how to open a port ?

[fizan]

Hi Team,

how to open a port ? i want to start a new application temp as well as across the reboot?

[Steven E. Protter · Accepted solution]

Shalom,

check /etc/services for port reservations.

netstat -an

See if there is a service listening on the port.

If nothing is there, you can set it up.

The service will need the following to auto start.

start up script in /sbin/init.d

Soft links to the startup script in /sbin/rc3.d starting with an S for start up and K for kill in /sbin/rc2.d

SEP

[Torsten.]

If you search the forum for "how to open a port ?" you get 50 results or more.

A port is "open" if "something" is listening to this port.

[Mel Burslan]

You can not "open" a port from the operating system per-se. There is no such command as

open port 22

Although, running

/usr/sbin/sshd

when run, will start listing to port 22, which means, in your word, the port is open. I am ashamed to say that I am not a C programmer but if you want to "open" a port, you will need to employ sockets programming using C.

Opening a port usually means nothing without the application listening to port doing something though.

[Torsten.]

If you would explain your question better, you can get a more specific answer.


However, there are some classic methods to "open" a port.

Write an application that

- creates a socket
- bind this socket to an address (port)
- listen to this address

If a clients wants to connect, accept this attempt. Now server and client can communicate.

An example for this is a web server, bound to port 80, always running and listening for connection attempts.


The inetd is doing the same (more or less), but if a connection attempt arrieves, it is starting the appropriate process.
An example is telnetd.



So in general, if a process is listening on a specific port, you can consider this port as "open". If nothing is listening, it's in your wording "closed".


IMHO the terms "open" and "closed" are wrong and misleading.