How To Setup Port Forwarding

If you are considering hosting services (web service, FTP service, game server) on your home network computers accessible from the Internet, then port forwarding is a prerequisite. Port forwarding, also called port mapping, is a networking process, NAT/PAT, to allow a remote computer, from the internet, to be redirected to a port listening on a private network where a service is running.The NAT (Network Address Translation) / PAT (Port Address Translation) mechanism is enabled at the router level. Let’s look at the picture below as an example.

The “red” line represents the Internet traffic. A workstation makes a connection to my WAN IP address provided by my ISP, on the port 3389 (RDP). At the router level, the port value is checked against the existing port forwarding rules.

Once the rule is found, the router will “forward” the request to the local IP address (192.168.110.2) associated with the port, illustrated by “blue” line.

Prerequisites:
Have a Dynamic DNS setup.
Static IP on the servers or workstations hosting the services.
Workstations have their firewall is disabled or add an exception rule.

Steps Overview
1- Identify the host LAN IP address, from the command line or via the network interface properties
2- Identify the port listener associated with the service
3- Set up the port forwarding rule in the router
4- Connect to the service from a remote client

1- Get the LAN IP addresses

From the Command line
“Run” > “Start” > “cmd”
Type ipconfig at the prompt and hit the ENTER key.
From the output, the value that we are looking for is the “IPv4 Address”. Write down the value.
Here, I only care about the workstation IP address, thus, “ipconfig”. The switch /all returns a lot more information, such as “Default Gateway”, DNS servers, MAC address, DHCP server, leases.

From the Network Adapter Settings.
Start > Control Panel > Network and Sharing Center > Change Adapter settings (#1)

Right Click on the active Network interface (#1) and “Status” (#2)

Click on “Details” (#1) and (#2) is the value for the local IP address

Identify the port.

Every server type application utilizes a port which is a unique value.
When a service is setup to accept connections from clients over a network, the architecture is called “client-server” model.

For a client to connect, and authenticates, to a service (hosted on a networked device), two pieces of information are required. The host IP address and the listening port.

For instance, terminal services or Remote Desktop Protocol listens on port 3389, Web Server (IIS or Apache) on port 80, FTP service on port 21.

The last information good to know is the protocol, TCP, UDP or both. When in doubt, select “Both” or “TCP/UDP”.

Setting up the port forwarding rule.

The set up occurs within the router. From the main menu, look for “Applications” and “Port Forwarding ” or “Port Range Forward”.

Based on my router, here are the detailed explanation for each value from top to bottom.

“Sequence Number”: This is just an incremental value

“Rule Name”: Mostly for record purposes. Come up with something explanatory in case the rule needs to be updated.

“Rule Enable”: In some cases, there may be a need to temporary disable the port forwarding rule.

“External Interface“: This field is specific to my router. It supports a Mobile WAN as a failover. In the event my internet service is unavailable, I could connect a USB mobile broadband and keep my network online.

“Protocol“: Choices are, TCP, UDP. TCP/UDP. In doubt, choose TCP/UDP.

“External Port Range“: Ordinary, the external port range matches the Internal port range. However for security purposes, we may want to change the external port value. I will discuss that point in the best practices section. What’s important for now is to understand the port forwarding concept.

“Internal IP“: Self explanatory, enter here the IP address of the application or service host.

“Internal Port Range“: Port value of the service running on the internal IP host. The services were going to make available and port number associated are, FTP 21, IIS 80 and RDP 3389.

Connection from a remote client to the local service.

Remote Desktop Protocol:

From the remote computer, start the RDP client.
In Windows 7:
“Start” > “Run” > “mstsc”
[Alternative] “Start” > “All Programs” > “Accessories” > “Remote Desktop Connection”

Enter the computer hostname or public IP address (#1), click “Connect” (#2). On the next dialog box, (#3) is optional, check it if unsure then click “Connect” (#4)

If you are getting the authentication box, it means that the connection has been established and the port forwarding is functional. Enter your credentials (#1), it is NOT recommended to check “Remember my credentials” and this is valid for any form of authentication. If you happened to leave your computer unattended, a third party could gain access without the need to enter the username/password combo.
Click “OK” (#3) to authenticate and access the resource.

Success! I was able to establish a remote desktop control into my workstation from the internet.

Best practices:

Setting up port forwarding or port mapping will open up your network to the internet. Access to your computer resources from the internet requires two pieces of information, the WAN IP address or name server and the port number.

You do not want to advertize your IP address, in forums, social media networks.

You do not want to use the application or service default port as the incoming port on the WAN side. It is easy to guess the service based on the port. Once I know what service is behind that port, I know what client I would use and try to gain access to that resource.

In my example, if the port 3389 is opened, it is likely that the computer accepts RDP connections, therefore I would use a RDP client to connect. I still need to authenticate before I can access the workstation.

The idea is to pick a random port for the WAN incoming request. For this tutorial, I set the RDP port value to “4000”. Although the port is opened from the internet, it would take a lot of guessing to find out what is the real service I am running on the home network side.

Normally, to RDP into a computer there is no need to specify the port, but since it was changed from 3389, then it must be entered as shown in the screenshot below.

Conclusion:

Once you understand the port forwarding or port mapping concept, your data and computer resources can be accessed from anywhere as long as an internet connection is available.

Keep in mind that if your resources are accessible from the outside, you want to monitor your server(s) for unusual behavior, such as slowness, higher bandwidth usage, increase of disk space usage, connections logs (from the router), security logs (from Windows event viewer) and keep your antivirus up to date.

Michael Nguyen
Categories: Computers
Tags: networking