Such a thing is impossible by design of the IP-stack. Let me elaborate:

First, what is a port: a port is an address on layer 4 of the network stack - just like the IP-address is an address on layer 3 (and the MAC address on layer 2). Other than the IP address it identifies a service rather than a host. There are 2 bytes identifying the port and therefore there are 65535 possible ports. Think of it like appartments to a house: the IP address identifies the house itself, the port number identifies the appartment. if you ring at different doors, different people will open, even if you are still in the same house. Some might not open at all, because the apartment is empty.

Communication now works in this way: behind any port(-number) a daemon can listen and offer some service - or not. If a daemon indeed listens it will pick up what comes its way and somehow react - by answering, by doing something, establishing a connection, whatever. Some ports, especially the ones up to # 1023, are for offering standard services: telnet, ftp, http, ssh, lpd, .... and so on. These are called "well known services" and only root can start a process using these ports. Usually a symbolic name is defined for these ports - see "/etc/services" - but this is not necessary.

You can even try this with a printer: if you have a network printer it sure runs a "lpd" daemon. This listens at port 515 per default. Using your telnet client (it allows to configure the port used) try:

Code:

telnet printer.yournetwork.com 515

Of course you will not get any telnet connection - the lpd speaks lpd and not telnet - but you will probably get some banner, identifying the printer. Something like "HP JetDirect Line Printer daemon v1.23 .....", maybe listing some EPROM software revision or so before it closes the connection.

If no daemon is listening, there is simply no answer. If you attempt to initiate a communication with a remote port where no daemon listens you get a "connection refused" answer usually, but this comes from your system, not the remote one - the remote one is simply not saying anything at all.

You see, there is no "closed" ports because ports can neither be closed nor opened. They are just listened at - or not. The daemon listening itself can of course terminate a certain connection and send your system the message "connection closed" - but this does not "close" any port, it just terminates the status that to a certain port someone is listening - if using some specific port was part of the session setup. Some daemons are contacted at a common port, then set up connections at different ports (above 1024) and listen again at the common port for new sessions.

I hope this helps.

bakunin