7.2. General Network Configuration

7.2.1. 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.

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

for NIC in /sys/class/net/* ; do
    INTERFACE=${NIC##*/} udevadm test --action=add $NIC
done

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

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:

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.

7.2.2. 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 meaningful to the administrator such as the device 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.

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=192.168.1.1
GATEWAY=192.168.1.2
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 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 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 PREFIX variable according to your specific subnet.

7.2.3. 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.