Adds systemd readme file

Author: simonemainardi

README.systemd.md

@@ -0,0 +1,90 @@
+nprobe is controlled using utility `systemctl` on operating systems
+and distributions that use the `systemd` service manager.
+
+Upon successful package installation, the nprobe service is
+automatically started on the loopback interface. The service uses a
+configuration file that is located at `/etc/nprobe/nprobe.conf` and
+that is populated with some defaults during installation. The
+configuration file can be edited and extended with any configuration
+option supported by nprobe. A service restart is required after
+configuration file modifications.
+
+The nprobe service is always started on boot by default. The service
+must be disabled to prevent this behavior.
+
+## The nprobe service configuration file
+
+The configuration file is located at `/etc/nprobe/nprobe.conf`.
+
+## Controlling nprobe
+
+To start, stop and restart the nprobe service type:
+
+```
+# systemctl start nprobe
+# systemctl stop nprobe
+# systemctl restart nprobe
+```
+
+To prevent nprobe from starting on boot type:
+
+```
+# systemctl disable nprobe
+```
+
+To start nprobe on boot, assuming it has previously been disabled,
+type:
+
+```
+# systemctl enable nprobe
+```
+
+To check the status of the service, including its output and PID, type:
+
+```
+# systemctl status nprobe
+```
+
+## Instantiated nprobe services
+
+There are circumstances under which multiple instances of the nprobe
+service may run on the same host. To manage a particular `<instance>`
+of the service, append an `@<instance>` to the `nprobe` service name.
+
+Typically, `<instance>` corresponds to an interface name (e.g., `eno1`)
+or to `none` when nprobe is used in collector mode. This convention
+allows an easy identification of the purpose of each
+service. Nonetheless, any string is acceptable as value for `<instance>`.
+
+The `<instance>` uniquely identifies a service and its corresponding
+configuration file that is located under
+`/etc/nprobe/nprobe-<instance>.conf`.
+
+For example, to start two nprobe services, one on interface `eno1` and
+another to collect netflow, one can add two configuration files:
+
+```
+/etc/nprobe/nprobe-eno1.conf
+/etc/nprobe/nprobe-none.conf
+```
+
+And then start the services with:
+
+```
+# systemctl start nprobe@eno1
+# systemctl start nprobe@none
+```
+
+Optionally, one may want to start the services on boot with:
+
+```
+# systemctl enable nprobe@eno1
+# systemctl enable nprobe@none
+```
+
+The status of the services above can be controlled with:
+
+```
+# systemctl status nprobe@eno1
+# systemctl status nprobe@none
+```