[mongodb] Error: network error while attempting to run command 'isMaster' on host '127.0.0.1:27017'

[Question]

 .> mongo

MongoDB shell version v3.6.5 connecting to: mongodb://127.0.0.1:27017 2018-06-26T17:37:13.313+0530 I NETWORK [thread1] Socket recv() An established connection was aborted by the software in your host machine. 127.0.0.1:27017 2018-06-26T17:37:13.313+0530 I NETWORK [thread1] SocketException: remote: (NONE):0 error: SocketException socket exception [RECV_ERROR] server [127.0.0.1:27017] 2018-06-26T17:37:13.313+0530 E QUERY [thread1] Error: network error while attempting to run command 'isMaster' on host '127.0.0.1:27017' : connect@src/mongo/shell/mongo.js:251:13 @(connect):1:6 exception: connect failed

[Answer]

See Mongod logs to find the root cause. For me it was leaking connections from one of the clients, this is what I found in logs:  NETWORK [listener] connection refused because too many open connections:

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