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On the Windows platform, what native options to I have to check if a port (3306, for example) on my local machine (as in localhost), is being blocked?

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1  
Just to clarify, do you mean blocked, as in blocked by a firewall or gateway, or do you mean already in use by something else? – squillman Jun 16 '09 at 14:34
    
The Audit My PC Firewall test allows you to test a specific port or a range of ports from an external source. – Peter Stuer Jun 16 '09 at 15:38
up vote 67 down vote accepted

Since you are on the Windows machine, these things can be done,

  • Execute the following command and look for a ":3306" listener (you did not mention UDP/TCP). This will confirm there is something running on the port.

    netstat -a -n

  • After this, if you are expecting incoming connections on this port and feel that the firewall may be blocking them, you could start windows firewall logging and check the logs for dropped connections

    • Go to the Windows Firewall, Advanced settings
    • Click on the Settings button next to "Local Area Connection"
    • Select "Log dropped packets"
    • Look at the log file location (if not present define one)
    • Click OK
    • Now, when the connection attempt is made (assuming you know when this is done), look at the log file for a drop on port 3306.
    • If this is seen, you will want to add an exception for this port.
  • There is one more command to check the firewall state
    (Update for Windows 7 users -- as referred by Nick below -- use netsh advfirewall firewall)

    netsh firewall show state

    • this will list the blocked ports as well as active listening ports with application associations
  • This command will dump the Windows firewall configuration detail

    netsh firewall show config


If you have an active block (incoming connections are being dropped by firewall) after you start logging, you should see that in the log.

If you are running an application/service that is listening on 3306, the firewall config should show it to be Enabled. If this is not seen, you have probably missed adding an exception with the firewall to allow this app/service.

Finally, port 3306 is typically used for MySQL. So, I presume you are running MySQL server on this windows machine. You should therefore see a listener for 3306 accepting incoming connections. If you do not see that, you need to work with your application (MySQL) to get that started first.

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2  
+1 for great details – Sage Jun 16 '09 at 15:39
1  
Starting with Windows Vista the "netsh firewall" command is deprecated. It recommends you use "netsh advfirewall firewall" instead and references article go.microsoft.com/fwlink/?linkid=121488 – Nick DeVore May 18 '11 at 15:31
1  
To parse output at the command line, add |find "3306" to the command, e.g. C:\Windows\System32>netstat -an |find "3306" – Cees Timmerman Jul 5 '13 at 8:20

If you can telnet to the port from the local machine (using the external IP address), but not from another machine - then it is being blocked somewhere between.

Note that a firewall on your local machine could prevent even the first action.

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Note that telnet is not being installed anymore on Win7 and newer systems. – Alexis Wilke Apr 18 '15 at 19:12
    
@AlexisWilke That's not correct. Telnet can be installed – deepdive Jun 18 '15 at 1:50
    
To install telnet from the command line: dism /online /Enable-Feature /FeatureName:TelnetClient – Jason Massey Jan 21 at 20:08

NETSTAT will tell you if the port is listening but it will not tell you if the port is open to the outside world. What I mean by this is that NETSTAT may show that the 0.0.0.0 is LISTENING on port 3306 but a firewall may still be blocking that port which is preventing outside connections; so it isn't sufficient to rely on NETSTAT alone.

The best way to check if a port is blocked is to do a port scan from the client machine.

There are many ways to do a port scan but since you mentioned being on Windows then I will suggest the Microsoft command line utility PortQry and the Graphical version PortQryUI

Using a PortScan utility you will get one of 3 results.

  • Listening means the server is listening on the specified port
  • Filtered means it received a TCP acknowledgement packet with the Reset flag set which likely indicates a firewall or software issue
  • Not Listening means it didn't receive a response at all

telnet is another command line option that is usually installed on the OS by default. This command line utility can be used a quick way to see if a port responds to a network request.

To use telnet you would simply issue the following command from a command prompt:

telnet localhost 3306

The command above should give you a quick indication if the port 3306 on the localhost is responding.

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protected by Michael Hampton Apr 4 at 0:20

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