elasticsearch/logstash — pull request #1352 by kanarip (2014-05-09 23:02:33+00:00)
Title: Allow underscores (_) in the HOSTNAME grok pattern

While of course it is not conform the relevant RFCs, reverse DNS records tend to be out of one's control – and underscores start getting used more often.

See also ticket https://logstash.jira.com/browse/LOGSTASH-2198