systemd.unit(5) for the common options of all unit configuration files. The common configuration items are configured in the generic [Unit] and [Install] sections. A separate [Target] section does not exist, since no target-specific options may be configured. Target units do not offer any additional functionality on top of the generic functionality provided by units. They exist merely to group units via dependencies (useful as boot targets), and to establish standardized names for synchronization points used in dependencies between units. Among other things, target units are a more flexible replacement for SysV runlevels in the classic SysV init system. (And for compatibility reasons special target units such as runlevel3.target exist which are used by the SysV runlevel compatibility code in systemd. See systemd.special(7) for details).When adding dependencies to other units, it's important to check if they set DefaultDependencies=. Service units, unless they set DefaultDependencies=no, automatically get a dependency on sysinit.target. In this case, both emergency.target and systemd-networkd.service have DefaultDependencies=no, so they are suitable for use in this target, and do not pull in sysinit.target. You can now switch into this emergency mode by running systemctl isolate emergency-net.target or by passing the option systemd.unit=emergency-net.target on the kernel command line. Other units can have WantedBy=emergency-net.target in the [Install] section. After they are enabled using systemctl enable, they will be started before emergency-net.target is started. It is also possible to add arbitrary units as dependencies of emergency.target without modifying them by using systemctl add-wants. systemd(1), systemctl(1), systemd.unit(5), systemd.special(7), systemd.directives(7)
•Target units will automatically complement all configured dependencies of type Wants= or Requires= with dependencies of type After= unless DefaultDependencies=no is set in the specified units. Note that Wants= or Requires= must be defined in the target unit itself — if you for example define Wants=some.target in some.service, the automatic ordering will not be added.
•Target units automatically gain Conflicts= and Before= dependencies against shutdown.target.
# emergency-net.target [Unit] Description=Emergency Mode with Networking Requires=emergency.target systemd-networkd.service After=emergency.target systemd-networkd.service AllowIsolate=yes