networkd.conf, networkd.conf.d - Global Network configuration files
These configuration files control global network parameters. Currently the DHCP Unique Identifier (DUID).
The default configuration is defined during compilation, so a configuration file is only needed when it is necessary to deviate from those defaults. By default, the configuration file in /etc/systemd/ contains commented out entries showing the defaults as a guide to the administrator. This file can be edited to create local overrides.
When packages need to customize the configuration, they can install configuration snippets in /usr/lib/systemd/*.conf.d/. Files in /etc/ are reserved for the local administrator, who may use this logic to override the configuration files installed by vendor packages. The main configuration file is read before any of the configuration directories, and has the lowest precedence; entries in a file in any configuration directory override entries in the single configuration file. Files in the *.conf.d/ configuration subdirectories are sorted by their filename in lexicographic order, regardless of which of the subdirectories they reside in. When multiple files specify the same option, for options which accept just a single value, the entry in the file with the lexicographically latest name takes precedence. For options which accept a list of values, entries are collected as they occur in files sorted lexicographically. It is recommended to prefix all filenames in those subdirectories with a two-digit number and a dash, to simplify the ordering of the files.
To disable a configuration file supplied by the vendor, the recommended way is to place a symlink to /dev/null in the configuration directory in /etc/, with the same filename as the vendor configuration file.
This section configures the DHCP Unique Identifier (DUID) value used by DHCP protocol. DHCPv6 client protocol sends the DHCP Unique Identifier and the interface Identity Association Identifier (IAID) to a DHCP server when acquiring a dynamic IPv6 address. DHCPv4 client protocol sends IAID and DUID to the DHCP server when acquiring a dynamic IPv4 address if ClientIdentifier=duid. IAID and DUID allows a DHCP server to uniquely identify the machine and the interface requesting a DHCP IP. To configure IAID and ClientIdentifier, see systemd.network(5).
The following options are understood:
Specifies how the DUID should be generated. See RFC 3315  for a description of all the options.
The following values are understood:
If "DUIDType=vendor", then the DUID value will be generated using "43793" as the vendor identifier (systemd) and hashed contents of machine-id(5). This is the default if DUIDType= is not specified.
link-layer-time, link-layer, uuid
Those values are parsed and can be used to set the DUID type field, but DUID contents must be provided using DUIDRawData=.
In all cases, DUIDRawData= can be used to override the actual DUID value that is used.
Specifies the DHCP DUID value as a single newline-terminated, hexadecimal string, with each byte separated by ":". The DUID that is sent is composed of the DUID type specified by DUIDType= and the value configured here.
The DUID value specified here overrides the DUID that systemd-networkd generates using the machine-id from the /etc/machine-id file. To configure DUID per-network, see systemd.network(5). The configured DHCP DUID should conform to the specification in RFC 3315  , RFC 6355  . To configure IAID, see systemd.network(5).
Example 1. A DUIDType=vendor with a custom value
This specifies a 14 byte DUID, with the type DUID-EN ("00:02"), enterprise number 43793 ("00:00:ab:11"), and identifier value "f9:2a:c2:77:29:f9:5c:00".
systemd(1), systemd.network(5), machine-id(1)