Dynamic port use

Ethan Blanton elb at pidgin.im
Wed May 14 11:35:51 EDT 2008

Mayank Jain Nawal spake unto us the following wisdom:
> At most of the places chatting is not allowed, to stop users from doing that 
> port numbers are generally blocked. So are we still using port numbers for 
> identifying that the traffic belong to specific application (I guess in 
> pidgin we are are using port numbers). 

What are you trying to say?  This email does not make sense.

Pidgin uses a random local port, and for every protocol the remote
port is configurable.  However, servers only listen on particular
ports.  For some (such as AOL's OSCAR servers) any port can be used,
for others (most XMPP servers) only a very specific port can be used.
There is nothing the client can do about this.

> Possible solution to this is use of Dynamic ports. User should not be 
> restricted to use a particular port for certain application, the 
> identification of packet should be on the basis of certain unique string or 
> certain signature which should be present in every packet, not on the basis 
> of port which a particular application uses.   

I think you need to look into a decent networking textbook.  Packet
classification is handled by the operating system, not the
application, and for TCP (which all IM protocols supported by Pidgin
use for their primary negotiations) the port numbers of a connection
are fixed from connection establishment forward.

If my comments do not address your concerns, please try to clarify
your concerns.  As I said, they do not really make sense.


The laws that forbid the carrying of arms are laws [that have no remedy
for evils].  They disarm only those who are neither inclined nor
determined to commit crimes.
		-- Cesare Beccaria, "On Crimes and Punishments", 1764
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