MongoDB 3.2 authentication failed

Well, you'll need to take couple of steps in sequence to create user successfully.

First of all, you need to create an administrator user. I prefer creating super user.

> use admin
> db.createUser({user: "root", pwd: "123456", roles:["root"]})

Restart your MongoDB server and enable authentication with --auth flag.

> mongod --auth --port 27017 --dbpath /var/lib/mongodb

Once your server is up, connect to it as administrator

> mongo <host:port> -u "root" -p "123456" --authenticationDatabase "admin"

Once you are connected, create normal user. Assuming your user database name is cd2.

> use cd2
> db.createUser({user: "cd2", pwd: "cd2", roles:["dbOwner"]})

If you see success messsage, disconnect from mongo shell and reconnect with new user credentials.

> mongo <host:port>/cd2 -u "cd2" -p "cd2"

If you log in through shell, make sure your create user under db "admin", NOT under a customized db. In your case you switched to "c2d".

Here is what I have tried (Log in as "admin")

1. This one will work:

$ mongo -u admin -p --authenticationDatabase "admin"
> use admin
> db.createUser(
  {
    user: "user007",
    pwd: "YourP@ssw0rd",
    roles: [
       { role: "readWrite", db: "yourdb" },
    ]
  }
)

Output 1:

root@mongo-server:/# mongo -u admin -p --authenticationDatabase "admin"
MongoDB shell version v4.0.6
Enter password:
connecting to: mongodb://127.0.0.1:27017/?authSource=admin&gssapiServiceName=mongodb
MongoDB server version: 4.0.6
----
> use admin
switched to db admin
> db.createUser(
...   {
...     user: "user007",
...     pwd: "YourP@ssw0rd",
...     roles: [
...        { role: "readWrite", db: "yourdb" },
...     ]
...   }
... )
Successfully added user: {
        "user" : "user007",
        "roles" : [
                {
                        "role" : "readWrite",
                        "db" : "yourdb"
                }
        ]
}

root@mongo-server:/# mongo -u user007 -p YourP@ssw0rd

connecting to: mongodb://127.0.0.1:27017/?gssapiServiceName=mongodb
Implicit session: session { "id" : UUID("ceabf440-c584-4518-90f5-cc845eaec3b3") }
MongoDB server version: 4.0.6
---
>

2. This one will fail:

$ mongo -u admin -p --authenticationDatabase "admin"
> use yourdb
> db.createUser(
  {
    user: "user007",
    pwd: "YourP@ssw0rd",
    roles: [
       { role: "readWrite", db: "yourdb" },
    ]
  }
)

Output 2:

root@mongo-server:/# mongo -u admin -p --authenticationDatabase "admin"
MongoDB shell version v4.0.6
Enter password:
connecting to: mongodb://127.0.0.1:27017/?authSource=admin&gssapiServiceName=mongodb
MongoDB server version: 4.0.6
----
> use yourdb
switched to db yourdb
> db.createUser(
...   {
...     user: "user007",
...     pwd: "YourP@ssw0rd",
...     roles: [
...        { role: "readWrite", db: "yourdb" },
...     ]
...   }
... )
Successfully added user: {
        "user" : "user007",
        "roles" : [
                {
                        "role" : "readWrite",
                        "db" : "yourdb"
                }
        ]
}
>

root@mongo-server:/# mongo -u user007 -p YourP@ssw0rd
MongoDB shell version v4.0.6
connecting to: mongodb://127.0.0.1:27017/?gssapiServiceName=mongodb
2019-12-06T04:28:34.630+0800 E QUERY    [js] Error: Authentication failed. :
connect@src/mongo/shell/mongo.js:343:13
@(connect):1:6
exception: connect failed

Our experience with this issue on MongoDB (3.2.4) is that it seems a bug or a undocumented feature. If you attempt to create the users and roles from a remote shell (not localhost), the users are created but as soon as you leave the shell they are gone (they are transient to that session) and not really persisted.

Solution: Just try to create your users (without authentication enabled initially on MongoDB) and do it directly on the console of your DB server (localhost).