When I had installed Jenkins on Amazon Linux AMI following steps mentioned in http://bhargavamin.com/how-to-do/install-jenkins-on-amazon-linux-aws/
After installation
The problem here is pretty simple, if you do curl http://127.0.0.1:8080
on your host machine, you should get a message stating:
Authentication required
This means that jenkins has anonymous priviledges which is not allowing any connection.
To solve this issue, you will have to do two changes:
Add Jenkins user to root group: sudo usermod -a -G root jenkins
Make Jenkins listen to all external IPs by editing file /etc/sysconfig/jenkins
and changing the JENKINS_LISTEN_ADDRESS="0.0.0.0"
Once this is done, restart the jenkins server and then try to install plugins for jenkins.