How to Distinguish Between ISP Blocking and Server Port Status for Port 53

When testing network connectivity, such as checking if port 53 (commonly used for DNS) on a server like 219.156.123.221 is accessible, you may need to determine whether the port is open on the server, blocked by your Internet Service Provider (ISP), or affected by local firewall settings. A successful connection (e.g., TcpTestSucceeded: True in PowerShell) suggests the port is open and accessible, but if you suspect the ISP might be blocking it while the server keeps it open, specific steps can help you differentiate these scenarios. This guide outlines methods to clarify the situation.

Understanding the Baseline

Using PowerShell’s Test-NetConnection -ComputerName 219.156.123.221 -Port 53, a result of TcpTestSucceeded: True indicates:

• The server (219.156.123.221) has port 53 open.
• Your ISP isn’t blocking outbound connections to port 53.
• Your local firewall allows the connection.

If the result were False, you’d need to investigate further to distinguish between ISP blocking and the server’s port status. Below are steps to confirm if the ISP is blocking port 53 while the server keeps it open.

1. Switch Networks for Comparison

Steps:

• Disconnect from your current network (e.g., Wi-Fi) and connect to a different one, such as a mobile hotspot (using cellular data), a friend’s Wi-Fi, or another ISP’s network.
• Run the following in PowerShell:

  Test-NetConnection -ComputerName 219.156.123.221 -Port 53
  

Analysis:

• Current network False, new network True: Your ISP likely blocks port 53 outbound, while the server’s port 53 is open (accessible via other networks).
• Both networks True: ISP isn’t blocking port 53, and the server port is open.
• Both networks False: The server may not have port 53 open, or other factors (e.g., server firewall) are at play.

Why It Works:

Different networks, especially those with different ISPs, help isolate ISP-specific blocking.

2. Test with a VPN

Steps:

• Enable a VPN service (e.g., NordVPN, ExpressVPN) and connect to a server in another region or country to bypass your ISP’s restrictions.
• Run the test again:

  Test-NetConnection -ComputerName 219.156.123.221 -Port 53
  

Analysis:

• Without VPN False, with VPN True: Strong evidence your ISP blocks port 53 outbound, while the server’s port 53 is open.
• Both True: ISP isn’t blocking, and the server port is open.
• Both False: The server’s port 53 may not be open, or the VPN restricts traffic.

Note:

Use a reliable VPN that doesn’t limit port 53 traffic.

3. Test a Public Server’s Port 53

Steps:

• Test a known open port 53 on a public DNS server, like Google’s 8.8.8.8 or Cloudflare’s 1.1.1.1:

  Test-NetConnection -ComputerName 8.8.8.8 -Port 53
  

Analysis:

• 8.8.8.8 True, 219.156.123.221 False: Your ISP may selectively block port 53 for specific IPs (e.g., 219.156.123.221), while the server port could still be open.
• Both True: ISP isn’t blocking port 53, and the server port is open.
• Both False: ISP may broadly block port 53 outbound.

Why It Works:

Public DNS servers are reliable benchmarks, as their port 53 is typically open.

4. Use Online Port Scanning Tools

Steps:

• Visit tools like https://www.yougetsignal.com/tools/open-ports/ or https://ping.eu/port-chk/.
• Enter the IP address 219.156.123.221 and port 53 to scan.

Analysis:

• Tool shows Open, your test False: Your ISP blocks port 53 outbound, but the server’s port 53 is open (external tools bypass your ISP).
• Tool shows Open, your test True: ISP isn’t blocking, and the server port is open.
• Tool shows Closed: The server’s port 53 may not be open, requiring further server-side checks.

Caution:

Scanning someone else’s server without permission may be illegal; ensure you have authorization for 219.156.123.221.

5. Contact Relevant Parties

ISP Inquiry:

• Ask your ISP if they block port 53 outbound, either generally or for specific IPs like 219.156.123.221.
• Some ISPs restrict DNS ports to redirect traffic to their own servers.

Server Admin Check:

• If you have access to 219.156.123.221, confirm port 53’s status.
• On the server (if accessible), run:

  netstat -tuln | grep 53
  

• If port 53 is listed as listening, the server isn’t blocking it.

Key Indicators of ISP Blocking, Server Not Blocking

• Your test (Test-NetConnection) returns False on current network.
• Tests via other networks or VPN return True.
• Online tools report port 53 as “Open” for 219.156.123.221.
• Public DNS servers (e.g., 8.8.8.8) may return False (if ISP blocks broadly) or True (if blocking is IP-specific).

Important Notes

• Permissions: Scanning ports on servers like 219.156.123.221 without authorization may be illegal; ensure you have permission.
• Local Firewall: Check Windows Defender Firewall (Control Panel > System and Security > Windows Defender Firewall > Outbound Rules) or third-party security software for port 53 restrictions.
• Server Functionality: An open port 53 doesn’t guarantee a working DNS service; test with:

  Resolve-DnsName -Name example.com -Server 219.156.123.221
  

Conclusion

Distinguishing between an ISP blocking port 53 and a server keeping it open requires comparing results across networks, using VPNs, testing public servers, and leveraging online tools. If your current test shows True, it suggests no ISP blocking and an open port, but the above steps can confirm this under different conditions. If needed, consult your ISP or the server admin for clarity.