Reports on the mailing list indicate problems installing when Jenkins is offline and cannot retrieve the update site metadata. Needs to be investigated and fixed.

https://groups.google.com/d/msg/jenkinsci-users/fEWFVUj0UVY/28nW1zotJQAJ

Possibly obsolete when https://github.com/jenkinsci/jenkins/pull/2042 by kzantow is merged, unclear.