serial numbers work):</para>
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232 |
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233 | <screen role="nodump"><userinput>udevadm info -a -p /sys/class/video4linux/video0</userinput></screen>
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234 |
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235 | <para>Then write rules that create the symlinks, e.g.:</para>
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236 |
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237 | <screen role="nodump"><userinput>cat > /etc/udev/rules.d/83-duplicate_devs.rules << "EOF"
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238 | <literal>
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239 | # Persistent symlinks for webcam and tuner
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240 | KERNEL=="video*", ATTRS{idProduct}=="1910", ATTRS{idVendor}=="0d81", SYMLINK+="webcam"
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241 | KERNEL=="video*", ATTRS{device}=="0x036f", ATTRS{vendor}=="0x109e", SYMLINK+="tvtuner"
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242 | </literal>
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243 | EOF</userinput></screen>
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244 |
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245 | <para>The result is that <filename>/dev/video0</filename> and
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246 | <filename>/dev/video1</filename> devices still refer randomly to the tuner
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247 | and the web camera (and thus should never be used directly), but there are
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248 | symlinks <filename>/dev/tvtuner</filename> and
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249 | <filename>/dev/webcam</filename> that always point to the correct
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250 | device.</para>
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251 |
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252 | </sect2>
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253 |
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254 | </sect1>
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