agent.jar is meant to be launched by Jenkins, typically through a remote shell like ssh/rsh.
Jenkins master then communicates with this slave through stdin and stdout.
agent.jar is not meant to initiate a connection to the master on its own, so if you are trying to run
it from cron or as a service, you are misunderstanding how this works.
% java -jar agent.jar WARNING: Are you running agent.jar from an interactive console? If so, you are probably using it incorrectly. See https://wiki.jenkins.io/display/JENKINS/Launching+agent+from+console
See Distributed builds for more details.