Some Mongo database servers do not give average accounts access to the admin database. This causes problems when attempting to make an authorized connection to a normal database.

When attempting to connect to such a machine, I get errors like this:

{{Jan 11, 2012 3:56:24 PM com.mongodb.DBTCPConnector fetchMaxBsonObjectSize
WARNING: null
java.io.IOException: couldn't connect to [/0.0.39.87:27017] bc:java.net.SocketException: Invalid argument or cannot assign requested address
at com.mongodb.DBPort._open(DBPort.java:206)
at com.mongodb.DBPort.go(DBPort.java:94)
at com.mongodb.DBPort.go(DBPort.java:75)
at com.mongodb.DBPort.findOne(DBPort.java:129)
at com.mongodb.DBPort.runCommand(DBPort.java:138)
at com.mongodb.DBTCPConnector.fetchMaxBsonObjectSize(DBTCPConnector.java:409)
at com.mongodb.DBTCPConnector.checkMaster(DBTCPConnector.java:396)
at com.mongodb.DBTCPConnector.call(DBTCPConnector.java:192)
at com.mongodb.DBApiLayer$MyCollection.__find(DBApiLayer.java:295)
at com.mongodb.DB.command(DB.java:152)
at com.mongodb.DB._doauth(DB.java:496)
at com.mongodb.DB.authenticate(DB.java:433)
at com.mongodb.casbah.MongoDB.authenticate(MongoDB.scala:74)}}

I can't say what the exact solution is, but it appears that fetchMaxBsonObjectSize() returns a null in this situation, with its unexpected presence (an int is expected) then causing the exception in later commands like DB.authenticate().