MongoDB Driver 3.6.3 throws `UnhandledPromiseRejectionWarning: MongoServerSelectionError: Server selection timed out after 30000 ms` if you connect to the MongoDB server that has a replication set when used with useUnifiedTopology.

 

Complete reproduction repo: https://github.com/rmehner/mongo-3.6.3-bug

Steps to reproduce:

1. Install MongoDB driver 3.6.3
2. Start MongoDB server 4.4.3 with `mongod --bind_ip_all --replSet rs0`
3. Connect to MongoDB via node:

```js

const { MongoClient, connect } = require("mongodb");

const client = new MongoClient("mongodb://localhost:27017",

{ useUnifiedTopology: true, }

);

(async () =>

{ const connection =await client.connect(); console.log("If you see this, the connection was successful �"); await connection.close() process.exit(0); }

)();
```

1. 1. Expected

Successful connection and the message "If you see this, the connection was successful �"

1. 1. Actual

UnhandledPromiseRejectionWarning: MongoServerSelectionError: Server selection timed out after 30000 ms

1. 1. Fixes

Either downgrade to driver version 3.6.2 or remove the useUnifiedTopology option in the connection options.

 

Thank you for looking into this