One of the more common networking issues we look at is where some or all ports between servers are being blocked. TCPView provides a more informative and conveniently. On Windows Server 2008, Vista, and XP, TCPView also reports the name of the process that owns the endpoint. TCPView is a Windows program that will show you detailed listings of all TCP and UDP endpoints on your system, including the local and remote addresses and state of TCP connections.
This will attempt a TCP connection to a specific hostname or IP on a specific port. For Server 2012 R2+The easiest method is the PowerShell command Test-NetConnection. Once you've confirmed the server is listening, either with netstat or by testing that some clients are able to connect, you can proceed with troubleshooting blocked ports.If you are having an issue and know the port number that's failing, such as RDP/3389 or HTTP/80, it's easy to test for connectivity to that specific port. This can be confirmed with the "netstat -na" command, where you'll see the port in a LISTENING state, or by testing from another device that is able to connect. This file can specify an.When testing for blocked ports, it's important to ensure that the server you're connecting to is actually listening on the given port.
This is where PortQryUI comes in handy. Multiple PortsIf there are multiple ports to test, it can be burdensome to use the above commands to test them repeatedly while troubleshooting. However if the command prompt clears/resets, that indicates that the port is not blocked.An example of a successful connection, where the server disconnects automatically:An example of a failed connection that indicates the TCP port is not open:There are some scenarios where Test-NetConnection and telnet aren't totally adequate for testing port availability. You may still get disconnected immediately, for example if the server expects your client to immediately send some specific data. When telnet successfully connects, you'll see the CMD screen reset. Here's an example of a successful test using HTTP port 80 against google.com:On older operating systems, you'll have to resort to using telnet to test port availability.
LISTENING indicates a successful connection.A failed or closed port is usually indicated by a status of FILTERED instead of LISTENING. For example, here we test the default AD ports to the DC named MGLABDC5. There are a number of ports that need to be tested, and PortQryUI makes it easy to test all of them.
The most common cause of this is that the port is being blocked. SYN_SENT indicates that the client sent the first part of the TCP handshake but never received a response. To find blocked ports in these cases, you can look for a connection in the SYN_SENT state on the client side. For example, many Windows services listen on random high-numbered ports.
When you compare them, you should see some packets leaving the client but never arriving on the destination server. After waiting a few seconds for the connections to be attempted, stop the packet captures. After starting the packet capture on both sides, attempt to connect from the client to the server a few times. Packet CapturesIf you need to document in-depth evidence of a port issue, you can run concurrent packet captures on the client and server using Wireshark, netsh, etc. This is what the client-side netstat shows in that case:SYN_SENT shows that this server is trying to initiate a connection with 10.250.0.12 on port 49665, but it never received an acknowledgement of the initial TCP packet.
Can the server connect to itself on the expected port? Test with telnet or Test-NetConnection using 127.0.0.1 or localhost. Is the server listening on the expected port? This can be confirmed with "netstat -na" on the server or through other evidence such as other clients successfully connecting. Troubleshooting ProcessGenerally speaking, troubleshooting blocked ports follows this process:
If you know the port number of the service that is failing, does Test-NetConnection or telnet show it is being blocked? Is the Windows Firewall blocking the connection? Temporarily disable on the server if needed to test. If using the hostname of the server, can the client successfully resolve it? Use "nslookup " to confirm.
These services have to be tested through other methods.
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