===== systemd-networkd =====

==== Introduction ====

Systemd-networkd is a system service that manages networks. Debian conventionally has the networking service networking.service configured as the default network manager. Which uses ///etc/network/interfaces// file to manage your network interfaces by means of //ifup// and //ifdown// commands.

Systemd-networkd is an alternative option to the Debian networking service — networking.service. Since Debian 12 (Bookworm) systemd-networkd is used to configure the network within the lxc container.

==== Required services ====
To use systemd-networkd, start/enable systemd-networkd.service. It is optional to also configure systemd-resolved, which is a network name resolution service to local applications, considering the following points:

  * It is important to understand how resolv.conf and systemd-resolved interact to properly configure the DNS that will be used, some explanations are provided in systemd-resolved.
  * systemd-resolved is required if DNS entries are specified in .network files.
  * systemd-resolved is also required if you want to obtain DNS addresses from DHCP servers or IPv6 router advertisements.
  * (by setting (DHCP= and/or IPv6AcceptRA= in the [Network] section, and UseDNS=yes (the default) in the corresponding section(s) [DHCPv4], [DHCPv6], [IPv6AcceptRA], see systemd.network(5)).
  * Note that systemd-resolved can also be used without systemd-networkd.

==== Configuration files ====
Systemd-networkd reads files from the following locations:

  * /usr/lib/systemd/network <-- Default files shipped with systemd. Lowest priority if filename matches
  * /run/systemd/network <-- Second Highest Priority. Overrules above files with same name
  * /etc/systemd/network <-- Highest Priority. Overrules above files with same name

==== Network Adapter naming ====
Systemd/udev automatically assigns predictable, stable network interface names for all local Ethernet, WLAN, and WWAN interfaces. e.g. enp1s0 is the wired adapter and wlp2s0 is the wireless adapter. Use //networkctl list// to list the devices on the system.
<code>

# networkctl list
--------------------------------------------
IDX LINK TYPE     OPERATIONAL SETUP     
  1 lo   loopback carrier     unmanaged
  2 eth0 ether    routable    configured

2 links listed.
--------------------------------------------
</code>


==== Configuration ====
All configurations in this section are stored as *.network in /etc/systemd/network/. For a full listing of options and processing order, see #Configuration files and systemd.network(5). There are three types of configuration files. They all use a format similar to systemd unit files.

  * **.network** files. They will apply a network configuration for a matching device
  * **.netdev** files. They will create a virtual network device for a matching environment
  * **.link** files. When a network device appears, udev will look for the first matching .link file

They all follow the same rules:
If all conditions in the [Match] section are matched, the profile will be activated
an empty [Match] section means the profile will apply in any case (can be compared to the * wildcard)
all configuration files are collectively sorted and processed in lexical order, regardless of the directory in which they live 
files with identical name replace each other

After making changes to a configuration file, restart systemd-networkd.service.

==== Links ====
  * https://manpages.debian.org/bookworm/systemd/systemd.network.5.en.html
  * https://wiki.archlinux.org/title/systemd-networkd