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198 lines
8.1 KiB
Markdown
198 lines
8.1 KiB
Markdown
# Port Forwarding Guide
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# What is a port
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An IP address identifies specific machine on a network,
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**a port** identifies specific **application** running on that machine.
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A port is **a number** between [1 - 65,535] that is assigned by the OS
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to any application that wants to communicate over the network.
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Can be choosen at random, can be hard set.
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# How Router's NAT works
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* **Allows outgoing** traffic on any port.
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* **Drops incoming** traffic unless it is a response to communication
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initialized from the inside.
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This is not really some security effort, the router just literally has no idea
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where to send it...
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<details>
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<summary><b>More unnecessary details</b></summary>
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NAT is implemented in your router.<br>
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It makes your **LAN side** devices able to connect with the
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outside world - **WAN side,** through one public IP.
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"The internet" *sees* just one device it communicates with,
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at least at this network level.
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#### LAN side initialized communication
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* You visit a website, let's say `youtube.com`
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* Your browser has some random port assigned by the OS,
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this will be the **source port**.
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The local IP address of the machine it runs on will be the **source IP**
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* Browser/OS ask DNS servers for IP address of `youtube.com`,
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the answer is `142.250.191.78` - **destination IP**
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* Youtube is a website, standard for https is using port `443` - **destination port.**
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* All requred information are there. Destination[ip & port] Source[ip & port].
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* Packets are send.
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* The browser now waits for a response at that random port.
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* Since the router is the **default gateway**, thats where the packets arrive.
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* The router saves all that info in its state table for a time, could be seconds,
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could be days depending on protocol and [state](https://serverfault.com/a/481909).
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* Router doing the NAT now replaces the **source IP address** of that one machine,
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with its own wan IP address,
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it might also change source port but that is not as important,
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and sends it out in the direction of the **destination IP**.
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* Response comes back, router knows it is a response because it's coming from the
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IP that it contacted recently and the **destination port** it uses is the same
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number that was used as the source port.
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* It checks the state table for the **original source IP and source port**,
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put them in, now as destination and off the packets go.
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* The browser receives response on its assigned port, from the IP it contacted.
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#### WAN side initialized communication
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* Want to connect to a jellyfin server to watch some movies from browser.
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* You know the IP address or the url.
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You also expect it to run on default port jellyfin uses `8096`
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* The browser makes the request.
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* The router sees it coming at port `8096`, but where does it send it?
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There is nothing in the state table, that would tell it.
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* So it drops it, unless there is a port forwarding rule that says
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that if something comes to port `8096` send it to this local ip address
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and at that port...
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Youtube explanation videos if you want deeper dive:
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* [NAT - Network Address Translation.](https://www.youtube.com/watch?v=RG97rvw1eUo)
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* [Public IP vs. Private IP and Port Forwarding](https://www.youtube.com/watch?v=92b-jjBURkw)
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---
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---
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</details>
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# Double NAT (CGNAT)
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<!--  -->
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**Bad News.**<br>
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It is very likely that even when you do everything 100% correctly,
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you still wont get your ports open.<br>
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The reason being that your machine is behind double NAT.
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**Your ISP** - internet service provider, has you behind its own NAT device
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and that WAN side of your router is not really "the internet", but ISPs LAN side.
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A way to try and check, is looking up your [public IP online](http://icanhazip.com/)
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then log in to your router and finding somewhere the IP address of your WAN interface.
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If they are the same then your are not behind double NAT and port forwarding
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will work straight away.<br>
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If they differ and some local IP is there, then there is still a chance it will work,
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but you wont know till you try.
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But if you are failing to make port forwarding work, it's time to call your ISP
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and ask about public IP, how much would it cost.
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It can be few extra € to your monthly bill.
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# Port forwarding
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Finally. Right?!
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You want to host something, lets say a minecraft server.<br>
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You set it all up, you give your public IP address to others and they try to connect,
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but your router blocks them. It's a connection initialized from the outside.
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So you need to tell your router/firewall to let through the traffic that comes to
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minecraft default port - `25565` and where to send it on your LAN,
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to the local IP of your minecraft server.
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#### Examples of port forward rule
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* [Asus](https://i.imgur.com/brs9Mr6.png)
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* [TPlink](https://i.imgur.com/FNS2xCj.png)
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* [ubiquiti](https://i.imgur.com/D04HVJc.png)
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How to actually create that port forward rule depends on router/firewall model.
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Generally what to expect
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* It would be called port forwarding, or a virtual server, or be under NAT section.
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* **The port** on which to expect traffic is obviously a core information,
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sometimes it is called a service port or an external port.
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* **IP address** is required, so that the router knows where on the LAN side
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to send traffic that comes to that external port.
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* The setup might offer option for **internal port**,
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this can be often left empty, or the same port number is put there.<br>
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It is there to give you option to run stuff on your LAN network on a different
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port than the one you open to the world. Like your webserver is `80`,
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but you open to the world port `12250` or whatever.
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* **The protocol** - TCP or UDP, if dunno **select both / all**<br>
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You don't need to fear you are opening too much, if there is no service
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running on that port for that protocol it is same as a closed port.
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# Testing if port forwarding works
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First you need to understand that unless there is some application running
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that answers on that port, all tests will come back as - closed port.
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For testing we can use websites that will test if a port is open at specified public IP.
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* [portchecker.co](https://portchecker.co/)
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* [yougetsignal.com](https://www.yougetsignal.com/tools/open-ports/)
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## Windows
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* [Find the local ip address](https://www.youtube.com/results?search_query=how+to+find+ip+address+windows)
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of the machine you are planning to use for the test.
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* Follow the instruction in Port forwarding section of this guide
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and forward port `666` to the IP of that machine.
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* Download [Port Listener](https://www.rjlsoftware.com/software/utility/portlistener/).
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* Run Port Listener, set port `666`, press Start.
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* If a windows firewall notification pops up with a question, answer yes.
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* Go to [portchecker.co](https://portchecker.co/), set the port to 666 and press Check.
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In windows it is also pretty useful knowing that you can go
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`Task Manager` > `Performance` > `Open Resource Monitor` > `Network` Tab
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There unroll `Listening Ports`
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and you should find there - `listener.exe` with port `666` and firewall status
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should be *allowed, not restricted*
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## Linux
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* find your ip address - `ip r`
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* Follow the instruction in Port forwarding section of this guide
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and forward port `666` to the IP of that machine.
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* try running netcat - `nc `<br>
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* if it is not installed, get it for your distro,
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for arch it's `openbsd-netcat`, for debian it's `netcat-openbsd`,
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for fedora it's `netcat`
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* execute `sudo nc -vv -l -p 666`
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* Go to [portchecker.co](https://portchecker.co/), set the port to 666 and press Check.
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## UDP port test
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UDP is kinda special cuz it's session-less, so you need to actually communicate
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through it to test it.
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* on a linux machine on LAN - `sudo nc -vv -u -l -p 666`
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* on a linux machine somewhere out there - `nc -u the_public_ip_goes_here 666`
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* write something and it should appear on the other side
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