sd-network enables network connectivity during the initramfs phase of the Linux boot process. This is achieved by starting the systemd network manager systemd-networkd. The install hook sd-network takes care of providing the initramfs image with all binaries, network drivers, configuration files required by systemd-networkd and enabling the systemd service systemd-networkd.service.

By default the network configuration files in and below /etc/systemd/network are copied unaltered into the initramfs image. Usually this setup is desired. But see below section Predictable Network Interface Names.

An important detail to keep in mind is the topic of predictable network interface names. In short, systemd (with the help of udev) takes care of naming all network interfaces in a consistent (predictable) way, overriding names assigned by the kernel (e.g. eth0, eth1, etc.). The latter names are assigned in a non-predictable way, i.e. a certain network interface may be assigned eth0 at one instance, while after the next reboot the very same network interface may be named eth2.

Unfortunately, by default these predictable network interface names are not available during initramfs phase. In other words, during early boot network interface names follow the non-predictable naming scheme with eth0, eth1, wlan0, wlan1, etc., while in the regular operating environment you will encounter network interface names like enp2s0, wlp0s20f3, etc.

This has the unpleasant consequence, that in many cases network configuration files in and below /etc/systemd/network that are used by systemd-networkd for the regular operating environment will not work during initramfs phase. There are several approaches how to deal with this complication.

With the following two approaches you can use the configuration files for systemd-networkd from your regular operating environment unaltered also for the initramfs phase.

Instead of identifying your network interfaces by their name (being unreliable during initramfs phase) you can use their MAC address or their persistent path. For example:

[Match]
MACAddress=00:1C:06:30:9F:15

or

[Match]
Path=platform-xhci-hcd.5.auto-usb-0:1:1.0

The latter can be determined with

udevadm info --query=property --property=ID_PATH /sys/class/net/<nic-name>

where <nic-name> is the network interface name as reported by ip link.

It is fairly easy to make predictable network interface names available during initramfs phase. Just add

/usr/lib/udev/rules.d/75-net-description.rules
/usr/lib/udev/rules.d/80-net-setup-link.rules
/usr/lib/systemd/network/99-default.link

to the array FILES in /etc/mkinitcpio.conf. This way you get the very same network interface names during initramfs phase as in your final operating environment.

Add sd-network to the array HOOKS in /etc/mkinitcpio.conf. The entry must be positioned somewhere after (right of) autodetect and after systemd. Apart from that the concrete position is irrelevant.

• SD_NETWORK_EXCLUDES: This is a bash array to specify filenames and globs (e.g. "wg*") that will be excludes when copying configuration files in and below /etc/systemd/network to the initramfs image.

  E.g. you may want to avoid that a wireguard tunnel is established during the initramfs phase. (The tunnel is of no use in this early phase of your system and the configuration files contain sensitive information that could be easily extracted from the initramfs image.) So specify SD_NETWORK_EXCLUDES somewhere in /etc/mkinitcpio.conf:

  SD_NETWORK_EXCLUDES=("wg*")

  This example assumes that the configuration of the wireguard tunnel has been stored in files with names beginning with wg.

  SD_NETWORK_EXCLUDES is a bash array so the parentheses are required. Entries have to be separated by blanks. Put all globs in quotes to prevent them from being expanded too early. Otherwise you may experience undesired behavior.

• SD_NETWORK_CONFIG: With this configuration in your /etc/mkinitcpio.conf you can specify an alternative source directory where the network configuration is copied from, e.g.:

  SD_NETWORK_CONFIG=/etc/systemd/network-initramfs

  Drop-in files ${SD_NETWORK_CONFIG}/*.d/*.conf are taken care of the same way as drop-in files for the default case (i.e. /etc/systemd/network/*.d/*conf).

  SD_NETWORK_CONFIG and SD_NETWORK_EXCLUDES can be used together (although this rarely makes sense).

• SD_NETWORKD_CONFIG: (Mind the extra D.) The configuration of systemd-networkd itself is located in /etc/systemd/networkd.conf (and optionally in drop-in files) and copied into the initramfs image. With SD_NETWORKD_CONFIG you can specify an alternative location where this configuration files are copied from.

  Drop-in files ${SD_NETWORKD_CONFIG}.d/*.conf are taken care of the same way as drop-in files for the default case (i.e. /etc/systemd/network.d/*conf).