How to connect a client?

[occasl]

I'm confused how to connect a client to the MongoDB catalog replica set offering. I understand how to create it but how would a client connect to the primary of the replica set? I've tried setting up HAProxy LB to connect to it but it just round robins between the nodes in the cluster. Since the cluster nodes don't expose 27017, you can't set-up the usual mongodb URL to connect to the primary consistently.