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How to Find Out If A Firewall Is Blocking Your Remote Control Software

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By Louis Machado
User-Submitted Article
(10 Ratings)

If you ever tried to help a user at another location, you have appreciated the usefulness of remote control software. But, you might not be able to connect to it after installation. When that happens, you must determine whether the user's firewall is causing the problem, or if it is some other issue with the user's computer. Both computers should be on the same site for you to use these tips; If your user's computer is offsite, this procedure will be useless.

The computer with the issue (Computer 1) should have an operating system that includes the Telnet utility, such as Windows XP. You will also need its IP address, or its DNS name. Computer 2 should have an operating system that includes the Telnet utility. There must be a reliable working network connection between the two.

Difficulty: Challenging
Instructions

    Test Your Connection to Computer 1 from Computer 2

  1. Step 1

    Find out the IP port number that your remote control software uses. You can get this information from your remote software settings in your user computer, its documentation or its technical support. Here are the more common IP port numbers:

    3389 - Microsoft's Remote Desktop connection Protocol (RDP)
    5631 - Symantec's PC Anywhere (default; it can be changed)
    5900 - Open Source Forge's VNC and its derivatives (ditto)
    4899 - Famatech's Remote Administrator (ditto)

  2. Step 2

    Click "Start." A toolbar or a window will come up.

  3. Step 3

    Click "Run." A window will come up with a place to type a command or other text.

  4. Step 4

    Click on the rectangle (or text field) and type this command:

    cmd

    Press "Enter," or click "OK"

    A (usually) black window with a blinking cursor will come up. This window has the title Command Prompt.

  5. Step 5

    Click the title bar or anywhere else inside the window, to select it.
    You should be able to type commands into it.

  6. Step 6

    In the Command Prompt window, type this command where the (IP address) is replaced by Computer 1's IP address or DNS name:

    start telnet (IP address) (TCP port number)

    Start allows the command you type next to come up in a separate window.
    Telnet is a (text) terminal emulation program that uses TCP/IP instead of a serial cable to connect to computers. (IP address) is replaced by your 4-number set, also known as the dotted quad. It is the Internet equivalent of a phone number. It can also be replaced by your DNS name.
    The last one is your TCP port number. This you will replace with your particular port number as applicable.

    Examples:

    start telnet 10.120.201.18 3389

    or:

    start telnet mail.yourfriend.local 4899

  7. Step 7

    These steps only test if your TCP port number is open on Computer 1 to the outside world. You may still have an issue with the remote control software that prevents you from connecting. If everything else is in order, rebooting Computer 1 usually takes care of the issue, at least for a while.

  8. Test Your Rmote Control Software in Computer 1 (steps are very similar to Section 1)

  9. Step 1

    On Computer 1, click "Start." A toolbar or window will come up.

  10. Step 2

    Click Run. A window will come up with a place to type a command or other text.

  11. Step 3

    Click on the rectangle and type this command:

    cmd

    Press "Enter," or click "OK."

    A (usually) black window will come up. In it it will have a blinking cursor. This window has the title Command Prompt.

  12. Step 4

    Type this command, and press "ENTER:"

    IPCONFIG

    As a response, you will get text such as this:

    Windows IP Configuration


    Ethernet adapter Wireless Network Connection:

    Connection-specific DNS Suffix . :
    IP Address. . . . . . . . . . . . : ###.###.###.###
    Subnet Mask . . . . . . . . . . . : 255.255.255.0
    Default Gateway . . . . . . . . . : ###.###.###.###

  13. Step 5

    Make a note of the set of four numbers (or the "dotted quad") in the IP Address line. Then type the command:

    start telnet (your IP Address) (your TCP port number)

    Continue to our next step.

  14. Step 6

    A second window will come up. The text

    Connecting to (blah blah blah) ...

    may show up. One of these things may happen:

    1- The text above lingers for more than half a second or so, and then the screen closes, so you are back to the previous window. This means Computer 1 has nothing listening on that particular port. Either your remote control software is not active, it cannot become active for some reason, or else it is listening in another TCP port.

    You can now contact your remote control software technical support and have them help you figure out what the problem is.

    2- The screen is blank except for a cursor. This means Computer 1 has some software listening on that TCP port. Hopefully, it is your remote control software who is listening.

    You can press ENTER or the letter q, and usually this window will close.
    If your results on Section 1 were that no connection can be made on this same TCP port, then there is a firewall or other software on Computer 1 blocking this port for inbound connections. You will need to look into Computer 1 to see if Windows Firewall or a third-party firewall software is causing this issue. Common culprits are Windows Firewall, Norton Security, or ZoneAlarm. Instead of disabling the firewall, it is better to tell it to accept inbound connections at that port.

    3.- In rare instances, the second window opens and closes almost immediately, and you are back to the first one. In this case, the port is actively refusing the connection. Either your remote control software is acting up, or some other software is listening to that TCP port. You will need to verify that your TCP port number is indeed correct.

    NOTES:
    These steps only test if your TCP port number is open on Computer 1. You may still have an issue with the remote control software itself that prevents you from connecting. If everything else is in order (authentication, etc), rebooting Computer 1 usually takes care of the issue, at least for a while. Reinstallation may be also an option. Otherwise, you will need to contact your remote control software tech support.

Tips & Warnings
  • Computer 1 should be able to surf the web, or at least have a valid IP address as shown by the command prompt utility IPCONFIG.
  • The computer with the remote control software (Computer 1) should have an operating system that includes the Telnet utility, such as Windows XP. You will also need its IP address, or its DNS name.
  • You could use the word localhost instead of your IP address. However, not all remote control software works on (binds to) this address (which is known as Loopback address) so tests may fail on you but your software may still work.
  • You can use this command in Computer 1 to obtain a list of listening ports: NBTSTAT -S. Using TELNET as described will give you a better feel for the health of your connection, though.
  • This article is an advanced topic, and requires knowledge of the TCP/IP family of protocols.
  • There are security issues in leaving remote control software running on any computer all the time, specially if it does not prompt the user for approval or otherwise warns the user about an incoming connection.

Comments  

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on 1/27/2008 Lots of information and resources here, thanks!

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