2. Linux From Scratch 12.0
3. chapter09_symlinks
4. Search

Filters

• Untranslated strings • state:empty
• Unfinished strings • state:<translated
• Translated strings • state:>=translated
• Strings marked for edit • state:needs-editing
• Strings with suggestions • has:suggestion
• Strings with variants • has:variant
• Strings with screenshots • has:screenshot
• Strings with labels • has:label
• Strings with context • has:context
• Unfinished strings without suggestions • state:<translated AND NOT has:suggestion
• Strings with comments • has:comment
• Strings with any failing checks • has:check
• Approved strings • state:approved
• Strings waiting for review • state:translated

state:empty

Position and priority

• Position and priority
• Position
• Priority
• Labels
• Source string
• Target string
• String age
• Last updated
• Number of words
• Number of comments
• Number of failing checks
• Key
• String location

	Translation	Source string	Target string
	Linux From Scratch 12.0/chapter09_symlinks — Greek	Managing Devices
	Linux From Scratch 12.0/chapter09_symlinks — Greek	Network Devices
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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—on a computer with two network cards made by Intel and Realtek, for instance—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.
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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.
	Linux From Scratch 12.0/chapter09_symlinks — Greek	Disabling Persistent Naming on the Kernel Command Line
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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"/>.
	Linux From Scratch 12.0/chapter09_symlinks — Greek	Creating Custom Udev Rules
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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:
	Linux From Scratch 12.0/chapter09_symlinks — Greek	<userinput>bash /usr/lib/udev/init-net-rules.sh</userinput>
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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:
	Linux From Scratch 12.0/chapter09_symlinks — Greek	<userinput>cat /etc/udev/rules.d/70-persistent-net.rules</userinput>
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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.
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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.
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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:
	Linux From Scratch 12.0/chapter09_symlinks — Greek	<literal>SUBSYSTEM=="net"</literal> - This tells udev to ignore devices that are not network cards.
	Linux From Scratch 12.0/chapter09_symlinks — Greek	<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).
	Linux From Scratch 12.0/chapter09_symlinks — Greek	<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.
	Linux From Scratch 12.0/chapter09_symlinks — Greek	<literal>ATTR{address}</literal> - The value of this keyword is the NIC's MAC address.
	Linux From Scratch 12.0/chapter09_symlinks — Greek	<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.
	Linux From Scratch 12.0/chapter09_symlinks — Greek	<literal>NAME</literal> - The value of this keyword is the name that udev will assign to this interface.
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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.
	Linux From Scratch 12.0/chapter09_symlinks — Greek	CD-ROM Symlinks
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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.
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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.
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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.
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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.
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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.
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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:
	Linux From Scratch 12.0/chapter09_symlinks — Greek	<userinput>udevadm test /sys/block/hdd</userinput>
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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.
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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>):
	Linux From Scratch 12.0/chapter09_symlinks — Greek	<userinput>sed -e 's/"write_cd_rules"/"write_cd_rules <replaceable>mode</replaceable>"/' \ -i /etc/udev/rules.d/83-cdrom-symlinks.rules</userinput>
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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.
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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.
	Linux From Scratch 12.0/chapter09_symlinks — Greek	Dealing with Duplicate Devices
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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>.
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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):
	Linux From Scratch 12.0/chapter09_symlinks — Greek	<userinput>udevadm info -a -p /sys/class/video4linux/video0</userinput>
	Linux From Scratch 12.0/chapter09_symlinks — Greek	Then write rules that create the symlinks, e.g.:
	Linux From Scratch 12.0/chapter09_symlinks — Greek	<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>
	Linux From Scratch 12.0/chapter09_symlinks — Greek	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.