The server responds with the RST/ACK flag after the client sends the SYN flag

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I have been understanding why the X server responds to the TCP request of the Y client with the RST/ACK flag. I think the problem may be one of the 3 below.

  1. The port on the server side is closed;
  2. A package is sent with a problem and the server forces it to resend a new request;
  3. Have some Firewall blocking these requests.

After the above points, the question is:

  1. How do I know that the port on the Server side is closed? I emphasize that there are times when the connection is successful...

  2. How to know through Wireshak that the package is poorly formed?

  3. The firewall bus is internal to the client network or on the server network?

tcpclient

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Most likely there is no application waiting for connections on that port on the target server. The fact that "sometimes" the connection is successful indicates that "sometimes" the application in the destination is in the air.

It could be some firewall problem, but then it’s hard to figure out, it’s hard to know what’s in between, unless you control the environment as a whole.

It could also be poorly formed package by hardware problems, some network interface, but also do not believe in it.

My guess is that it is an operational problem even: not always the application in the destination is in the air.

  • Hello Joseph X, thank you for your hunch. In fact it is also what I suspect...

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