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255 lines
13 KiB
XML
255 lines
13 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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<!ENTITY % general-entities SYSTEM "../general.ent">
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%general-entities;
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]>
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<sect1 id="ch-config-symlinks">
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<?dbhtml filename="symlinks.html"?>
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<title>Managing Devices</title>
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<sect2 revision="sysv">
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<title>Network Devices</title>
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<para>Udev, by default, names network devices according to Firmware/BIOS
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data or physical characteristics like the bus, slot, or MAC address. The
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purpose of this naming convention is to ensure that network devices are
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named consistently and not based on the time the network card was
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discovered. For example, on a computer having two network cards made by
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Intel and Realtek, the network card manufactured by Intel may become eth0
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and the Realtek card becomes eth1. In some cases, after a reboot the cards
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could get renumbered the other way around.</para>
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<para>In the new naming scheme, typical network device names would then
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be something like enp5s0 or wlp3s0. If this naming convention is not
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desired, the traditional naming scheme or a custom scheme can be
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implemented.</para>
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<sect3>
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<title>Disabling Persistent Naming on the Kernel Command Line</title>
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<para>The traditional naming scheme using eth0, eth1, etc can be
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restored by adding <userinput>net.ifnames=0</userinput> on the
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kernel command line. This is most appropriate for those systems
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that have only one ethernet device of the same type. Laptops
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often have multiple ethernet connections that are named eth0 and
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wlan0 and are also candidates for this method. The command line
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is passed in the GRUB configuration file.
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See <xref linkend="grub-cfg"/>.</para>
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</sect3>
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<sect3>
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<title>Creating Custom Udev Rules</title>
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<para>The naming scheme can be customized by creating custom udev
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rules. A script has been included that generates the initial rules.
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Generate these rules by running:</para>
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<screen role="install"><userinput>bash /usr/lib/udev/init-net-rules.sh</userinput></screen>
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<para> Now, inspect the
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<filename>/etc/udev/rules.d/70-persistent-net.rules</filename> file, to
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find out which name was assigned to which network device:</para>
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<screen role="nodump"><userinput>cat /etc/udev/rules.d/70-persistent-net.rules</userinput></screen>
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<note><para>In some cases such as when MAC addresses have been assigned to
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a network card manually or in a virtual environment such as Qemu or Xen,
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the network rules file may not have been generated because addresses
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are not consistently assigned. In these cases, this method cannot
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be used.</para></note>
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<para>The file begins with a comment block followed by two lines for each
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NIC. The first line for each NIC is a commented description showing its
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hardware IDs (e.g. its PCI vendor and device IDs, if it's a PCI card),
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along with its driver in parentheses, if the driver can be found. Neither
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the hardware ID nor the driver is used to determine which name to give an
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interface; this information is only for reference. The second line is the
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udev rule that matches this NIC and actually assigns it a name.</para>
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<para>All udev rules are made up of several keys, separated by commas and
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optional whitespace. This rule's keys and an explanation of each of them
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are as follows:</para>
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<itemizedlist>
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<listitem>
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<para><literal>SUBSYSTEM=="net"</literal> - This tells udev to ignore
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devices that are not network cards.</para>
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</listitem>
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<listitem>
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<para><literal>ACTION=="add"</literal> - This tells udev to ignore this
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rule for a uevent that isn't an add ("remove" and "change" uevents also
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happen, but don't need to rename network interfaces).</para>
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</listitem>
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<listitem>
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<para><literal>DRIVERS=="?*"</literal> - This exists so that udev will
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ignore VLAN or bridge sub-interfaces (because these sub-interfaces do
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not have drivers). These sub-interfaces are skipped because the name
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that would be assigned would collide with their parent devices.</para>
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</listitem>
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<listitem>
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<para><literal>ATTR{address}</literal> - The value of this key is the
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NIC's MAC address.</para>
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</listitem>
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<listitem>
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<para><literal>ATTR{type}=="1"</literal> - This ensures the rule only
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matches the primary interface in the case of certain wireless drivers
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which create multiple virtual interfaces. The secondary interfaces are
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skipped for the same reason that VLAN and bridge sub-interfaces are
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skipped: there would be a name collision otherwise.</para>
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</listitem>
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<listitem>
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<para><literal>NAME</literal> - The value of this key is the name that
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udev will assign to this interface.</para>
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</listitem>
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</itemizedlist>
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<para>The value of <literal>NAME</literal> is the important part. Make sure
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you know which name has been assigned to each of your network cards before
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proceeding, and be sure to use that <literal>NAME</literal> value when
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creating your configuration files below.</para>
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</sect3>
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</sect2>
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<sect2 revision="sysv">
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<title>CD-ROM symlinks</title>
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<para>Some software that you may want to install later (e.g., various
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media players) expect the <filename class="symlink">/dev/cdrom</filename>
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and <filename class="symlink">/dev/dvd</filename> symlinks to exist, and
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to point to a CD-ROM or DVD-ROM device. Also, it may be convenient to put
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references to those symlinks into <filename>/etc/fstab</filename>. Udev
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comes with a script that will generate rules files to create these symlinks
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for you, depending on the capabilities of each device, but you need to
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decide which of two modes of operation you wish to have the script use.</para>
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<para>First, the script can operate in <quote>by-path</quote> mode (used by
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default for USB and FireWire devices), where the rules it creates depend on
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the physical path to the CD or DVD device. Second, it can operate in
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<quote>by-id</quote> mode (default for IDE and SCSI devices), where the
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rules it creates depend on identification strings stored on the CD or DVD
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device itself. The path is determined by udev's <command>path_id</command>
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script, and the identification strings are read from the hardware by its
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<command>ata_id</command> or <command>scsi_id</command> programs, depending
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on which type of device you have.</para>
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<para>There are advantages to each approach; the correct approach to use
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will depend on what kinds of device changes may happen. If you expect the
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physical path to the device (that is, the ports and/or slots that it plugs
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into) to change, for example because you plan on moving the drive to a
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different IDE port or a different USB connector, then you should use the
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<quote>by-id</quote> mode. On the other hand, if you expect the device's
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identification to change, for example because it may die, and you would
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replace it with a different device with the same capabilities and which
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is plugged into the same connectors, then you should use the
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<quote>by-path</quote> mode.</para>
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<para>If either type of change is possible with your drive, then choose a
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mode based on the type of change you expect to happen more often.</para>
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<!-- If you use by-id mode, the symlinks will survive even the transition
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to libata for IDE drives, but that is not for the book. -->
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<important><para>External devices (for example, a USB-connected CD drive)
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should not use by-path persistence, because each time the device is plugged
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into a new external port, its physical path will change. All
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externally-connected devices will have this problem if you write udev rules
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to recognize them by their physical path; the problem is not limited to CD
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and DVD drives.</para></important>
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<para>If you wish to see the values that the udev scripts will use, then
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for the appropriate CD-ROM device, find the corresponding directory under
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<filename class="directory">/sys</filename> (e.g., this can be
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<filename class="directory">/sys/block/hdd</filename>) and
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run a command similar to the following:</para>
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<screen role="nodump"><userinput>udevadm test /sys/block/hdd</userinput></screen>
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<para>Look at the lines containing the output of various *_id programs.
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The <quote>by-id</quote> mode will use the ID_SERIAL value if it exists and
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is not empty, otherwise it will use a combination of ID_MODEL and
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ID_REVISION. The <quote>by-path</quote> mode will use the ID_PATH value.</para>
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<para>If the default mode is not suitable for your situation, then the
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following modification can be made to the
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<filename>/etc/udev/rules.d/83-cdrom-symlinks.rules</filename> file,
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as follows (where <replaceable>mode</replaceable> is one of
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<quote>by-id</quote> or <quote>by-path</quote>):</para>
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<screen role="nodump"><userinput>sed -e 's/"write_cd_rules"/"write_cd_rules <replaceable>mode</replaceable>"/' \
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-i /etc/udev/rules.d/83-cdrom-symlinks.rules</userinput></screen>
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<para>Note that it is not necessary to create the rules files or symlinks
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at this time because you have bind-mounted the host's
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<filename class="directory">/dev</filename> directory into the LFS system
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and we assume the symlinks exist on the host. The rules and symlinks will
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be created the first time you boot your LFS system.</para>
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<para>However, if you have multiple CD-ROM devices, then the symlinks
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generated at that time may point to different devices than they point to on
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your host because devices are not discovered in a predictable order. The
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assignments created when you first boot the LFS system will be stable, so
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this is only an issue if you need the symlinks on both systems to point to
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the same device. If you need that, then inspect (and possibly edit) the
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generated <filename>/etc/udev/rules.d/70-persistent-cd.rules</filename>
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file after booting, to make sure the assigned symlinks match what you need.</para>
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</sect2>
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<sect2>
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<title>Dealing with duplicate devices</title>
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<para>As explained in <xref linkend="ch-config-udev"/>, the order in
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which devices with the same function appear in
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<filename class="directory">/dev</filename> is essentially random.
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E.g., if you have a USB web camera and a TV tuner, sometimes
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<filename>/dev/video0</filename> refers to the camera and
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<filename>/dev/video1</filename> refers to the tuner, and sometimes
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after a reboot the order changes.
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For all classes of hardware except sound cards and network cards, this is
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fixable by creating udev rules for custom persistent symlinks.
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The case of network cards is covered separately in
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<xref linkend="ch-config-network"/>, and sound card configuration can
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be found in <ulink url="&blfs-book;postlfs/devices.html">BLFS</ulink>.</para>
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<para>For each of your devices that is likely to have this problem
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(even if the problem doesn't exist in your current Linux distribution),
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find the corresponding directory under
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<filename class="directory">/sys/class</filename> or
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<filename class="directory">/sys/block</filename>.
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For video devices, this may be
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<filename
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class="directory">/sys/class/video4linux/video<replaceable>X</replaceable></filename>.
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Figure out the attributes that identify the device uniquely (usually,
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vendor and product IDs and/or serial numbers work):</para>
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<screen role="nodump"><userinput>udevadm info -a -p /sys/class/video4linux/video0</userinput></screen>
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<para>Then write rules that create the symlinks, e.g.:</para>
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<screen role="nodump"><userinput>cat > /etc/udev/rules.d/83-duplicate_devs.rules << "EOF"
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<literal>
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# Persistent symlinks for webcam and tuner
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KERNEL=="video*", ATTRS{idProduct}=="1910", ATTRS{idVendor}=="0d81", SYMLINK+="webcam"
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KERNEL=="video*", ATTRS{device}=="0x036f", ATTRS{vendor}=="0x109e", SYMLINK+="tvtuner"
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</literal>
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EOF</userinput></screen>
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<para>The result is that <filename>/dev/video0</filename> and
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<filename>/dev/video1</filename> devices still refer randomly to the tuner
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and the web camera (and thus should never be used directly), but there are
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symlinks <filename>/dev/tvtuner</filename> and
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<filename>/dev/webcam</filename> that always point to the correct
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device.</para>
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</sect2>
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</sect1>
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