Mongo Super User won't connect to another database other than admin

Why is that?

$ mongo admin -u admin -p password --host 192.168.99.100
MongoDB shell version: 3.2.9
connecting to: 192.168.99.100:27017/admin
> show users
{
    "_id" : "admin.admin",
    "user" : "admin",
    "db" : "admin",
    "roles" : [
        {
            "role" : "userAdminAnyDatabase",
            "db" : "admin"
        },
        {
            "role" : "dbAdminAnyDatabase",
            "db" : "admin"
        },
        {
            "role" : "readWriteAnyDatabase",
            "db" : "admin"
        }
    ]
}
> db^C
bye
cesco@laptop: ~/code/go/src/bitbucket.org/cescoferraro/cluster/containers on develop [!$]
$ mongo iot -u admin -p password --host 192.168.99.100
MongoDB shell version: 3.2.9
connecting to: 192.168.99.100:27017/iot
2016-09-25T22:22:36.829-0300 E QUERY    [thread1] Error: Authentication failed. :
DB.prototype._authOrThrow@src/mongo/shell/db.js:1441:20
@(auth):6:1
@(auth):1:2

exception: login failed

Although the admin user you've created has global permissions through *AnyDatabase roles, the user credentials only exist in the admin database. As per your example, you will fail to authenticate if matching user credentials don't exist in the database you are trying to access.

To authenticate with the mongo shell using credentials stored in a different database from the one which you are trying to access (admin vs iot) you can either:

1) Specify the database with user credentials using --authenticationDatabase:

mongo iot --authenticationDatabase admin -u admin -p password --host 192.168.99.100

2) Specify the default database as /admin and then change to the desired target database:

$ mongo -u admin -p password 192.168.99.100/admin

 MongoDB shell version: 3.2.9
 connecting to: 192.168.99.100/admin

 > use iot
 switched to db iot

The first example is the most typical approach.

NOTE: If you want to specify host and database as per the second example, you have to provide the database connection string without using the --host parameter.

StackExchange Database Administrators Q#150615, answer score: 5