Two customers reported on Intercom they could not connect to their MongoDB Atlas Cluster M0 instance using the MongoDB Go Driver.

I triaged their environment, ping, telnet, ports, connection string, whitelist all good. They can connect with the MongoShell but not the MongoDB Go Driver. I suspected this was SRV/DNS related but I got the same error with both the SRV and Standard Connection strings. 

I've reproduced their issue.

Loaner-2-Dublin:GoCode mongodb$ go run main.go
2019/05/13 11:46:52 server selection error: server selection timeout
current topology: Type: ReplicaSetNoPrimary
Servers:
Addr: mongodbcluster-shard-00-00-cqyvf.mongodb.net:27017, Type: Unknown, State: Connected, Average RTT: 0, Last error: (CommandNotFound) command not found
Addr: mongodbcluster-shard-00-01-cqyvf.mongodb.net:27017, Type: Unknown, State: Connected, Average RTT: 0, Last error: (CommandNotFound) command not found
Addr: mongodbcluster-shard-00-02-cqyvf.mongodb.net:27017, Type: Unknown, State: Connected, Average RTT: 0, Last error: (CommandNotFound) command not found
exit status 1

And diagnosed that this error is only produced when connecting to the M0 Cluster Tier. I haven't checked M2/M5. I get this result when connecting to M10

Loaner-2-Dublin:GoCode mongodb$ go run main.go
Connected to MongoDB!
Connection to MongoDB closed.

Attached is:

• Full reproduction of the issue with links to the clusters, port, telnet tests, MongoShell connection.
• Go Main Code based on the MongoDB Go Driver Tutorial posted on March 20, 2019, Password in connection string has been redacted https://www.mongodb.com/blog/post/mongodb-go-driver-tutorial-part-1-connecting-using-bson-and-crud-operations/