https://community.cloudera.com/t5/Support-Questions/Null-Pointer-Exception-in-ambari-server-logs-after-ambari/m-p/230535#M192385 <P><A rel="user" href="https://community.cloudera.com/users/17123/rafaelgomezg.html" nodeid="17123">@Rafael Gomez</A></P><P>Additionally, we see that you are getting the following error:</P><PRE>Unable to connect to: <A href="https://hadoop-m:8441/agent/v1/register/hadoop-m.c.hdp-1-163209.internal" target="_blank">https://hadoop-m:8441/agent/v1/register/hadoop-m.c.hdp-1-163209.internal</A></PRE><P> Which indicates either you have not setup the Hostname for all your ambari agent hosts properly or they are not able to communicate to ambari server on the mentioned ports. So please check if from the ambari agent machines you are able to access the below mentioned port of ambari server?</P><PRE># telnet hadoop-m 8441 # telnet hadoop-m 8440</PRE><P>.</P><P> Also every host in the cluster should be configured with a proper FQDN/hostname so please check if you have the proper entry inside the /etc/hosts file inside every ambari agent machine. Also they should return proper FQDN when you run the following command on ambari-agent hosts:</P><PRE># hostname -f</PRE><P>.</P><P>Also pleas echeck from ambari agent machine if you are able to connect to ambari server using openssl command and able to see the certificate of ambari server?</P><PRE># openssl s_client -connect hadoop-m:8440 -tls1_2 OR # openssl s_client -connect hadoop-m:8440 AND # openssl s_client -connect hadoop-m:8441 -tls1_2 OR # openssl s_client -connect hadoop-m:8441 </PRE><P>.</P> Tue, 16 May 2017 22:53:07 GMT jsensharma 2017-05-16T22:53:07Z