I developed an app in Java (which is working perfectly; with this app you can scan TCP/UDP ports -for testing purposes only-), but meanwhile when I was writing the code I read several documentation (wiki) about sockets. I thought open port is mean, a protocol where the client/server can establish/bind a connection even if there are nothing on the server side which can handle/response data. I thought open port is mean (if there are no routers/firewalls) a protocol which is ready to be used.
I was testing my application on my local server machine, but at first I thought my application is not working. It was working, but there was no any software which could response to my queries. After I opened my APACHE server and I wrote a simple UDP server (which can handle/response to any query on a specific port) the application was able to find those ports.
I thought listening port is a port which is used and it's opened. What does listening port is meaning?
If I send UDP packets to a remote host and there are no closed ports (router or firewall) what happens to the packet? Does the data is written in the buffer? Or it simply ignores/denies the packet.