9.5. General Network Configuration

#### Linux From Scratch - Version 12.4

### Chapter 9. System Configuration

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# 9.5. General Network Configuration

## 9.5.1. Creating Network Interface Configuration Files

The files in `/etc/sysconfig/` usually determine which interfaces are brought up and down by the network script. This directory should contain a file for each interface to be configured, such as `ifconfig.xyz`, where “xyz” describes the network card. The interface name (e.g. eth0) is usually appropriate. Each file contains the attributes of one interface, such as its IP address(es), subnet masks, and so forth. The stem of the filename must be ifconfig.

### 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 or ls /sys/class/net after you have booted your system.

The interface names depend on the implementation and configuration of the udev daemon running on the system. The udev daemon for LFS (installed in Section 8.76, “Udev from Systemd-257.8”) will not run until the LFS system is booted. So the interface names in the LFS system cannot always be determined by running those commands on the host distro, even in the chroot environment.

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 in italics must be changed in each file, to set the interfaces up correctly.

If the `ONBOOT` variable is set to `yes` the System V network script will bring up the Network Interface Card (NIC) during the system boot process. If set to anything besides `yes`, the NIC will be ignored by the network script and will not be started automatically. Interfaces 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 filename extension must match this value.

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 specifies the number of bits used in the subnet. Each segment of an IP address is 8 bits. If the subnet's netmask is 255.255.255.0, then it is using the first three segments (24 bits) to specify the network number. If the netmask is 255.255.255.240, the subnet is 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.

## 9.5.2. Creating the /etc/resolv.conf File

The system will need some means of obtaining 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

The Google Public IPv4 DNS addresses are 8.8.8.8 and 8.8.4.4.

## 9.5.3. Configuring the System Hostname

During the boot process, the file `/etc/hostname` is used for establishing the system's hostname.

Create the `/etc/hostname` file and enter a hostname by running:

    echo "<lfs>" > /etc/hostname

`<lfs>` needs to be replaced with the name given to the computer. Do not enter the Fully Qualified Domain Name (FQDN) here. That information goes in the `/etc/hosts` file.

## 9.5.4. Customizing the /etc/hosts File

Decide on a fully-qualified domain name (FQDN), and possible aliases for use in the `/etc/hosts` file. If using static IP addresses, you'll also need to decide on an IP address. The syntax for a hosts file entry is:

    IP_address myhost.example.org aliases

Unless the computer is to be visible to the Internet (i.e., there is a registered domain and a valid block of assigned IP addresses—most users do not have this), make sure that the IP address is in the private network IP address range. Valid ranges are:

    Private Network Address Range      Normal Prefix
    10.0.0.1 - 10.255.255.254           8
    172.x.0.1 - 172.x.255.254           16
    192.168.y.1 - 192.168.y.254         24

x can be any number in the range 16-31. y can be any number in the range 0-255.

A valid private IP address could be 192.168.1.2.

If the computer is to be visible to the Internet, a valid FQDN can be the domain name itself, or a string resulted by concatenating a prefix (often the hostname) and the domain name with a “.” character. And, you need to contact the domain provider to resolve the FQDN to your public IP address.

Even if the computer is not visible to the Internet, a FQDN is still needed for certain programs, such as MTAs, to operate properly. A special FQDN, `localhost.localdomain`, can be used for this purpose.

Create the `/etc/hosts` file by running:

    cat > /etc/hosts << "EOF"
    # Begin /etc/hosts

    127.0.0.1 localhost.localdomain localhost
    127.0.1.1 <FQDN> <HOSTNAME>
    <192.168.1.2> <FQDN> <HOSTNAME> [alias1] [alias2 ...]
    ::1       localhost ip6-localhost ip6-loopback
    ff02::1   ip6-allnodes
    ff02::2   ip6-allrouters

    # End /etc/hosts
    EOF

The `<192.168.1.2>`, `<FQDN>`, and `<HOSTNAME>` values need to be changed for specific uses or requirements (if assigned an IP address by a network/system administrator and the machine will be connected to an existing network). The optional alias name(s) can be omitted.

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