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Default Services
What is a Service?
For the purposes of this document, a "service" is defined as one or more of:
A daemon or process that is invoked by hardware activation (i.e. started via a udev rule).
Note that this includes processes which are not persistent. If something started by a systemd service unit runs for a short period of time and then exits, it is still a service. An example would be `+iptables+`. This also includes services in the user session (i.e. started per-user by the `systemd --user` manager).
Enabling Services by Default
Only services that meet all criteria below MAY be enabled by default on package installation.
Must not alter other services
Installation of the package providing the unit auto-started by this preset MUST NOT change the behavior of any other service running (or potentially running) on the system.
Must not require manual configuration to function
The service MUST NOT require configuration before it starts properly. If the end-user/administrator must make some specific configuration change before the service is able to start without error then it MUST NOT be enabled by default.
Must not fail under normal operating conditions
The service MUST NOT, under normal operating conditions, exit with an error causing systemd to mark the unit as failed. A service which is started by default is permitted to fail under exceptional conditions. For example, a service could start when appropriate hardware is present, but would still be allowed to fail if that hardware is somehow malfunctioning. Or a service could fail to start with an error if a configuration file has been locally modified to be syntactically incorrect.
Must not listen for outside connections
The service MUST NOT listen on a network socket for connections originating on a separate physical or virtual machine.
D-BUS services and hardware-activated services generally meet this requirement.
Hardware Support Services
Some hardware requires some additional service to be started in order to be useful. This may come in the form of a non-persistent setup process or in the form of a continuously-running service. If the service can be hardware activated to only start when the relevant hardware is present and do nothing when not present, and otherwise it meets the above requirements, then it SHOULD be enabled by default upon package installation.
If the service cannot be hardware activated, but it is possible to configure it such that it will exit without error and without marking the service as "failed" according to systemd, then it SHOULD be enabled by default upon package installation. This clean exit may be accomplished through https://www.freedesktop.org/software/systemd/man/systemd.unit.html#ConditionArchitecture=[systemd conditionals], by having the service (or a wrapper script) perform hardware detection and exit without indicating an error, or via other similar means.
Approved Exceptions