[authentication] How to secure MongoDB with username and password

This is what I did on Ubuntu 18.04:

$ sudo apt install mongodb
$ mongo
> show dbs
> use admin
> db.createUser({  user: "root",  pwd: "rootpw",  roles: [ "root" ]  })  // root user can do anything
> use lefa
> db.lefa.save( {name:"test"} )
> db.lefa.find()
> show dbs
> db.createUser({  user: "lefa",  pwd: "lefapw",  roles: [ { role: "dbOwner", db: "lefa" } ]  }) // admin of a db
> exit
$ sudo vim /etc/mongodb.conf
auth = true
$ sudo systemctl restart mongodb
$ mongo -u "root" -p "rootpw" --authenticationDatabase  "admin"
> use admin
> exit
$ mongo -u "lefa" -p "lefapw" --authenticationDatabase  "lefa"
> use lefa
> exit