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.

The rules were pre-generated in the build instructions for udev (systemd) in the last chapter. 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. Создание файлов конфигурации сетевых интерфейсов

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 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.i If omitted, the PREFIX defaults to 24.

For more information see the ifup man page.

7.2.3. Создание файла /etc/resolv.conf

Если система будет подключена к сети Интернет, ей понадобится информация о DNS (Domain Name Service, службе доменных имен) для преобразования имен доменов Интернета в IP-адреса и наоборот. Лучшим способом предоставить ей эту информацию будет указать адреса DNS-серверов, которые Вам выдал провайдер или администратор сети, в файле /etc/resolv.conf. Создадим этот файл командой:

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

domain <Имя вашего домена>
nameserver <IP-адрес первичного сервера имен>
nameserver <IP-адрес вторичного сервера имен>

# Конец /etc/resolv.conf
EOF

Строка domain может быть опущена или заменена на search. Обратитесь к странице справки по resolv.conf за более подробной информацией.

Замените <IP-адрес сервера имен> IP-адресом самого подходящего на Ваш взгляд DNS-сервера. Зачастую стоит указать более одного сервера (вторичные сервера требуются для обеспечения надежности). Если Вы хотите указать только один DNS-сервер, удалите вторую строку nameserver из файла. IP-адрес может также принадлежать роутеру Вашей локальной сети.