To keep your Factorio server safe from troublemakers who might wreck everything you've built, it's a good idea to keep an eye on who can get in. Make sure to slap a password on it to keep the bad guys out.
Setting a Factorio Server Password
Factorio servers come with a large array of Server settings which can be configured on the command line (See here for a full list of Factorio Server CLI Commands). One of these commands is the server-game-password argument.
For users on ServerFlex, this can be easily configured in your server settings:
This setting will add a password requirement to users joining your server. Instead of users being able to type in the IP address, port and then join, they will be prompted to enter a password.
We recommend rotating this password frequently and change it immediately if you suspect users who shouldn't be allowed to join, have a copy.