Running a Minecraft server in offline mode allows players to connect without authenticating with Mojang's servers. This can be useful for LAN parties, testing environments, or when an internet connection is unavailable. However, it's essential to implement security measures to prevent unauthorized access and ensure a safe gaming environment.

Requirements:

1. Minecraft Server Software: Download the server software from the official Minecraft website or use a third-party alternative like Spigot or Paper.

2. Java: Ensure that Java is installed on your system since Minecraft servers run on Java.

Setting Up an Offline Mode Server:

1. Download and Install Minecraft Server:

   - Go to the official Minecraft website or a trusted third-party site.

   - Download the server software that corresponds to your desired Minecraft version.

   - Create a new folder for your server and place the downloaded server file in it.

   - Run the server file to generate the necessary files.

2. Configure Server Properties:

   - Open the `server.properties` file using a text editor.

   - Set `online-mode` to `false`. This allows players to connect without a Mojang account.

   - Adjust other server settings as needed.

3. Start the Server:

   - Run the server by executing the server file (e.g., `java -jar minecraft_server.jar`).

   - Allow the server to complete its initial setup.

4. Share Server IP:

   - Players can now connect using your local IP address or external IP if hosting online.

   - Provide the IP address to others on your LAN or, if applicable, open the necessary ports on your router for online play.

Securing the Offline Mode Server:

1. Whitelist:

   - While connected to the server enable whitelist by typing: /whitelist on.

   - After that add the players by typing: /whitelist add player.

   - Or optionally create a whitelist by editing the `whitelist.json` file. Add the Minecraft usernames of players allowed to connect.

   - This prevents unauthorized users from joining your server.

2. Firewall Settings:

   - Configure your firewall to only allow traffic on the necessary ports (default is 25565). Block other ports to enhance security.

3. Regular Backups:

   - Regularly back up your server files to prevent data loss in case of unexpected issues or unauthorized access.

4. Plugin-Based Security (Optional):

   - Use server plugins like Vault, PermissionsEx, or LuckPerms to manage player permissions and enhance security.

   - Plugins can help you control who can build, break blocks, or use certain commands.

5. Anti-Botting Plugin:

   - Implement an anti-botting plugin like NoCheatPlus to detect and prevent automated actions that could disrupt the gameplay experience.

6. Common Problems:

   - Authentication Issues: Ensure that the `online-mode` property is set to `false` in the `server.properties` file.

   - Firewall Blocks: Check your firewall settings to ensure that the server port (default 25565) is open.

   - Plugin Compatibility: Verify that plugins are compatible with your server version. Outdated or incompatible plugins can cause issues.

   - Progress is Deleted: Check for proper backup procedures, and investigate whether players are accidentally resetting their progress.

   - Cannot Join Server: Ensure that the server is running, and players are using the correct IP address.

   - Login with any Account: Implement stricter authentication measures, and consider using plugins like AuthMeReloaded to prevent unauthorized logins.

Always check plugin compatibility with your server version, and read reviews or documentation to ensure they meet your specific needs. Regularly update plugins to benefit from bug fixes and security updates. Remember that a well-maintained and secure server provides a better gaming experience for all players.