7.3. General Network Configuration

This section only applies if a network card is to be configured.

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, you will need to remove the network symlinks from all run-level directories (/etc/rc.d/rc*.d) after the bootscripts are installed in Section 7.2, “LFS-Bootscripts-20140404”.

7.3.1. Creating stable names for network interfaces

If there is only one network interface in the system to be configured, this section is optional, although it will never be wrong to do it. In many cases (e.g. a laptop with a wireless and a wired interface), accomplishing the configuration in this section is necessary.

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 eth0 and the Realtek card becomes eth1. 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.

If using the traditional network interface names such as eth0 is desired, generate a custom Udev rule:

bash /lib/udev/init-net-rules.sh

Now, inspect the /etc/udev/rules.d/70-persistent-net.rules file, to find out which name was assigned to which network device:

cat /etc/udev/rules.d/70-persistent-net.rules
[Note]

Note

In some cases such as when MAC addresess have been assigned to a network card manually or in a virtual environment such as Xen, the network rules file may not have been generated because addresses are not consistently assigned. In these cases, just continue to the next section.

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.

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:

  • SUBSYSTEM=="net" - This tells Udev to ignore devices that are not network cards.

  • ACTION=="add" - 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).

  • DRIVERS=="?*" - 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.

  • ATTR{address} - The value of this key is the NIC's MAC address.

  • ATTR{type}=="1" - 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.

  • KERNEL=="eth*" - This key was added to the Udev rule generator to handle machines that have multiple network interfaces, all with the same MAC address (the PS3 is one such machine). If the independent interfaces have different basenames, this key will allow Udev to tell them apart. This is generally not necessary for most Linux From Scratch users, but does not hurt.

  • NAME - The value of this key is the name that Udev will assign to this interface.

The value of NAME 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 NAME value when creating your configuration files below.

7.3.2. Creating Network Interface Configuration Files

Which interfaces are brought up and down by the network script depends on the files in /etc/sysconfig/. This directory should contain a file for each interface to be configured, such as ifconfig.xyz, where xyz is required to be a Network Card Interface name (e.g. eth0). Inside this file are attributes to this interface, such as its IP address(es), subnet masks, and so forth. It is necessary that the stem of the filename be ifconfig.

[Note]

Note

If the procedure in the previous section was not used, Udev will assign network card interface names based on system physical characteristics such as enp2s1. If you are not sure what your interface name is, you can always run ip link after you have booted your system. Again, it is important that ifconfig.xyz is named after correct network card interface name (e.g. ifconfig.enp2s1 or ifconfig.eth0) or your network interface will not be initialized during the boot process.

The following command creates a sample file for the eth0 device with a static IP address:

cd /etc/sysconfig/
cat > ifconfig.eth0 << "EOF"
ONBOOT=yes
IFACE=eth0
SERVICE=ipv4-static
IP=192.168.1.2
GATEWAY=192.168.1.1
PREFIX=24
BROADCAST=192.168.1.255
EOF

The values of these variables must be changed in every file to match the proper setup.

If the ONBOOT variable is set to yes the System V network script will bring up the Network Interface Card (NIC) during booting of the system. If set to anything but yes the NIC will be ignored by the network script and not be automatically brought up. The interface can be manually started or stopped with the ifup and ifdown commands.

The IFACE variable defines the interface name, for example, eth0. It is required for all network device configuration files.

The SERVICE 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 /lib/services/ directory allows other IP assignment methods. This is commonly used for Dynamic Host Configuration Protocol (DHCP), which is addressed in the BLFS book.

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

The PREFIX variable contains 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 PREFIX variable according to your specific subnet. If omitted, the PREFIX defaults to 24.

For more information see the ifup man page.

7.3.3. Configuring the Network Interface Card at boot (systemd)

Enabling of the network interface card configuration in systemd is done per interface. To enable network interface card configuration at boot, run:

systemctl enable ifupdown@eth0

To disable a previously enabled network interface card configuration at boot, run:

systemctl disable ifupdown@eth0

To manually start the network interface card configuration, run:

systemctl start ifupdown@eth0

Replace eth0 with the correct network interface card name as described on the beginning of this page.

[Note]

Note

The network card can also be started or stopped with the traditional ifup <device> or ifdown <device> commands.

7.3.4. Creating the /etc/resolv.conf File

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 /etc/resolv.conf. Create the file by running the following:

cat > /etc/resolv.conf << "EOF"
# Begin /etc/resolv.conf

domain <Your Domain Name>
nameserver <IP address of your primary nameserver>
nameserver <IP address of your secondary nameserver>

# End /etc/resolv.conf
EOF

The domain statement can be omitted or replaced with a search statement. See the man page for resolv.conf for more details.

Replace <IP address of the nameserver> 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 nameserver line from the file. The IP address may also be a router on the local network.

[Note]

Note

The Google Public IPv4 DNS addresses are 8.8.8.8 and 8.8.4.4.