To start logstash as an agent during startup, add

    logstash_enable="YES"

to your /etc/rc.conf.

Extra options can be found in startup script.

NOTE: Since version 1.5.0, logstash is started in agent mode.
The Kibana web application is no longer embedded in this Logstash
distribution.

NOTE: Since version 2, logstash does not support an embedded
ElasticSearch instance.