Disabling online mode on your Minecraft server means that players who connect to your server won't be verified and can login with any username. Minecraft skins also won't be visible by default. This means that users who have cracked versions of Minecraft can join your server. This can be a security risk to your server, so modify the setting at your own risk.
1. Open the File Manager
2. Navigate to /minecraft/
3. Right click server.properties
and click Edit
4. Find the line that says online-mode
and set it to false
5. Restart the server