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Is there a TCP port range which I can be sure will not be used by anything else for traffic between two machines?

If I am reading RFC6335 correctly, I can be sure that no other applications will use specific ports in the Dynamic/Private/Ephemeral range of 49152-65535. However, if I understand correctly, it also states (section 8.1.2) that an application shouldn't assume that any given port in that range is available at any given time, which would mean I can't be 100% sure that it will be unused all the time.

Specifically, I want to assign specific SSH traffic to a different port for the security benefit and so I can classify it differently for QoS purposes, and not have to worry about changing the port in the future.

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Like Frederik said, you can never be 100% sure. There are only 65535 ports to choose from, and a zillion (real number!) applications out there all trying not to run on the same port number as each other. – jwbensley Sep 5 '12 at 20:46
up vote 1 down vote accepted

Once a service is started and listening on a port, no other program will be able to bind to that port number. So start your service on whatever port you want; you have nothing to worry about.

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Thanks - that's the key bit of info I had missed... that just by listening on a port no other application will then try to use that port. So, by using a port from the dynamic range, as long as it is free when I start up sshd, then I can be confident it will not by hijacked by anything else (assuming anything else conforms to RFC6335, I suppose). – rowatt Sep 6 '12 at 7:25

You can never ever be 100% sure that a certain port is never used on any system - that is simply not possible.

However, if you check your environment, you should be able to find a port(range) that is not being used by anything else, and use a port in that range.

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