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Port Forwarding how to
You want to open something on your network to the world.
What is a port
Ports solves the problem on how to make many applications communicate over the network simultaneously.
A port is a number between 0 - 65,535 that gets assigned to any application that wants to communicate over the network. This number is then added to every packet that is transmitted by that application. The system knows that any respone packets with that port number are to be send to that one application.
How firewall works
A firewall knows direction and state of traffic.
- It allows outgoing communication on any port.
- But the incoming traffic gets dropped on all ports, unless it is a response to communication initialized from the inside.
More details
- When you visit some website you initialize the communication.
- Your browser picks a random port as the source port and sends a request at some IP using a well known https port 443 - the destination port
- Then the browser is waiting for a response at that random port.
- This traffic goes through your firewall and all that info is kept in its state table.
- This allows firewall to know that when packets start coming from that IP, with that source port number now being a destination port, it is a response and let it through.
Port forwarding
What if you want to host something, lets say a minecraft server.
You set it all up, you have your IP address known to others and they try to connect,
but your firewall blocks them. Its a connection initialized from the outside.
So you need to tell your router/firewall to let through traffic that comes to
minecraft default port - 25565 and where to send it on your LAN,
to the local IP of your minecraft server.
Examples of port forward rule
How to actually create that port forward rule depends on router/firewall model. It can be easy, it can be bit complicated.
Generally what to expect
- would be called port forwarding or a virtual server
- IP address is a core information, it is your LAN side machine IP on which your stuff runs.
- Another essential is the port on which to expect traffic, sometimes called a service port or an external port
- it might offer option for internal port, this can be often left empty if port on which your stuff on server is running is the same as the one you are opening to the world. But this gives you option to open port 3333 of firewall but on your LAN machine have port 80 actually being used.
- protocol - TCP or UDP, if dunno select both / all, its safer for the initial setup and testing
Testing if it works
Windows
There are sites that will test if your port is open, but you need to run some service at that port.
Linux
- netcat