We just ran into an issue joining a new server to our MongoDB replica set powering Where’s it Up & ShotSherpa. We copied the data over to give it a good starting point, then added it to the replica set. We waited a while, it still hadn’t joined, looking at the tail of the log showed many connection lines, nothing telling. When I restarted MongoDB I saw:

I tried pinging the server’s hostname to ensure it was successful. No problem. I tried connecting to the server using the mongo command line on the primary server (`mongo server.example.com:27017`). No problem.

Eventually I copied the mongo hostname from the replica set configuration, and tried to use it to connect to mongo on the new secondary. No dice! As it turns out the NAT hadn’t been configured to allow that hostname to work locally. A few new entries in our /etc/hosts file later, and our server was joined successfully.

MongoDB Tip:

When MongoDB has trouble connecting, try the connection strings listed in the replica set config from all relevant hosts.