Windows Troubleshooting Tip – Verifying TCP Connections

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Windows Troubleshooting Tip – Verifying TCP Connections

Like This Blog 0 Mike Danseglio
Added by December 30, 2013

I’ve published a number of articles on following a simple and straightforward troubleshooting methodology. I advocate a simple and focused approach shown in Figure 1. You can see that symptom identification is the first step, followed by root cause analysis and problem resolution.

Figure 1. MikeDan’s Quick and Dirty Troubleshooting Methodology.

There’s lots of great tools built right in to Windows that will actually help when narrowing down network communications problems.

Why Check TCP Connections?

You might already use a tool like ping to verify fundamental network connectivity. But ping is connectionless and frequently filtered by routers and firewalls. You are probably familiar with tracert as well, which is subject to the same limitations as it also uses connectionless network communications that can be blocked or filtered. To troubleshoot problems establishing a TCP connection you need to use a tool that understands TCP and uses the 3-way handshake to connect to a target hostname and TCP port.

My favorite tool for this task is Telnet.

What is Telnet?

Telnet is designed as a command-line tool to remotely connect and administer a computer. There are two components: a Telnet Client that runs on the client computer, and a Telnet Server that accepts incoming connections, requires authentication, accepts commands and responds with results. Telnet uses TCP port 23 by default.

How Do I Use Telnet to Troubleshoot TCP Connection Issues?

The great thing about the Telnet client application is that it lets you test a connection to any destination TCP port. The destination host doesn’t need the Telnet Server to be installed. All you need to know is the number of a TCP port that accepts incoming connections… which should be the one you’re already troubleshooting.

For my example, here’s how to use the Telnet client application to test a connection to a Simple Mail Transfer Protocol (SMTP) server. I’m testing a problem connecting from a Windows 7 computer to the outbound SMTP server, which uses port 587 for SMTP.

Note that these steps are written for Windows 7 but will work for virtually any version from Windows 2000 onwards.

1. Open a command prompt and type telnet 587 and then press Enter. This specifies both the destination host and the destination port. The screenshot is taken before I press Enter, because when I do, a successful connection typically clears the screen.


Figure 1. The Telnet command-prompt input.

2. The great thing about Telnet is that I either immediately get an answer or I don’t! I don’t need to interact with any response. You can see in Figure 2 that the server responds immediately and identifies itself.


Figure 2. A successful connection with Telnet.

That’s it! The beauty of this technique is that is works quickly and uses a built-in tool.

Optional: Installing Telnet in Windows 7

Windows 7 does not always install the built-in Telnet client by default. This may depend on how you deployed Windows, the various configuration options you’ve chosen, other software you’ve installed, and so on. If you need to install the Telnet Client in Windows 7, the steps are:

1. Launch Control Panel.

2. Click on Programs and Features.

3. In the left pane, click Turn Windows features on or off.

4. Scroll down until you see Telnet Client.


5. If this option is not checked, check it, and then click OK.

Now you should be able to open a command prompt and launch Telnet.


Mike Danseglio -CISSP / CEH
Interface Technical Training – Technical Director and Instructor

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