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I am trying to open a port in my Linux machine but I am having some trouble. I looked up some information about port forwarding and I found this What is port forwarding and what is it used for? and jcrowfordor's post (the long post with a picture) helped me understand it better.

But I still have some trouble opening a port. I looked up tutorials and videos and it always says:

  • 1-Forward the port in the router
  • 2-Download ufw
  • 3-Create a rule in ufw that allows inbound traffic to that port

I did all that and it didn't work. Maybe I am doing something wrong? I still have some doubts about port forwarding so I think that clearing those doubts might help me figure out what I am doing wrong.

Also, the whole firewall configuring+router portforwarding might be unnecessary, because I have a Linux Mint VM that accepts inbound traffic to any port I bind using netcat (I use sudo nc -l portnumber). Why is that? I haven't configured anything, except for disabling the Linux Mint's firewall. If I disable the other Linux's firewall it still refuses connections.

My doubts regarding port forwarding are:

  1. When a computer from a network wants to communicate with another computer in that network does the packet it sends to the router count as inbound traffic? If so, in order for the destination computer to receive it I have to forward a port on the router, right?
  2. What if I want 2 or more computers to be able to receive connections on the same port? Do I have to forward that port in the router for all those computers?
  3. If yes, how does the router know to which computer it should send the inbound traffic? What happens when it comes from the network and when it comes from out of the network?
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Mark Read
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2 Answers2

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1) no. it goes straight to the LAN server without crossing the router (it never gets recieved on the routers WAN). Most commercial routers will not let an internal host connect to the routers WAN. Those that do support whats called NAT Hairpinning. To test a NAT port forward rule without hairpinning, you must do so from another network.

2) If you want tcp\80 on two differant boxes to forward, you will have to use differant external port numbers so that WAN:80 -> PC1:80 and WAN:81 -> PC2:80. Port numbers on teh WAN side must be unique, or it could not tell which server you want to use, and NAT forwarding is always unicast, so you could not send the packet to both internal servers.

3) NAT rules require you to specify the local IP address of the LAN server that will recieve the packets incomming on the port.

Frank Thomas
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  • So, that means that in order for 2 computers in the same network to communicate in a specific port I don't have to do any port forwarding on the router? That's one less problem. Thanks for answering. – Mark Read Oct 21 '15 at 17:13
  • that is correct. Port forwarding is specifically the process for exposing the servers port to entities that are not on the same LAN as the server. – Frank Thomas Oct 21 '15 at 17:14
  • @PeterSmith Okay, thanks. Do you have any suggestion as to why my Linux VM might be refusing connections? – Mark Read Oct 21 '15 at 17:15
  • well, that depends on how the VM connects to the physical LAN. many VMSs define a new network only within the host PC, so generally it is recommend that if you want to make a VM accessible to the physical LAN, that you connect the VM to the network using a "Bridged" connection. what VMMS are you using (virtualbox, vmware, etc)? – Frank Thomas Oct 21 '15 at 17:18
  • I am using VMWare Player and the machines are in Bridged Mode. – Mark Read Oct 21 '15 at 17:19
  • ok, then port forwarding will not be necessary for your LAN systems to access the server. First, on the linux box, run `sudo netstat -ntlup` to confirm that you have a process listening on the port you expect. Then make sure UFW is allowing the port. Consider using gufw if you are not comfortable with admining the firewall fromt he CLI. – Frank Thomas Oct 21 '15 at 17:21
  • I think I found out what is causing the problem. When I use sudo netstat -ntlup nothing is listening on the port I bound. When I try it with Linux Mint, I can see that the process is listening on the port I bound. Why is this happening? – Mark Read Oct 21 '15 at 17:29
  • This is odd, when I use nc -l port and the netstat -ntlup it shows that nc is listening but never on the port I chose, instead it listens in ports like 47954 – Mark Read Oct 21 '15 at 17:33
  • is the port perhaps already in use? I generally recommend picking a port above 10000 and below 40000 to minimize teh risk of conflict. – Frank Thomas Oct 21 '15 at 18:24
  • Nope, same result. I am going to post this question – Mark Read Oct 22 '15 at 12:16
  • I solved the problem. You are not going to believe it. I had to use nc -l -p port instead of nc -l port. Now it works fine. Oh well. – Mark Read Oct 22 '15 at 12:23
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Solution to the netcat problem:

Use nc -l -p port instead of nc -l port. That simple.

Mark Read
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