Chapter 39. Systemd network targets and services

Systemd maintains the network and network-online target units. The special units such as NetworkManager-wait-online.service, have WantedBy=network-online.target and Before=network-online.target parameters. If enabled, these units get started with network-online.target and delay the target to be reached until some form of network connectivity is established. They delay the network-online target until the network is connected.

The network-online target starts a service, which adds substantial delays to further execution. Systemd automatically adds dependencies with Wants and After parameters for this target unit to all the System V (SysV) init script service units with a Linux Standard Base (LSB) header referring to the $network facility. The LSB header is metadata for init scripts. You can use it to specify dependencies. This is similar to the systemd target.

The network target does not significantly delay the execution of the boot process. Reaching the network target means that the service that is responsible for setting up the network has started. However, it does not mean that a network device was configured. This target is important during the shutdown of the system. For example, if you have a service that was ordered after the network target during bootup, then this dependency is reversed during the shutdown. The network does not get disconnected until your service has been stopped. All mount units for remote network file systems automatically start the network-online target unit and order themselves after it.

The synchronous legacy network scripts iterate through all configuration files to set up devices. They apply all network-related configurations and ensure that the network is online.

The NetworkManager-wait-online service waits with a timeout for the network to be configured. This network configuration involves plugging-in an Ethernet device, scanning for a Wi-Fi device, and so forth. NetworkManager automatically activates suitable profiles that are configured to start automatically. The failure of the automatic activation process due to a DHCP timeout or similar event might keep NetworkManager busy for an extended period of time. Depending on the configuration, NetworkManager retries activating the same profile or a different profile.

When the startup completes, either all profiles are in a disconnected state or are successfully activated. You can configure profiles to auto-connect. The following are a few examples of parameters that set timeouts or define when the connection is considered active:

Red Hat Enterprise Linux installs systemd service files in the /usr/lib/systemd/system/ directory. This procedure creates a drop-in snippet for a service file in /etc/systemd/system/<service_name>.service.d/ that is used together with the service file in /usr/lib/systemd/system/ to start a particular service after the network is online. It has a higher priority if settings in the drop-in snippet overlap with the ones in the service file in /usr/lib/systemd/system/.