Logstash-filter-elasticsearch plugin - specify index to search


I'm using for the first time the logstash-filter-elasticsearch plugin version 2.1.1.
How can I specify which index to query?

What is the default for this? searching all indexes or only the current index?

An index option for selecting the index(es) to search was added in v3.0.0 of the logstash-filter-elasticsearch filter. Prior to that I think it searched all indexes.