A white list is your primary means of preventing unauthorized entry into your server. Any Minecraft usernames that you put in this list will be allowed to enter your server, while anyone not on it will be denied access.

ACTIVATING/DEACTIVATING THE WHITE LIST

In order to activate/deactivate the white list, you must first log in to your game panel.
Once in, click on "Configuration files". In the server.properties file, you can enable or disable the whitelist.
Make sure to restart your server after saving the settings on that page.

ADDING USERS TO THE WHITE LIST - Method 1 - Using the Server's console (Web Console)

You can add a user via console or in the game itself.

You will need to click on "Web Console" in the game panel in the navigation menu.

You can add users to the white list by typing into the console the following: whitelist add username

All Whitelist commands:

whitelist add <targets> (Java Edition)

Adds the player profile(s) to the whitelist. The player does not need to be online.

whitelist list (Java Edition)

Displays all player profiles in the whitelist.

whitelist off(Java Edition)

Disables the server's use of a whitelist.

whitelist on(Java Edition)

Enables the server's use of a whitelist.

whitelist reload(Java Edition)

Reloads the list of player profiles in whitelist.json from disk (used when whitelist.json has been modified outside of Minecraft).

whitelist remove <targets> (Java Edition)

Removes the player profile(s) from the whitelist. The player does not need to be online.

ADDING USERS TO THE WHITE LIST - Method 2 - Using the whitelist.json configuration file

You can edit the whitelist.json file by either editing it in the configuration files menu. 

IMPORTANT: Before you can add a user to the whitelist, make sure to first allow that user to join your server with the whitelist off so that the server "knows" their UUID/username.
Once that is done, you will then be able to re-enable whitelisting and add them as described below.

EXAMPLE SYNTAX / FORMAT FOR THE WHITELIST.json (You can get uuid's from here http://mcuuid.net/)

whitelist.json

[
   {
      "uuid":"uuid-goes-here",
      "name":"notch"
   },
   {
      "uuid":"uuid-goes-here",
      "name":"user1"
   },
   {
      "uuid":"uuid-goes-here",
      "name":"player1"
   }
]

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