[7152faa] | 1 | <?xml version="1.0" encoding="UTF-8"?>
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[b06ca36] | 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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[966b175] | 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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[afcfd74] | 8 | <sect1 id="ch-config-symlinks">
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[966b175] | 9 | <?dbhtml filename="symlinks.html"?>
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| 10 |
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[bf58c1e] | 11 | <title>Managing Devices</title>
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| 12 |
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[efd5d1f] | 13 | <sect2 revision="sysv">
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[bf58c1e] | 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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[8972a36] | 20 | named consistently, not based on when the network card was
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| 21 | discovered. In older versions of Linux—on a computer with two
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| 22 | network cards made by Intel and Realtek, for instance—the
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| 23 | network card manufactured by Intel might have become eth0
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| 24 | while the Realtek card became eth1. After a reboot, the cards
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| 25 | would sometimes get renumbered the other way around.</para>
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| 26 |
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| 27 | <para>In the new naming scheme, typical network device names are
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| 28 | something like enp5s0 or wlp3s0. If this naming convention is not
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| 29 | desired, the traditional naming scheme, or a custom scheme, can be
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[bf58c1e] | 30 | implemented.</para>
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| 31 |
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| 32 | <sect3>
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| 33 | <title>Disabling Persistent Naming on the Kernel Command Line</title>
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[c34b4fb] | 34 |
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[8972a36] | 35 | <para>The traditional naming scheme using eth0, eth1, etc. can be
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[c34b4fb] | 36 | restored by adding <userinput>net.ifnames=0</userinput> on the
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[8972a36] | 37 | kernel command line. This is most appropriate for systems
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| 38 | that have just one ethernet device of a particular type. Laptops
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| 39 | often have two ethernet connections named eth0 and
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[c75f261] | 40 | wlan0; such laptops can also use this method.</para>
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[bf58c1e] | 41 | </sect3>
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| 42 |
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| 43 | <sect3>
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| 44 | <title>Creating Custom Udev Rules</title>
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[c34b4fb] | 45 |
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[a3d0817] | 46 | <para>The naming scheme can be customized by creating custom udev
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[bf58c1e] | 47 | rules. A script has been included that generates the initial rules.
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| 48 | Generate these rules by running:</para>
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| 49 |
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[d7a9421] | 50 | <screen role="install"><userinput>bash /usr/lib/udev/init-net-rules.sh</userinput></screen>
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[bf58c1e] | 51 |
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[8aa7fde] | 52 | <para> Now, inspect the
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[bf58c1e] | 53 | <filename>/etc/udev/rules.d/70-persistent-net.rules</filename> file, to
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| 54 | find out which name was assigned to which network device:</para>
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| 55 |
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| 56 | <screen role="nodump"><userinput>cat /etc/udev/rules.d/70-persistent-net.rules</userinput></screen>
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| 57 |
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[8972a36] | 58 | <note><para>In some cases, such as when MAC addresses have been assigned to
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| 59 | a network card manually, or in a virtual environment such as Qemu or Xen,
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| 60 | the network rules file may not be generated because addresses
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[bf58c1e] | 61 | are not consistently assigned. In these cases, this method cannot
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| 62 | be used.</para></note>
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[c34b4fb] | 63 |
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[8972a36] | 64 | <para>The file begins with a comment block, followed by two lines for each
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[bf58c1e] | 65 | NIC. The first line for each NIC is a commented description showing its
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| 66 | hardware IDs (e.g. its PCI vendor and device IDs, if it's a PCI card),
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[8972a36] | 67 | along with its driver (in parentheses, if the driver can be found). Neither
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[bf58c1e] | 68 | the hardware ID nor the driver is used to determine which name to give an
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| 69 | interface; this information is only for reference. The second line is the
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[a3d0817] | 70 | udev rule that matches this NIC and actually assigns it a name.</para>
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[c34b4fb] | 71 |
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[8972a36] | 72 | <para>All udev rules are made up of several keywords, separated by commas and
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| 73 | optional whitespace. Here are the keywords, and an explanation of each one:</para>
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[c34b4fb] | 74 |
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[bf58c1e] | 75 | <itemizedlist>
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| 76 | <listitem>
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[a3d0817] | 77 | <para><literal>SUBSYSTEM=="net"</literal> - This tells udev to ignore
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[bf58c1e] | 78 | devices that are not network cards.</para>
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| 79 | </listitem>
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| 80 | <listitem>
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[a3d0817] | 81 | <para><literal>ACTION=="add"</literal> - This tells udev to ignore this
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[bf58c1e] | 82 | rule for a uevent that isn't an add ("remove" and "change" uevents also
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| 83 | happen, but don't need to rename network interfaces).</para>
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| 84 | </listitem>
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| 85 | <listitem>
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[a3d0817] | 86 | <para><literal>DRIVERS=="?*"</literal> - This exists so that udev will
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[bf58c1e] | 87 | ignore VLAN or bridge sub-interfaces (because these sub-interfaces do
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| 88 | not have drivers). These sub-interfaces are skipped because the name
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[8972a36] | 89 | that would be assigned would collide with the parent devices.</para>
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[bf58c1e] | 90 | </listitem>
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| 91 | <listitem>
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[8972a36] | 92 | <para><literal>ATTR{address}</literal> - The value of this keyword is the
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[bf58c1e] | 93 | NIC's MAC address.</para>
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| 94 | </listitem>
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| 95 | <listitem>
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| 96 | <para><literal>ATTR{type}=="1"</literal> - This ensures the rule only
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[a3d0817] | 97 | matches the primary interface in the case of certain wireless drivers
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[bf58c1e] | 98 | which create multiple virtual interfaces. The secondary interfaces are
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| 99 | skipped for the same reason that VLAN and bridge sub-interfaces are
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| 100 | skipped: there would be a name collision otherwise.</para>
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| 101 | </listitem>
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| 102 | <listitem>
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[8972a36] | 103 | <para><literal>NAME</literal> - The value of this keyword is the name that
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[a3d0817] | 104 | udev will assign to this interface.</para>
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[bf58c1e] | 105 | </listitem>
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| 106 | </itemizedlist>
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[c34b4fb] | 107 |
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[bf58c1e] | 108 | <para>The value of <literal>NAME</literal> is the important part. Make sure
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| 109 | you know which name has been assigned to each of your network cards before
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| 110 | proceeding, and be sure to use that <literal>NAME</literal> value when
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[8972a36] | 111 | creating your network configuration files.</para>
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[bf58c1e] | 112 |
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[3608380f] | 113 | <para>Even if the custom udev rule file is created, udev may still
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| 114 | assign one or more alternative names for a NIC based on physical
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[eea548f] | 115 | characteristics. If a custom udev rule would rename some NIC using
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[3608380f] | 116 | a name already assigned as an alternative name of another NIC, this
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| 117 | udev rule will fail. If this issue happens, you may create the
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| 118 | <filename>/etc/udev/network/99-default.link</filename> configuration
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| 119 | file with an empty alternative assignment policy, overriding the
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| 120 | default configuration file
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| 121 | <filename>/usr/lib/udev/network/99-default.link</filename>:</para>
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| 122 |
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| 123 | <screen role="nodump"><userinput>sed -e '/^AlternativeNamesPolicy/s/=.*$/=/' \
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| 124 | -i /usr/lib/udev/network/99-default.link \
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| 125 | > /etc/udev/network/99-default.link</userinput></screen>
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[bf58c1e] | 126 | </sect3>
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| 127 |
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| 128 | </sect2>
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[966b175] | 129 |
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[efd5d1f] | 130 | <sect2 revision="sysv">
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[966b175] | 131 |
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[8972a36] | 132 | <title>CD-ROM Symlinks</title>
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[966b175] | 133 |
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| 134 | <para>Some software that you may want to install later (e.g., various
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[8972a36] | 135 | media players) expects the <filename class="symlink">/dev/cdrom</filename>
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[84dbfdac] | 136 | and <filename class="symlink">/dev/dvd</filename> symlinks to exist, and
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| 137 | to point to a CD-ROM or DVD-ROM device. Also, it may be convenient to put
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| 138 | references to those symlinks into <filename>/etc/fstab</filename>. Udev
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| 139 | comes with a script that will generate rules files to create these symlinks
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| 140 | for you, depending on the capabilities of each device, but you need to
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| 141 | decide which of two modes of operation you wish to have the script use.</para>
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| 142 |
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[8536c02] | 143 | <para>First, the script can operate in <quote>by-path</quote> mode (used by
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| 144 | default for USB and FireWire devices), where the rules it creates depend on
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| 145 | the physical path to the CD or DVD device. Second, it can operate in
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| 146 | <quote>by-id</quote> mode (default for IDE and SCSI devices), where the
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[a3d0817] | 147 | rules it creates depend on identification strings stored on the CD or DVD
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| 148 | device itself. The path is determined by udev's <command>path_id</command>
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[8536c02] | 149 | script, and the identification strings are read from the hardware by its
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[84dbfdac] | 150 | <command>ata_id</command> or <command>scsi_id</command> programs, depending
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| 151 | on which type of device you have.</para>
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| 152 |
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[8972a36] | 153 | <para>There are advantages to each approach; the correct approach
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| 154 | depends on what kinds of device changes may happen. If you expect the
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[84dbfdac] | 155 | physical path to the device (that is, the ports and/or slots that it plugs
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| 156 | into) to change, for example because you plan on moving the drive to a
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| 157 | different IDE port or a different USB connector, then you should use the
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| 158 | <quote>by-id</quote> mode. On the other hand, if you expect the device's
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[8972a36] | 159 | identification to change, for example because it may die, and you intend
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| 160 | to replace it with a different device that
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| 161 | plugs into the same connectors, then you should use the
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[84dbfdac] | 162 | <quote>by-path</quote> mode.</para>
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| 163 |
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| 164 | <para>If either type of change is possible with your drive, then choose a
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| 165 | mode based on the type of change you expect to happen more often.</para>
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| 166 |
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| 167 | <!-- If you use by-id mode, the symlinks will survive even the transition
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| 168 | to libata for IDE drives, but that is not for the book. -->
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| 169 |
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[80640a49] | 170 | <important><para>External devices (for example, a USB-connected CD drive)
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| 171 | should not use by-path persistence, because each time the device is plugged
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| 172 | into a new external port, its physical path will change. All
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[a3d0817] | 173 | externally-connected devices will have this problem if you write udev rules
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[80640a49] | 174 | to recognize them by their physical path; the problem is not limited to CD
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| 175 | and DVD drives.</para></important>
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[84dbfdac] | 176 |
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[a3d0817] | 177 | <para>If you wish to see the values that the udev scripts will use, then
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[84dbfdac] | 178 | for the appropriate CD-ROM device, find the corresponding directory under
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[966b175] | 179 | <filename class="directory">/sys</filename> (e.g., this can be
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| 180 | <filename class="directory">/sys/block/hdd</filename>) and
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| 181 | run a command similar to the following:</para>
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| 182 |
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[61e63d3] | 183 | <screen role="nodump"><userinput>udevadm test /sys/block/hdd</userinput></screen>
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[84dbfdac] | 184 |
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| 185 | <para>Look at the lines containing the output of various *_id programs.
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| 186 | The <quote>by-id</quote> mode will use the ID_SERIAL value if it exists and
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| 187 | is not empty, otherwise it will use a combination of ID_MODEL and
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| 188 | ID_REVISION. The <quote>by-path</quote> mode will use the ID_PATH value.</para>
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| 189 |
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[8536c02] | 190 | <para>If the default mode is not suitable for your situation, then the
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| 191 | following modification can be made to the
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[0a28f5f4] | 192 | <filename>/etc/udev/rules.d/83-cdrom-symlinks.rules</filename> file,
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[8536c02] | 193 | as follows (where <replaceable>mode</replaceable> is one of
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| 194 | <quote>by-id</quote> or <quote>by-path</quote>):</para>
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[84dbfdac] | 195 |
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[a3d0817] | 196 | <screen role="nodump"><userinput>sed -e 's/"write_cd_rules"/"write_cd_rules <replaceable>mode</replaceable>"/' \
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| 197 | -i /etc/udev/rules.d/83-cdrom-symlinks.rules</userinput></screen>
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[966b175] | 198 |
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[84dbfdac] | 199 | <para>Note that it is not necessary to create the rules files or symlinks
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[a3d0817] | 200 | at this time because you have bind-mounted the host's
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| 201 | <filename class="directory">/dev</filename> directory into the LFS system
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[651719a] | 202 | and we assume the symlinks exist on the host. The rules and symlinks will
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| 203 | be created the first time you boot your LFS system.</para>
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[a3b689f] | 204 |
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[651719a] | 205 | <para>However, if you have multiple CD-ROM devices, then the symlinks
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| 206 | generated at that time may point to different devices than they point to on
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[a3d0817] | 207 | your host because devices are not discovered in a predictable order. The
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[651719a] | 208 | assignments created when you first boot the LFS system will be stable, so
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| 209 | this is only an issue if you need the symlinks on both systems to point to
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| 210 | the same device. If you need that, then inspect (and possibly edit) the
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| 211 | generated <filename>/etc/udev/rules.d/70-persistent-cd.rules</filename>
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[8972a36] | 212 | file after booting, to make sure the assigned symlinks match your needs.</para>
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[966b175] | 213 |
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| 214 | </sect2>
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| 215 |
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| 216 | <sect2>
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| 217 |
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[8972a36] | 218 | <title>Dealing with Duplicate Devices</title>
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[966b175] | 219 |
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[afcfd74] | 220 | <para>As explained in <xref linkend="ch-config-udev"/>, the order in
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[966b175] | 221 | which devices with the same function appear in
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| 222 | <filename class="directory">/dev</filename> is essentially random.
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| 223 | E.g., if you have a USB web camera and a TV tuner, sometimes
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| 224 | <filename>/dev/video0</filename> refers to the camera and
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| 225 | <filename>/dev/video1</filename> refers to the tuner, and sometimes
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[a3d0817] | 226 | after a reboot the order changes.
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[966b175] | 227 | For all classes of hardware except sound cards and network cards, this is
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[8972a36] | 228 | fixable by creating udev rules to create persistent symlinks.
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[966b175] | 229 | The case of network cards is covered separately in
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[afcfd74] | 230 | <xref linkend="ch-config-network"/>, and sound card configuration can
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[0ee07e5] | 231 | be found in <ulink url="&blfs-book;postlfs/devices.html">BLFS</ulink>.</para>
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[966b175] | 232 |
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| 233 | <para>For each of your devices that is likely to have this problem
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| 234 | (even if the problem doesn't exist in your current Linux distribution),
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| 235 | find the corresponding directory under
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| 236 | <filename class="directory">/sys/class</filename> or
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| 237 | <filename class="directory">/sys/block</filename>.
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| 238 | For video devices, this may be
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| 239 | <filename
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| 240 | class="directory">/sys/class/video4linux/video<replaceable>X</replaceable></filename>.
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| 241 | Figure out the attributes that identify the device uniquely (usually,
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| 242 | vendor and product IDs and/or serial numbers work):</para>
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| 243 |
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[61e63d3] | 244 | <screen role="nodump"><userinput>udevadm info -a -p /sys/class/video4linux/video0</userinput></screen>
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[966b175] | 245 |
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| 246 | <para>Then write rules that create the symlinks, e.g.:</para>
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| 247 |
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[8857ace2] | 248 | <screen role="nodump"><userinput>cat > /etc/udev/rules.d/83-duplicate_devs.rules << "EOF"
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[966b175] | 249 | <literal>
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| 250 | # Persistent symlinks for webcam and tuner
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[a3d0817] | 251 | KERNEL=="video*", ATTRS{idProduct}=="1910", ATTRS{idVendor}=="0d81", SYMLINK+="webcam"
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| 252 | KERNEL=="video*", ATTRS{device}=="0x036f", ATTRS{vendor}=="0x109e", SYMLINK+="tvtuner"
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[966b175] | 253 | </literal>
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| 254 | EOF</userinput></screen>
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| 255 |
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| 256 | <para>The result is that <filename>/dev/video0</filename> and
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| 257 | <filename>/dev/video1</filename> devices still refer randomly to the tuner
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| 258 | and the web camera (and thus should never be used directly), but there are
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| 259 | symlinks <filename>/dev/tvtuner</filename> and
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| 260 | <filename>/dev/webcam</filename> that always point to the correct
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| 261 | device.</para>
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| 262 |
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| 263 | </sect2>
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| 264 |
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| 265 | </sect1>
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