Re: destination port changes every time

[Dan Ritter <dsr@randomstring.org>]

On Thu, May 10, 2012 at 06:03:48PM +0100, abdelkader belahcene wrote:
> *Hi,
> I wrote a client/server  programs,   the server listen on port 5000   and
> of course the client connected on that port , I check the communication via
> the sniffex.c program from pcap (adapting something) ,   I noticed that
> destination port ( used by the client)  is not always the same and
> completely different from the source port.
> Here is an output example:
> 
> Device: lo                    Filter expression: ip
> Packet number 1:
>        From: 127.0.0.1
>          To: 127.0.0.1
>    Protocol: TCP                   ---->The client send a request
>    Src port: 49611                  ---->The port is randomly choosed  by
> the client ??
>    Dst port: 5000
> 
> Packet number 2:
>        From: 127.0.0.1
>          To: 127.0.0.1
>    Protocol: TCP                -----------> The answer from the server
>    Src port: 5000
>    Dst port: 49611
> 
> When I stopped the client and ran it again I  got another Src port from the
> client
> The question is : why the port opened by the client is not the same, and
> takes one randomly ??

That's the way TCP/IP works. Consult any handy reference, such
as TCP/IP Illustrated by W. Richard Stevens.

-dsr-


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