source: chapter09/symlinks.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
  <!ENTITY % general-entities SYSTEM "../general.ent">
  %general-entities;
]>

<sect1 id="ch-config-symlinks">
  <?dbhtml filename="symlinks.html"?>

  <title>Managing Devices</title>

  <sect2 revision="sysv">

    <title>Network Devices</title>

    <para>Udev, by default, names network devices according to Firmware/BIOS
    data or physical characteristics like the bus, slot, or MAC address.  The
    purpose of this naming convention is to ensure that network devices are
    named consistently, not based on when the network card was
    discovered.  In older versions of Linux&mdash;on a computer with two 
    network cards made by Intel and Realtek, for instance&mdash;the
    network card manufactured by Intel might have become eth0
    while the Realtek card became eth1. After a reboot, the cards
    would sometimes get renumbered the other way around.</para>

    <para>In the new naming scheme, typical network device names are
    something like enp5s0 or wlp3s0.  If this naming convention is not
    desired, the traditional naming scheme, or a custom scheme, can be
    implemented.</para>

    <sect3>
      <title>Disabling Persistent Naming on the Kernel Command Line</title>

      <para>The traditional naming scheme using eth0, eth1, etc. can be
      restored by adding <userinput>net.ifnames=0</userinput> on the
      kernel command line.  This is most appropriate for systems
      that have just one ethernet device of a particular type.  Laptops
      often have two ethernet connections named eth0 and
      wlan0; such laptops can also use this method.  The command line
      is in the GRUB configuration file.
      See <xref linkend="grub-cfg"/>.</para>
    </sect3>

    <sect3>
      <title>Creating Custom Udev Rules</title>

      <para>The naming scheme can be customized by creating custom udev
      rules.  A script has been included that generates the initial rules.
      Generate these rules by running:</para>

<screen role="install"><userinput>bash /usr/lib/udev/init-net-rules.sh</userinput></screen>

      <para> Now, inspect the
      <filename>/etc/udev/rules.d/70-persistent-net.rules</filename> file, to
      find out which name was assigned to which network device:</para>

<screen role="nodump"><userinput>cat /etc/udev/rules.d/70-persistent-net.rules</userinput></screen>

      <note><para>In some cases, such as when MAC addresses have been assigned to
      a network card manually, or in a virtual environment such as Qemu or Xen,
      the network rules file may not be generated because addresses
      are not consistently assigned.  In these cases, this method cannot
      be used.</para></note>

      <para>The file begins with a comment block, followed by two lines for each
      NIC. The first line for each NIC is a commented description showing its
      hardware IDs (e.g. its PCI vendor and device IDs, if it's a PCI card),
      along with its driver (in parentheses, if the driver can be found). Neither
      the hardware ID nor the driver is used to determine which name to give an
      interface; this information is only for reference. The second line is the
      udev rule that matches this NIC and actually assigns it a name.</para>

      <para>All udev rules are made up of several keywords, separated by commas and
      optional whitespace. Here are the keywords, and an explanation of each one:</para>

      <itemizedlist>
        <listitem>
          <para><literal>SUBSYSTEM=="net"</literal> - This tells udev to ignore
          devices that are not network cards.</para>
        </listitem>
        <listitem>
          <para><literal>ACTION=="add"</literal> - This tells udev to ignore this
          rule for a uevent that isn't an add ("remove" and "change" uevents also
          happen, but don't need to rename network interfaces).</para>
        </listitem>
        <listitem>
          <para><literal>DRIVERS=="?*"</literal> - This exists so that udev will
          ignore VLAN or bridge sub-interfaces (because these sub-interfaces do
          not have drivers). These sub-interfaces are skipped because the name
          that would be assigned would collide with the parent devices.</para>
        </listitem>
        <listitem>
          <para><literal>ATTR{address}</literal> - The value of this keyword is the
          NIC's MAC address.</para>
        </listitem>
        <listitem>
          <para><literal>ATTR{type}=="1"</literal> - This ensures the rule only
          matches the primary interface in the case of certain wireless drivers
          which create multiple virtual interfaces. The secondary interfaces are
          skipped for the same reason that VLAN and bridge sub-interfaces are
          skipped: there would be a name collision otherwise.</para>
        </listitem>
        <listitem>
          <para><literal>NAME</literal> - The value of this keyword is the name that
          udev will assign to this interface.</para>
        </listitem>
      </itemizedlist>

      <para>The value of <literal>NAME</literal> is the important part. Make sure
      you know which name has been assigned to each of your network cards before
      proceeding, and be sure to use that <literal>NAME</literal> value when
      creating your network configuration files.</para>

      <para>Even if the custom udev rule file is created, udev may still
      assign one or more alternative names for a NIC based on physical
      characteristics.  If a custom udev rule would rename some NIC using
      a name already assigned as an alternative name of another NIC, this
      udev rule will fail.  If this issue happens, you may create the
      <filename>/etc/udev/network/99-default.link</filename> configuration
      file with an empty alternative assignment policy, overriding the
      default configuration file
      <filename>/usr/lib/udev/network/99-default.link</filename>:</para>

<screen role="nodump"><userinput>sed -e '/^AlternativeNamesPolicy/s/=.*$/=/'  \
    -i /usr/lib/udev/network/99-default.link \
     > /etc/udev/network/99-default.link</userinput></screen>
    </sect3>

  </sect2>

  <sect2 revision="sysv">

    <title>CD-ROM Symlinks</title>

    <para>Some software that you may want to install later (e.g., various
    media players) expects the <filename class="symlink">/dev/cdrom</filename>
    and <filename class="symlink">/dev/dvd</filename> symlinks to exist, and
    to point to a CD-ROM or DVD-ROM device. Also, it may be convenient to put
    references to those symlinks into <filename>/etc/fstab</filename>. Udev
    comes with a script that will generate rules files to create these symlinks
    for you, depending on the capabilities of each device, but you need to
    decide which of two modes of operation you wish to have the script use.</para>

    <para>First, the script can operate in <quote>by-path</quote> mode (used by
    default for USB and FireWire devices), where the rules it creates depend on
    the physical path to the CD or DVD device. Second, it can operate in
    <quote>by-id</quote> mode (default for IDE and SCSI devices), where the
    rules it creates depend on identification strings stored on the CD or DVD
    device itself. The path is determined by udev's <command>path_id</command>
    script, and the identification strings are read from the hardware by its
    <command>ata_id</command> or <command>scsi_id</command> programs, depending
    on which type of device you have.</para>

    <para>There are advantages to each approach; the correct approach
    depends on what kinds of device changes may happen. If you expect the
    physical path to the device (that is, the ports and/or slots that it plugs
    into) to change, for example because you plan on moving the drive to a
    different IDE port or a different USB connector, then you should use the
    <quote>by-id</quote> mode. On the other hand, if you expect the device's
    identification to change, for example because it may die, and you intend
    to replace it with a different device that 
    plugs into the same connectors, then you should use the
    <quote>by-path</quote> mode.</para>

    <para>If either type of change is possible with your drive, then choose a
    mode based on the type of change you expect to happen more often.</para>

<!-- If you use by-id mode, the symlinks will survive even the transition
     to libata for IDE drives, but that is not for the book. -->

    <important><para>External devices (for example, a USB-connected CD drive)
    should not use by-path persistence, because each time the device is plugged
    into a new external port, its physical path will change. All
    externally-connected devices will have this problem if you write udev rules
    to recognize them by their physical path; the problem is not limited to CD
    and DVD drives.</para></important>

    <para>If you wish to see the values that the udev scripts will use, then
    for the appropriate CD-ROM device, find the corresponding directory under
    <filename class="directory">/sys</filename> (e.g., this can be
    <filename class="directory">/sys/block/hdd</filename>) and
    run a command similar to the following:</para>

<screen role="nodump"><userinput>udevadm test /sys/block/hdd</userinput></screen>

    <para>Look at the lines containing the output of various *_id programs.
    The <quote>by-id</quote> mode will use the ID_SERIAL value if it exists and
    is not empty, otherwise it will use a combination of ID_MODEL and
    ID_REVISION. The <quote>by-path</quote> mode will use the ID_PATH value.</para>

    <para>If the default mode is not suitable for your situation, then the
    following modification can be made to the
    <filename>/etc/udev/rules.d/83-cdrom-symlinks.rules</filename> file,
    as follows (where <replaceable>mode</replaceable> is one of
    <quote>by-id</quote> or <quote>by-path</quote>):</para>

<screen role="nodump"><userinput>sed -e 's/"write_cd_rules"/"write_cd_rules <replaceable>mode</replaceable>"/' \
    -i /etc/udev/rules.d/83-cdrom-symlinks.rules</userinput></screen>

    <para>Note that it is not necessary to create the rules files or symlinks
    at this time because you have bind-mounted the host's
    <filename class="directory">/dev</filename> directory into the LFS system
    and we assume the symlinks exist on the host. The rules and symlinks will
    be created the first time you boot your LFS system.</para>

    <para>However, if you have multiple CD-ROM devices, then the symlinks
    generated at that time may point to different devices than they point to on
    your host because devices are not discovered in a predictable order. The
    assignments created when you first boot the LFS system will be stable, so
    this is only an issue if you need the symlinks on both systems to point to
    the same device. If you need that, then inspect (and possibly edit) the
    generated <filename>/etc/udev/rules.d/70-persistent-cd.rules</filename>
    file after booting, to make sure the assigned symlinks match your needs.</para>

  </sect2>

  <sect2>

    <title>Dealing with Duplicate Devices</title>

    <para>As explained in <xref linkend="ch-config-udev"/>, the order in
    which devices with the same function appear in
    <filename class="directory">/dev</filename> is essentially random.
    E.g., if you have a USB web camera and a TV tuner, sometimes
    <filename>/dev/video0</filename> refers to the camera and
    <filename>/dev/video1</filename> refers to the tuner, and sometimes
    after a reboot the order changes.
    For all classes of hardware except sound cards and network cards, this is
    fixable by creating udev rules to create persistent symlinks.
    The case of network cards is covered separately in
    <xref linkend="ch-config-network"/>, and sound card configuration can
    be found in <ulink url="&blfs-book;postlfs/devices.html">BLFS</ulink>.</para>

    <para>For each of your devices that is likely to have this problem
    (even if the problem doesn't exist in your current Linux distribution),
    find the corresponding directory under
    <filename class="directory">/sys/class</filename> or
    <filename class="directory">/sys/block</filename>.
    For video devices, this may be
    <filename
    class="directory">/sys/class/video4linux/video<replaceable>X</replaceable></filename>.
    Figure out the attributes that identify the device uniquely (usually,
    vendor and product IDs and/or serial numbers work):</para>

<screen role="nodump"><userinput>udevadm info -a -p /sys/class/video4linux/video0</userinput></screen>

    <para>Then write rules that create the symlinks, e.g.:</para>

<screen role="nodump"><userinput>cat &gt; /etc/udev/rules.d/83-duplicate_devs.rules &lt;&lt; "EOF"
<literal>
# Persistent symlinks for webcam and tuner
KERNEL=="video*", ATTRS{idProduct}=="1910", ATTRS{idVendor}=="0d81", SYMLINK+="webcam"
KERNEL=="video*", ATTRS{device}=="0x036f",  ATTRS{vendor}=="0x109e", SYMLINK+="tvtuner"
</literal>
EOF</userinput></screen>

    <para>The result is that <filename>/dev/video0</filename> and
    <filename>/dev/video1</filename> devices still refer randomly to the tuner
    and the web camera (and thus should never be used directly), but there are
    symlinks <filename>/dev/tvtuner</filename> and
    <filename>/dev/webcam</filename> that always point to the correct
    device.</para>

 </sect2>

</sect1>