Both of these tools have the following output in their --help string:

--authenticationDatabase arg          user source (defaults to dbname)

I interpret this instruction as having two possible meanings (neither of which work):

1. dbname could refer to another command-line parameter. But there is no such other parameter to these two tools. The tools perform a global function so a default database has no meaning.
2. dbname could mean the literal string "dbname". If I create a user/pass for a database named "dbname" it works when I specify it as the --authenticationDatabase but not if I leave that parameter out.

I am forced to conclude that --authenticationDatabase is actually a required parameter for these two tools when authentication is enabled.

If there is a default database to auth against, I have not been able to find it. "admin" does not work. "test" does not work. "dbname" does not work.

The help output for these two tools should be adjusted and a sensible default should be given (or auth param made mandatory).