ufw enable by default blocks all ports. but nomachine port is still open even after ufw enable

When configuring the Uncomplicated Firewall (UFW) on a Linux system, it's important to understand how it functions concerning ports and services, particularly when working with remote access tools like NoMachine. Below is an analysis of the situation where NoMachine's port remains open even after UFW has been enabled, which by default blocks all incoming ports.

Understanding UFW Default Behavior

UFW is designed to provide an easy interface for managing a firewall, with the default configuration set to deny all incoming connections. This means that when you enable UFW, all ports should be blocked unless explicitly allowed by specific rules.

Default Policy: The default policy for UFW is to deny all incoming traffic. This means no services are accessible unless configured otherwise.
Explicit Rules Needed: Users must create explicit rules to allow specific services or ports. For example, running sudo ufw allow <port> or sudo ufw allow <service> would permit traffic through the specified port/service.

NoMachine and Port Accessibility

NoMachine, a remote desktop tool, utilizes specific ports to provide access. If its port remains open after enabling UFW, there are several possible explanations:

1. UFW Configuration

No Explicit UFW Rules: If you have not defined a rule for NoMachine in UFW, and you still find its port open, it could be because UFW is not the only firewall handling incoming traffic. In some cases, installations might use tools like iptables which can conflict with UFW settings.
Resetting UFW Configuration: If UFW is behaving unexpectedly, consider resetting UFW to its default settings with the command:
```
sudo ufw reset
```
This command will clear all existing rules and set UFW back to its default state.

2. Check for Conflicting Firewalls

Additional Firewall Services: There might be another firewall service running on your system, such as iptables, that allows certain traffic regardless of UFW settings. To check if iptables is configured to allow the port used by NoMachine, you might run:
```
sudo iptables -L -n
```

3. Verify Port Status

Netstat Check: You can verify which ports are listening with:
```
sudo netstat -tuln
```
This command will display active connections and listening ports, helping you identify if NoMachine is still accessible.

4. NoMachine Configuration

Built-In Firewall Options: NoMachine has its own firewall settings that might enable access regardless of the UFW settings. Review the NoMachine server settings to ensure it is not configured to bypass local firewall rules.

Resolving Open Ports Issue

To ensure tighter control over port access and potentially close any unintended access:

Review and Adjust UFW Rules: Depending on your requirements for NoMachine, create or modify existing UFW rules while ensuring to only allow necessary ports:
```
sudo ufw allow from any to any port <nomachine_port>
```
Monitor IPTable and UFW: Regularly monitor both iptables and UFW to ensure they are aligned with your security policies.
Testing Access: Utilize tools like nc (netcat) to test port accessibility from different devices in your network.

Conclusion

If NoMachine’s port remains open despite enabling UFW, thoroughly check your UFW rules, potential conflicts with other firewall settings, and inspect the NoMachine configurations. By ensuring UFW is properly configured and observing potential interference from other tools, you can maintain a secure firewall setup tailored to your needs. Regular checks and adjustments can help ensure that your server is not inadvertently exposed to unwanted access. For further assistance, consider consulting UFW documentation or community forums for specific scenarios.