#### Linux From Scratch - Version 12.3-systemd

### Chapter 9. System Configuration

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# 9.2. General Network Configuration

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

## 9.2.1. Network Interface Configuration Files

Starting with version 209, systemd ships a network configuration daemon called systemd-networkd which can be used for basic network configuration. Additionally, since version 213, DNS name resolution can be handled by systemd-resolved in place of a static `/etc/resolv.conf` file. Both services are enabled by default.

### Note

If you will not use systemd-networkd for network configuration (for example, when the system is not connected to network, or you want to use another utility like NetworkManager for network configuration), disable a service to prevent an error message during boot:

    systemctl disable systemd-networkd-wait-online

Configuration files for systemd-networkd (and systemd-resolved) can be placed in `/usr/lib/systemd/network` or `/etc/systemd/network`. Files in `/etc/systemd/network` have a higher priority than the ones in `/usr/lib/systemd/network`. There are three types of configuration files: `.link`, `.netdev` and `.network` files. For detailed descriptions and example contents of these configuration files, consult the systemd.link(5), systemd.netdev(5), and systemd.network(5) manual pages.

### 9.2.1.1. Network Device Naming

Udev normally assigns network card interface names based on physical system 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.

### Note

The interface names depend on the implementation and configuration of the udev daemon running on the system. The udev daemon for LFS (systemd-udevd, installed in Section 8.76, “Systemd-257.3”) will not run unless the LFS system is booted. So it's unreliable to determine the interface names being used in LFS system by running those commands on the host distro, even though you are in the chroot environment.

For most systems, there is only one network interface for each type of connection. For example, the classic interface name for a wired connection is eth0. A wireless connection will usually have the name wifi0 or wlan0.

If you prefer to use the classic or customized network interface names, there are three alternative ways to do that:

- Mask udev's `.link` file for the default policy:

      ln -s /dev/null /etc/systemd/network/99-default.link

- Create a manual naming scheme, for example by naming the interfaces something like `internet0`, `dmz0`, or `lan0`. To do that, create `.link` files in /etc/systemd/network/ that select an explicit name or a better naming scheme for your network interfaces. For example:

      cat > /etc/systemd/network/10-ether0.link << "EOF"
      [Match]
      # Change the MAC address as appropriate for your network device
      MACAddress=12:34:45:78:90:AB

      [Link]
      Name=ether0
      EOF

  See systemd.link(5) for more information.

- In /boot/grub/grub.cfg, pass the option `net.ifnames=0` on the kernel command line.

### 9.2.1.2. Static IP Configuration

The command below creates a basic configuration file for a Static IP setup (using both systemd-networkd and systemd-resolved):

    cat > /etc/systemd/network/10-eth-static.network << "EOF"
    [Match]
    Name=<network-device-name>

    [Network]
    Address=192.168.0.2/24
    Gateway=192.168.0.1
    DNS=192.168.0.1
    Domains=<Your Domain Name>
    EOF

Multiple DNS entries can be added if you have more than one DNS server. Do not include DNS or Domains entries if you intend to use a static `/etc/resolv.conf` file.

### 9.2.1.3. DHCP Configuration

The command below creates a basic configuration file for an IPv4 DHCP setup:

    cat > /etc/systemd/network/10-eth-dhcp.network << "EOF"
    [Match]
    Name=<network-device-name>

    [Network]
    DHCP=ipv4

    [DHCPv4]
    UseDomains=true
    EOF

## 9.2.2. 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`.

### 9.2.2.1. systemd-resolved Configuration

### Note

If using methods incompatible with systemd-resolved to configure your network interfaces (ex: ppp, etc.), or if using any type of local resolver (ex: bind, dnsmasq, unbound, etc.), or any other software that generates an `/etc/resolv.conf` (ex: a resolvconf program other than the one provided by systemd), the systemd-resolved service should not be used.

To disable systemd-resolved, issue the following command:

    systemctl disable systemd-resolved

When using systemd-resolved for DNS configuration, it creates the file `/run/systemd/resolve/stub-resolv.conf`. And, if `/etc/resolv.conf` does not exist, it will be created by systemd-resolved as a symlink to `/run/systemd/resolve/stub-resolv.conf`. So it's unnecessary to create a `/etc/resolv.conf` manually.

### 9.2.2.2. Static resolv.conf Configuration

If a static `/etc/resolv.conf` is desired, create it by running the following command:

    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 server most appropriate for your 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. Another option is to use the Google Public DNS service using the IP addresses below as nameservers.

### Note

The Google Public IPv4 DNS addresses are `8.8.8.8` and `8.8.4.4` for IPv4, and `2001:4860:4860::8888` and `2001:4860:4860::8844` for IPv6.

## 9.2.3. 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 is put in the `/etc/hosts` file.

## 9.2.4. 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 192.168.1.1.

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 using the following command:

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

    <192.168.0.2> <FQDN> [alias1] [alias2] ...
    ::1       ip6-localhost ip6-loopback
    ff02::1   ip6-allnodes
    ff02::2   ip6-allrouters

    # End /etc/hosts
    EOF

The `<192.168.0.2>` and `<FQDN>` 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, and the `<192.168.0.2>` line can be omitted if you are using a connection configured with DHCP or IPv6 Autoconfiguration, or using `localhost.localdomain` as the FQDN.

The `/etc/hostname` does not contain entries for `localhost`, `localhost.localdomain`, or the hostname (without a domain) because they are handled by the `myhostname` NSS module, read the man page nss-myhostname(8) for details.

The ::1 entry is the IPv6 counterpart of 127.0.0.1 and represents the IPv6 loopback interface.

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