From the docker plugin wiki page

https://wiki.jenkins-ci.org/display/JENKINS/Docker+Plugin

"If your host needs to allow connections from a jenkins instance hosted on a different machine, you will need to open up the TCP port"

However if you do not open the TCP port and do not configure docker to listen for TCP connections (-H tcp://127.0.0.1:4243) it does not work.

If you try to create a job and restrict the build on a docker container, it would stick waiting for an executor forever.

Once I opened the TCP port, jenkins and docker started to communicate.

NOTE: I am using CentOS on ec2 with jenkins and docker installed on it.