diff options
Diffstat (limited to 'chapter07/symlinks.xml')
| -rw-r--r-- | chapter07/symlinks.xml | 148 |
1 files changed, 146 insertions, 2 deletions
diff --git a/chapter07/symlinks.xml b/chapter07/symlinks.xml index bba84e369..1d4467a15 100644 --- a/chapter07/symlinks.xml +++ b/chapter07/symlinks.xml @@ -8,7 +8,151 @@ <sect1 id="ch-scripts-symlinks"> <?dbhtml filename="symlinks.html"?> - <title>Creating Custom Symlinks to Devices</title> + <title>Managing Devices</title> + + <sect2> + + <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="nodump"><userinput>bash /lib/udev/init-net-rules.sh</userinput></screen> + + <para> Now, inspect th + <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> + + <sect3> + <title>Custom Naming in Systemd</title> + + <para>Network interface names can also be customized with a set of + files spcific to systemd. A file with a name such as 10-eth0.link + in the /etc/systemd/network directory can set an interface name. All + files in the directory will be applied in lexical order. Files + in the /lib/systemd/network directory with the same name as those + in /etc/systemd/network will be overridden. See the man page + for systemd.link for a full explanation.</para> + + <para>An example file looks like:</para> + +<screen role="nodump">[Match] +MACAddress=12:34:56:78:9a:bc +Driver=brcmsmac +Path=pci-0000:02:00.0-* +Type=wlan +Virtualization=no +Host=my-laptop +Architecture=x86-64 + +[Link] +Name=wireless0 +MTUBytes=1450 +BitsPerSecond=10M +WakeOnLan=magic +MACAddress=cb:a9:87:65:43:21</screen> + + <para>The [Match] section specifies when to apply the rule. In + the example above, the entries can be shortened to the minimum + needed to uniquely identify the network device. Similarly, + the [Link] section only needs to specify the changes from the + default that are desired. In many cases, the only thing needed is + the Name entry.</para> + + </sect3> + + </sect2> <sect2> @@ -108,7 +252,7 @@ <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. + 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-root;view/svn/postlfs/devices.html">BLFS</ulink>.</para> |