How to Diagnose Firewall and Port Blocking Issues ? A Complete Guide (2025)

Table of Contents

• What Are Firewalls and How Do They Block Ports?
• Understanding Port Status: Open, Closed, Filtered
• Diagnosing Port Issues with Tools
• Diagnosing NAT Issues
• Steps to Diagnose and Fix Port Blocking
• Real-World Example: Diagnosing SSH Blockages
• Best Practices for Troubleshooting
• Conclusion
• Frequently Asked Questions (FAQs)

Diagnosing firewall and port blocking issues is a critical skill for system administrators, cybersecurity professionals, and network engineers. Whether you're dealing with connection timeouts, failed application deployments, or unexpected service behavior, understanding how ports and firewalls interact with NAT (Network Address Translation) can save hours of troubleshooting.

In this blog, we’ll explore step-by-step how to identify and resolve common port blocking issues using tools like nmap, telnet, and nc (netcat). We’ll also explain how NAT can impact port visibility and what that means for your security posture and connectivity.

What Are Firewalls and How Do They Block Ports?

Firewalls are security systems—either hardware-based, software-based, or a combination of both—that monitor and control incoming and outgoing network traffic. They determine which ports are open or closed based on pre-defined security rules.

Blocked ports can prevent access to websites, applications, email services, or any remote server depending on the protocol and port being used.

Common Causes of Port Blocking

• Misconfigured firewall rules

• ISP restrictions on certain ports

• NAT masking or port forwarding misconfigurations

• Operating system-level firewalls (e.g., iptables, Windows Defender Firewall)

  
  

• Cloud security group rules (e.g., AWS, Azure, GCP)

Understanding Port Status: Open, Closed, Filtered

Diagnosing Port Issues with Tools

1. Using Nmap

nmap is one of the most powerful tools for scanning ports and identifying whether a firewall or NAT is interfering with network traffic.

nmap -p 80,443,22 192.168.1.1

This scans common ports (HTTP, HTTPS, SSH) on a host. The output will tell you if the port is:

• open

• closed

• filtered

Example Advanced Command:

nmap -sS -Pn -p- --reason 192.168.1.100

• -sS: TCP SYN scan (stealth)

• -Pn: Skip ping

• -p-: Scan all 65535 ports

• --reason: Shows why Nmap made a decision about the port state

2. Using Telnet

telnet can help quickly check whether a port is reachable:

telnet yourdomain.com 443

• If the screen clears or you get a response like Connected, the port is open.

• If it hangs or returns an error, the port is closed or filtered.

Note: Telnet does not encrypt traffic; use only in trusted networks or for testing.

3. Using Netcat (nc)

nc is a versatile tool for testing connectivity:

nc -zv 192.168.1.1 22

• -z: Zero I/O mode (only scan)

• -v: Verbose

You’ll get output like:

Connection to 192.168.1.1 22 port [tcp/ssh] succeeded!

This is useful when scripting port checks across multiple hosts.

Diagnosing NAT Issues

Network Address Translation (NAT) can cause connectivity issues, especially with port forwarding or dynamic IPs. Here’s how to investigate:

Use nmap from an External Host

If scanning from within the network shows ports as open, but they’re filtered or closed from the outside, then NAT or firewall settings on the gateway/router are likely the issue.

Check Router Port Forwarding Settings

• Verify that the correct internal IP and port are mapped.

• Ensure that the firewall on the gateway allows inbound traffic.

Use Online Port Check Tools

You can use tools like canyouseeme.org to check port exposure from the public internet.

Steps to Diagnose and Fix Port Blocking

Real-World Example: Diagnosing SSH Blockage

You're unable to SSH into your Linux server hosted behind a NAT-enabled router.

Diagnosis Steps:

1. On local LAN:

   nmap -p 22 192.168.0.100
   

   → Port 22 is open.

2. From external device:

   nmap -p 22 your.public.ip
   

   → Port 22 is filtered.

3. Solution:

   • Log in to router admin page.

   • Set port forwarding: External Port 22 → Internal IP 192.168.0.100:22.

   • Allow inbound traffic on firewall.

Best Practices for Troubleshooting

• Always scan from both internal and external networks.

• Disable firewall temporarily only for testing (and re-enable afterward).

• Use secure methods (like SSH or HTTPS) and avoid exposing unnecessary ports.

• Document firewall and NAT rules for easy auditing.

Conclusion

Diagnosing firewall and port blocking issues doesn’t need to be overwhelming. By combining tools like nmap, telnet, and nc, along with a structured approach to checking firewall and NAT configurations, you can identify and resolve most connectivity problems. Proper visibility into open, closed, and filtered ports is key to ensuring both access and security in your network infrastructure.

FAQs

What is a blocked port?

A blocked port is a network port that is closed by a firewall or NAT, preventing inbound or outbound traffic through that port.

How can I check if a port is open?

You can use tools like nmap, telnet, or nc (netcat) to scan a port and verify whether it is open and accessible.

What is the difference between open, closed, and filtered ports?

• Open: Accepts connections.

• Closed: Rejects connections.

• Filtered: Blocked by firewall/NAT; no response.

What does nmap -p 80 do?

It scans port 80 on the target IP to see if it's open, closed, or filtered.

Why do some ports show as filtered in Nmap?

It usually means a firewall or router is silently dropping packets on that port.

How do I use netcat to check if a port is open?

Use: nc -vz to test TCP connectivity to a port.

Can Telnet be used for port testing?

Yes. Use: telnet , but it works only for TCP ports.

What is NAT and how does it affect ports?

NAT (Network Address Translation) can prevent external traffic from reaching internal systems unless port forwarding is configured.

How to fix NAT-related port blocking?

Log into your router/firewall and enable port forwarding for the required port.

What does “Connection refused” mean?

The port is closed or no service is listening on the specified port.

Is it safe to open ports on my firewall?

Only if you know the application and it’s secure. Never open ports without purpose.

Can Windows Firewall block specific ports?

Yes. You can create inbound or outbound rules to block or allow traffic on specific ports.

How do I check firewall rules on Linux?

Use iptables -L or firewalld/ufw commands depending on your firewall manager.

Why is a port open on LAN but not reachable over the internet?

Firewall or router settings might be blocking WAN access, or NAT isn’t configured properly.

What tools help visualize open and closed ports?

Tools like Nmap, Zenmap (GUI), and Netcat are widely used for visualizing port states.

How do I test UDP ports?

Use nmap -sU, or specialized tools like nping since Netcat doesn't support UDP by default.

How can I verify if a port is blocked on a remote server?

Use Nmap or Telnet from an external machine to test remote connectivity.

What is a stealth firewall?

A firewall that silently drops traffic, making ports appear “filtered” in scans.

Can antivirus software block ports?

Yes. Many AVs include firewalls that can interfere with port traffic.

What is a common sign of firewall blocking?

Timeouts, dropped packets, or services being inaccessible even when running.

Can ISP block certain ports?

Yes, especially ports like 25 (SMTP) or 445 (SMB) for security reasons.

How to open a port on a Windows machine?

Use Windows Defender Firewall settings > Advanced settings > New Rule.

What is ICMP and does it relate to port checking?

ICMP is used for pinging, not port testing, but some firewalls block both ICMP and port traffic.

What is a SYN scan in Nmap?

A stealthy scan (-sS) to detect open ports without completing a TCP handshake.

Should I allow ping requests?

For diagnostics yes, but from security perspective, it’s better to restrict them.

Can I test multiple ports at once?

Yes, use nmap -p 21,22,80,443 to test multiple ports.

How do I check if NAT is blocking my VPN port?

Use external scanners like canyouseeme.org or Nmap from an outside network.

Why is port 80 open but HTTP not working?

Possible causes: misconfigured server, firewall interference, or application crash.

Can routers have hidden firewall rules?

Yes, many consumer routers include default rules that block incoming ports.

Is there a GUI alternative to Nmap?

Zenmap provides a user-friendly graphical interface for Nmap.