Why mounting NFSv4 filesystem is returning "Permission denied" ?

Issue

• While configuring NFSv4 with kerberos, when at the final stage of the process while trying to mount the exported filesystem on the client, following errors were seen:

# mount -a 
Warning: rpc.idmapd appears not to be running.
         All uids will be mapped to the nobody uid.
Warning: rpc.gssd appears not to be running.
mount.nfs4: Permission denied

Both warnings above are not to be considered as they will show up even if both services are running. Nevertheless, its always good to double check.

Looking in /var/log/messages showed us these messages:

May 20 11:35:39 client rpc.gssd[4369]: ERROR: No usable machine credentials obtained 
May 20 11:35:39 client rpc.gssd[4369]: WARNING: Failed to obtain machine credentials for connection to server server.example.com

To obtain more verbose output, we enabled the rpcgssd verbose mode by uncommenting the RPCGSSDARGS="-vvv" in /etc/sysconfig/nfs and afterwards here is what we see in the /var/log/messages:

WARNING: Decrypt integrity check failed while getting initial ticket for principal 'nfs/client.example.com@EXAMPLE.COM' from keytab 'FILE:/etc/krb5.keytab' 
May 20 16:48:17 client rpc.gssd[5955]: ERROR: No usable machine credentials obtained 
May 20 16:58:31 client rpc.gssd[5955]: handling krb5 upcall 
May 20 16:58:31 client rpc.gssd[5955]: Using keytab file '/etc/krb5.keytab' 
May 20 16:58:31 client rpc.gssd[5955]: WARNING: Decrypt integrity check failed while getting initial ticket for principal 'nfs/client.example.com@EXAMPLE.COM' from keytab 'FILE:/etc/krb5.keytab' 
May 20 16:58:31 client rpc.gssd[5955]: ERROR: No usable machine credentials obtained 
May 20 16:58:31 client rpc.gssd[5955]: WARNING: Failed to obtain machine credentials for connection to server server.example.com 
May 20 16:58:31 client rpc.gssd[5955]: doing error downcall 
May 20 16:58:31 client rpc.gssd[5955]: destroying client clnt1d 
May 20 16:58:33 client rpc.gssd[5955]: destroying client clnt1c