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Port Forwarding/Port Triggering

By default, the router blocks inbound traffic from the Internet to your computers except for

replies to your outbound traffic. Create exceptions to this rule for these purposes:

•

To allow remote computers on the Internet to access a server on your local network.

•

To allow certain applications and games to work correctly when your router does not

recognize their replies.

Your router provides two features for creating these exceptions: port forwarding and port

triggering. The next sections provide background information to help you understand how

port forwarding and port triggering work, and the differences between the two.

Remote Computer Access Basics

When a computer on your network accesses a computer on the Internet, your computer

sends your router a message containing the source and destination address and process

information. Before forwarding your message to the remote computer, your router has to

modify the source information and create and track the communication session so that

replies can be routed back to your computer.

Here is an example of normal outbound traffic and the resulting inbound responses:

1.

You open a browser, and your operating system assigns port number 5678 to this

browser session.

2.

You type http://www.example.com into the URL field, and your computer creates a web page

request message with the following address and port information. The request message is

sent to your router.

Source address

. The IP address of your computer.

Source port number

. 5678, which is the browser session.

Destination address

. The IP address of www.example.com, which your computer finds

by asking a DNS server.

Destination port number

. 80, which is the standard port number for a web server

process.

3.

Your router creates an entry in its internal session table describing this communication

session between your computer and the web server at www.example.com. Before sending

the web page request message to www.example.com, your router stores the original

information and then modifies the source information in the request message, performing

Network Address Translation (NAT):

•

The source address is replaced with the public IP address of your router. This step is

necessary because your computer uses a private IP address that is not globally

unique and cannot be used on the Internet.

•

The source port number is changed to a number that is chosen by the router, such as

33333. This step is necessary because two computers could independently be using

the same session number.

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Your router then sends this request message through the Internet to the web server at

www.example.com.

4.

The web server at www.example.com composes a return message with the requested web

page data. The return message contains the following address and port information. The

web server then sends this reply message to your router.

Source address

. The IP address of www.example.com.

Source port number

. 80, which is the standard port number for a web server process.

Destination address

. The public IP address of your router.

Destination port number

. 33333.

5.

Upon receiving the incoming message, your router checks its session table to determine

whether an active session for port number 33333 exists. Finding an active session, the

router then modifies the message to restore the original address information that is replaced

by NAT. Your router sends this reply message to your computer, which displays the web

page from www.example.com. The message now contains the following address and port

information.

Source address

. The IP address of www.example.com.

Source port number

. 80, which is the standard port number for a web server process.

Destination address

. The IP address of your computer.

Destination port number

. 5678, which is the browser session that made the initial

request.

6.

When you finish your browser session, your router eventually detects a period of inactivity in

the communications. Your router then removes the session information from its session

table, and incoming traffic is no longer accepted on port number 33333.

Port Triggering to Open Incoming Ports

In the preceding example, requests are sent to a remote computer by your router from a

particular service port number, and replies from the remote computer to your router are

directed to that port number. If the remote server sends a reply to a different port number,

your router does not recognize it and discards it. However, some application servers (such as

FTP and IRC servers) send replies to multiple port numbers. Using the port triggering

function of your router, you can tell the router to open more incoming ports when a particular

outgoing port originates a session.

An example is Internet Relay Chat (IRC). Your computer connects to an IRC server at

destination port 6667. The IRC server not only responds to your originating source port, but

also sends an “identify” message to your computer on port 113. Using port triggering, you can

tell the router, “When you initiate a session with destination port 6667, you have to allow

incoming traffic also on port 113 to reach the originating computer.” Using steps similar to the

preceding example, the following sequence shows the effects of the port triggering rule you

have defined:

1.

You open an IRC client program to start a chat session on your computer.

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2.

Your IRC client composes a request message to an IRC server using a destination port

number of 6667, the standard port number for an IRC server process. Your computer then

sends this request message to your router.

3.

Your router creates an entry in its internal session table describing this communication

session between your computer and the IRC server. Your router stores the original

information, performs Network Address Translation (NAT) on the source address and port,

and sends this request message through the Internet to the IRC server.

4.

Noting your port triggering rule and having observed the destination port number of 6667,

your router creates an additional session entry to send any incoming port 113 traffic to your

computer.

5.

The IRC server sends a return message to your router using the NAT-assigned source port

(as in the previous example, say port 33333) as the destination port. The IRC server also

sends an identify message to your router with destination port 113.

6.

Upon receiving the incoming message to destination port 33333, your router checks its

session table to determine whether an active session for port number 33333 exists. Finding

an active session, the router restores the original address information that is replaced by

NAT and sends this reply message to your computer.

7.

Upon receiving the incoming message to destination port 113, your router checks its session

table and learns that an active session for port 113 exists and is associated with your

computer. The router replaces the destination IP address of the message with the IP

address of your computer and forwards the message to your computer.

8.

When you finish your chat session, your router eventually senses a period of inactivity in the

communications. The router then removes the session information from its session table,

and incoming traffic is no longer accepted on ports 33333 or 113.

To configure port triggering, you need to know which inbound ports the application needs.

Also, you need to know the number of the outbound port that triggers the opening of the

inbound ports. You can usually determine this information by contacting the publisher of the

application or user groups or newsgroups.

Note:

Only one computer at a time can use the triggered application.

Port Forwarding to Permit External Host Communications

In both of the preceding examples, your computer initiates an application session with a

server computer on the Internet. However, you need to allow a client computer on the Internet

to initiate a connection to a server computer on your network. Normally, your router ignores

any inbound traffic that is not a response to your own outbound traffic. You can configure

exceptions to this default rule by using the port forwarding feature.

A typical application of port forwarding can be shown by reversing the client-server

relationship from the previous web server example. In this case, a browser on a remote

computer accesses a web server running on a computer in your local network. Using port

forwarding, you can tell the router, “When you receive incoming traffic on port 80 (the

standard port number for a web server process), forward it to the local computer at

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192.168.1.123.” The following sequence shows the effects of the port forwarding rule you

have defined:

1.

The user of a remote computer opens a browser and requests a web page from

www.example.com, which resolves to the public IP address of your router. The remote

computer composes a web page request message with the following destination

information:

Destination address

. The IP address of www.example.com, which is the address of your

router.

Destination port number

. 80, which is the standard port number for a web server

process.

The remote computer then sends this request message through the Internet to your

router.

2.

Your router receives the request message and looks in its rules table for any rules covering

the disposition of incoming port 80 traffic. Your port forwarding rule specifies that incoming

port 80 traffic is forwarded to local IP address 192.168.1.123. Therefore, your router

modifies the destination information in the request message:

The destination address is replaced with 192.168.1.123.

Your router then sends this request message to your local network.

3.

Your web server at 192.168.1.123 receives the request and composes a return message

with the requested web page data. Your web server then sends this reply message to your

router.

4.

Your router performs Network Address Translation (NAT) on the source IP address, and

sends this request message through the Internet to the remote computer, which displays the

web page from www.example.com.

To configure port forwarding, you need to know which inbound ports the application needs.

Usually you can determine this information by contacting the publisher of the application or

the relevant user groups and news groups.

How Port Forwarding Differs from Port Triggering

The following points summarize the differences between port forwarding and port triggering:

•

Port triggering can be used by any computer on your network, although only one

computer can use it at a time.

•

Port forwarding is configured for a single computer on your network.

•

Port triggering requires specific outbound traffic to open the inbound ports, and the

triggered ports are closed after a period of no activity.

•

Port forwarding is always active and is never triggered.

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Set Up Port Forwarding



To set up port forwarding:

1.

Log in to the router as described in

Log In to Your Router

on page 17.

2.

From the main menu, select

Advanced > Port Forwarding/Triggering

.

By default, the

Port Forwarding

radio button is selected.

3.

You can select a service or create a custom service.

•

Select a service from the Service Name drop-down list, specify the computer’s IP

address, and click

Add

.

•

If you want to add a service that is not in the list, click the

Add Custom Service

button. Fill in the fields in the Add Custom Service screen and click

Apply

.

The added service appears in the list.

Set Up Port Triggering



To set up port triggering:

1.

Log in to the router as described in

Log In to Your Router

on page 17.