If you are connecting to a MongoDB Database on Aptible, either through your app or a Database Tunnel, you might hit an error such as this one: 
MongoDB shell version: 3.2.1
connecting to: 172.17.0.2:27017/db
2016-02-08T10:43:40.421+0000 E QUERY    [thread1] Error: network error while attempting to run command 'isMaster' on host '172.17.0.2:27017'  :
connect@src/mongo/shell/mongo.js:226:14
@(connect):1:6

exception: connect failed

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Cause

This error is usually caused by attempting to connect without SSL to a MongoDB server that requires it, which is the case on Aptible.

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Resolution

To solve the issue, connect to your MongoDB server over SSL.

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Clients

Connection URLs generated by Aptible include the ssl=true parameter, which should instruct your MongoDB client to connect over SSL. If your client does not connect over SSL despite this parameter, consult its documentation.

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CLI

📘 Make sure you use a hostname to connect to MongoDB databases when using a database tunnel. If you use an IP address for the host, certificate verification will fail. You can work with --sslAllowInvalidCertificates in your command line, but using a hostname is simpler and safer.
The MongoDB CLI client does not accept database URLs. Use the following to connect: 
mongo --ssl \
        --username aptible --password "$PASSWORD" \
        --host "$HOST" --port "$PORT"