lfs-book/chapter07/network.xml

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<sect1 id="ch-scripts-network">
  <?dbhtml filename="network.html"?>

  <title>Configuring the network Script</title>

  <indexterm zone="ch-scripts-network">
    <primary sortas="d-network">network</primary>
  <secondary>configuring</secondary></indexterm>

  <para>This section only applies if a network card is to be
  configured.</para>

  <para>If a network card will not be used, there is likely no need to
  create any configuration files relating to network cards. If that is
  the case, remove the <filename class="symlink">network</filename>
  symlinks from all run-level directories (<filename
  class="directory">/etc/rc.d/rc*.d</filename>).</para>

  <sect2>
    <title>Creating stable names for network interfaces</title>

    <para>With Udev and modular network drivers, the network interface numbering
    is not persistent across reboots by default, because the drivers are loaded
    in parallel and, thus, in random order. For example, on a computer having
    two network cards made by Intel and Realtek, the network card manufactured
    by Intel may become <filename class="devicefile">eth0</filename> and the
    Realtek card becomes  <filename class="devicefile">eth1</filename>. In some
    cases, after a reboot the cards get renumbered the other way around. To
    avoid this, Udev comes with a script and some rules to assign stable names
    to network cards based on their MAC address.</para>

    <para>Pre-generate the rules to ensure the same names get assigned to the
    same devices at every boot, including the first:</para>

<screen><userinput>/lib/udev/write_net_rules all_interfaces</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><userinput>cat /etc/udev/rules.d/70-persistent-net.rules</userinput></screen>

    <para>Each NIC takes up two lines in the file. The first line is a
    description of the NIC itself, 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. This line is a comment; neither
    the hardware ID nor the driver is used to determine which name to give an
    interface. 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 explanations 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>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>ATTRS{type}=="1"</literal> - Optional. This key will
        only be added if this NIC is a wireless NIC whose driver creates
        multiple virtual interfaces; it ensures the rule only matches the
        primary interface. The secondary interfaces are not matched for the
        same reason that VLAN and bridge sub-interfaces are not matched: there
        would be a name collision.</para>
      </listitem>
      <listitem>
        <para><literal>ATTRS{address}</literal> - The value of this key is the
        NIC's MAC address.</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>

  </sect2>

  <sect2>
    <title>Creating Network Interface Configuration Files</title>

    <para>Which interfaces are brought up and down by the network script
    depends on the files and directories in the <filename
    class="directory">/etc/sysconfig/network-devices</filename> hierarchy.
    This directory should contain a sub-directory for each interface to be
    configured, such as <filename>ifconfig.xyz</filename>, where
    <quote>xyz</quote> is a network interface name. Inside this directory
    would be files defining the attributes to this interface, such as its IP
    address(es), subnet masks, and so forth.</para>

    <para>The following command creates a sample <filename>ipv4</filename>
    file for the <emphasis>eth0</emphasis> device:</para>

<screen><userinput>cd /etc/sysconfig/network-devices &amp;&amp;
mkdir -v ifconfig.eth0 &amp;&amp;
cat &gt; ifconfig.eth0/ipv4 &lt;&lt; "EOF"
<literal>ONBOOT=yes
SERVICE=ipv4-static
IP=192.168.1.1
GATEWAY=192.168.1.2
PREFIX=24
BROADCAST=192.168.1.255</literal>
EOF</userinput></screen>

    <para>The values of these variables must be changed in every file to match
    the proper setup. If the <envar>ONBOOT</envar> variable is set to
    <quote>yes</quote> the network script will bring up the Network Interface
    Card (NIC) during booting of the system. If set to anything but
    <quote>yes</quote> the NIC will be ignored by the network script and not
    be brought up.</para>

    <para>The <envar>SERVICE</envar> variable defines the method used for
    obtaining the IP address. The LFS-Bootscripts package has a modular IP
    assignment format, and creating additional files in the <filename
    class="directory">/etc/sysconfig/network-devices/services</filename>
    directory allows other IP assignment methods. This is commonly used for
    Dynamic Host Configuration Protocol (DHCP), which is addressed in the
    BLFS book.</para>

    <para>The <envar>GATEWAY</envar> variable should contain the default
    gateway IP address, if one is present. If not, then comment out the
    variable entirely.</para>

    <para>The <envar>PREFIX</envar> variable needs to contain the number of
    bits used in the subnet. Each octet in an IP address is 8 bits. If the
    subnet's netmask is 255.255.255.0, then it is using the first three octets
    (24 bits) to specify the network number. If the netmask is 255.255.255.240,
    it would be using the first 28 bits.  Prefixes longer than 24 bits are
    commonly used by DSL and cable-based Internet Service Providers (ISPs).
    In this example (PREFIX=24), the netmask is 255.255.255.0. Adjust the
    <envar>PREFIX</envar> variable according to your specific subnet.</para>

  </sect2>

  <sect2 id="resolv.conf">
    <title>Creating the /etc/resolv.conf File</title>

    <indexterm zone="resolv.conf">
      <primary sortas="e-/etc/resolv.conf">/etc/resolv.conf</primary>
    </indexterm>

    <para>If the system is going to be connected to the Internet, it will
    need some means of Domain Name Service (DNS) name resolution to
    resolve Internet domain names to IP addresses, and vice versa. This is
    best achieved by placing the IP address of the DNS server, available
    from the ISP or network administrator, into
    <filename>/etc/resolv.conf</filename>. Create the file by running the
    following:</para>

<screen><userinput>cat &gt; /etc/resolv.conf &lt;&lt; "EOF"
<literal># Begin /etc/resolv.conf

domain <replaceable>&lt;Your Domain Name&gt;</replaceable>
nameserver <replaceable>&lt;IP address of your primary nameserver&gt;</replaceable>
nameserver <replaceable>&lt;IP address of your secondary nameserver&gt;</replaceable>

# End /etc/resolv.conf</literal>
EOF</userinput></screen>

    <para>Replace <replaceable>&lt;IP address of the nameserver&gt;</replaceable>
    with the IP address of the DNS most appropriate for the setup. There will
    often be more than one entry (requirements demand secondary servers for
    fallback capability). If you only need or want one DNS server, remove the
    second <emphasis>nameserver</emphasis> line from the file. The IP address
    may also be a router on the local network.</para>

  </sect2>

</sect1>