Why does my MongoDB Atlas connection fail?

Most commonly: 1) Your server IP is not whitelisted in Atlas Network Access. 2) Password has special characters not URL-encoded. 3) Using the wrong connection string format (use the SRV format from Atlas dashboard).

How do I connect to a MongoDB replica set?

List all members in the connection string: 'mongodb://host1:27017,host2:27017,host3:27017/mydb?replicaSet=myrs'. Or use SRV format which resolves members automatically: 'mongodb+srv://host/mydb'.

MongoDB Connection Errors — Authentication Failed and Connection String Issues

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Overview

Fix MongoDB connection errors including authentication failures, connection string parsing issues, replica set connectivity, and TLS configuration problems.

Key Details

MongoDB uses connection strings in URI format: mongodb://user:pass@host:port/database
Authentication database may differ from the application database (authSource parameter)
Replica sets require all members listed in the connection string or SRV DNS records
MongoDB Atlas uses SRV connection strings: mongodb+srv://user:pass@cluster.xxxxx.mongodb.net/
Special characters in passwords must be URL-encoded in the connection string

Common Causes

Authentication database mismatch — user created in 'admin' but connecting to application database
Special characters in password not URL-encoded (@ becomes %40, : becomes %3A)
Replica set member unreachable from the application server
MongoDB Atlas IP whitelist not including the application server's IP address

Steps

1URL-encode special characters in the password: use encodeURIComponent() in Node.js or equivalent
2Add authSource parameter: 'mongodb://user:pass@host:27017/mydb?authSource=admin'
3For MongoDB Atlas: whitelist your server IP in Atlas > Network Access > Add IP Address
4Test connection from command line: 'mongosh "mongodb://user:pass@host:27017/mydb?authSource=admin"'
5Check MongoDB logs: '/var/log/mongodb/mongod.log' for authentication and connection error details

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Frequently Asked Questions

authSource specifies which database contains the user credentials. By default, users are created in the 'admin' database. Add ?authSource=admin to your connection string if your user is in admin but you are connecting to a different database.