Major - P3 After changing the DNS entry and IP for a shard (by removing it from /etc/hosts), the mongos process continued to look for the old IP. DNS should be re-resolved at "trying reconnect" step.
Partial logs attached, restarting the mongos process fixed the problem.
Tue Dec 27 19:35:54 [WriteBackListener] WriteBackListener exception : socket exception Tue Dec 27 19:35:54 [Balancer] reconnect mongoshard05:27017 failed couldn't connect to server mongoshard05:27017 Tue Dec 27 19:35:54 [Balancer] ReplicaSetMonitor::_checkConnection: caught exception mongoshard05:27017 socket exception Tue Dec 27 19:35:54 [Balancer] trying reconnect to mongoshard06:27017 Tue Dec 27 19:35:57 [WriteBackListener] WriteBackListener exception : socket exception Tue Dec 27 19:35:59 [Balancer] reconnect mongoshard06:27017 failed couldn't connect to server mongoshard06:27017 Tue Dec 27 19:35:59 [Balancer] ReplicaSetMonitor::_checkConnection: caught exception mongoshard06:27017 socket exception Tue Dec 27 19:35:59 [ReplicaSetMonitorWatcher] trying reconnect to mongoshard04:27017 Tue Dec 27 19:36:00 [Balancer] ~ScopedDbConnection: _conn != null Tue Dec 27 19:36:00 [Balancer] caught exception while doing balance: socket exception Tue Dec 27 19:36:04 [ReplicaSetMonitorWatcher] reconnect mongoshard04:27017 failed couldn't connect to server mongoshard04:27017 Tue Dec 27 19:36:04 [ReplicaSetMonitorWatcher] trying reconnect to mongoshard05:27017 Tue Dec 27 19:36:06 [WriteBackListener] WriteBackListener exception : socket exception Tue Dec 27 19:36:09 [ReplicaSetMonitorWatcher] reconnect mongoshard05:27017 failed couldn't connect to server mongoshard05:27017 Tue Dec 27 19:36:09 [ReplicaSetMonitorWatcher] trying reconnect to mongoshard06:27017 Tue Dec 27 19:36:11 [WriteBackListener] WriteBackListener exception : socket exception Tue Dec 27 19:36:12 [WriteBackListener] WriteBackListener exception : socket exception Tue Dec 27 19:36:14 [ReplicaSetMonitorWatcher] reconnect mongoshard06:27017 failed couldn't connect to server mongoshard06:27017 Tue Dec 27 19:36:15 [ReplicaSetMonitorWatcher] trying reconnect to mongoshard04:27017 Tue Dec 27 19:36:15 [WriteBackListener] WriteBackListener exception : socket exception Tue Dec 27 19:36:16 [WriteBackListener] WriteBackListener exception : socket exception Tue Dec 27 19:36:16 [WriteBackListener] WriteBackListener exception : socket exception Tue Dec 27 19:36:19 [WriteBackListener] WriteBackListener exception : socket exception Tue Dec 27 19:36:20 [ReplicaSetMonitorWatcher] reconnect mongoshard04:27017 failed couldn't connect to server mongoshard04:27017 Tue Dec 27 19:36:20 [ReplicaSetMonitorWatcher] ReplicaSetMonitor::_checkConnection: caught exception mongoshard04:27017 socket exception Tue Dec 27 19:36:20 [ReplicaSetMonitorWatcher] trying reconnect to mongoshard05:27017 Tue Dec 27 19:36:22 [WriteBackListener] WriteBackListener exception : socket exception Tue Dec 27 19:36:23 [WriteBackListener] WriteBackListener exception : socket exception Tue Dec 27 19:36:25 [ReplicaSetMonitorWatcher] reconnect mongoshard05:27017 failed couldn't connect to server mongoshard05:27017 Tue Dec 27 19:36:25 [ReplicaSetMonitorWatcher] ReplicaSetMonitor::_checkConnection: caught exception mongoshard05:27017 socket exception Tue Dec 27 19:36:25 [ReplicaSetMonitorWatcher] trying reconnect to mongoshard06:27017