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- <?xml version="1.0" encoding="ISO-8859-1"?>
- <!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-scripts-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 and not based on the time the network card was
- discovered. For example, on a computer having two network cards made by
- Intel and Realtek, the network card manufactured by Intel may become eth0
- and the Realtek card becomes eth1. In some cases, after a reboot the cards
- get renumbered the other way around.</para>
-
- <para>In the new naming scheme, typical network device names would then
- be 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 those systems
- that have only one ethernet device of the same type. Laptops
- often have multiple ethernet connections that are named eth0 and
- wlan0 and are also candidates for this method. The command line
- is passed 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 /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 addresess have been assigned to
- a network card manually or in a virtual environment such as Qemu or Xen,
- the network rules file may not have been 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 keys, separated by commas and
- optional whitespace. This rule's keys and an explanation of each of them
- are as follows:</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 their parent devices.</para>
- </listitem>
- <listitem>
- <para><literal>ATTR{address}</literal> - The value of this key 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 key 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 configuration files below.</para>
- </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) expect 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 in 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 to use
- will depend 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 would
- replace it with a different device with the same capabilities and which
- is plugged 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 -i -e 's/"write_cd_rules"/"write_cd_rules <replaceable>mode</replaceable>"/' \
- /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 what you need.</para>
- </sect2>
- <sect2>
- <title>Dealing with duplicate devices</title>
- <para>As explained in <xref linkend="ch-scripts-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 to the opposite one.
- For all classes of hardware except sound cards and network cards, this is
- fixable by creating Udev rules for custom persistent symlinks.
- The case of network cards is covered separately in
- <xref linkend="ch-scripts-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 > /etc/udev/rules.d/83-duplicate_devs.rules << "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>
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