I have a long running Ambari configured cluster that has run into some problems. Several weeks ago we had a network upgrade and we lost the DNS routing that had previously been present. This caused the Ambari server VM to return "localhost" when running 'hostname -f'. As such, I expected this may cause Ambari to think that the Ambari REST API was running at localhost instead of the FQ hostname. Since then, I have updated the Ambari Server VM (via /etc/hosts) to return the correct FQ hostname when running 'hostname -f', restarted the ambari server, restarted the agents, but still the agents are looking for the REST API at 'locahost:8080'. The problem can be seen when trying to register a new host in the Ambari Server log: STDOUT: Host registration aborted. Ambari Agent host cannot reach Ambari Server 'localhost:8080'. Please check the network connectivity between the Ambari Agent host and the Ambari Server You can see above this that in the Bootstrapping hosts process that the 'server' is being set to 'localhost': using tmp dir /var/run/ambari-server/bootstrap/2 ambari: localhost; server_port: 8080; ambari version: 2.2.0.0; Is this hostname value cached somewhere? The agent can be manually registered by changing the ini file and specifying the Ambari server hostname, but this only helps with registration. All REST API calls from the Agent host still route to (for example) "locahost:8080/resources/..." so it doesn't solve the problem, and some component installation fails as a result. I've also looked for rogue Ambari processes and this does not appear to be the problem. Thanks for any help. ... View more