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Security
61
N300 Wireless ADSL2+ Modem Router DGN2200v4
Port Triggering to Open Incoming Ports
Some application servers (such as FTP and IRC servers) send replies to multiple port
numbers. Using the port triggering function of your modem router, you can tell the modem
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 modem router, “When you initiate a session with destination port 6667, you have to
also allow incoming traffic 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.
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 modem router.
3.
Your modem router creates an entry in its internal session table describing this
communication session between your computer and the IRC server. Your modem 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 modem 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 modem router using the NAT-assigned
source port (for example, port 33333) as the destination port. The IRC server also sends an
“identify” message to your modem router with destination port 113.
6.
Upon receiving the incoming message to destination port 33333, your modem router
checks its session table to determine whether there is an active session for port number
33333. Finding an active session, the modem router restores the original address
information replaced by NAT and sends this reply message to your computer.
7.
Upon receiving the incoming message to destination port 113, your modem router checks
its session table and learns that there is an active session for port 113, associated with your
computer. The modem router replaces the message’s destination IP address with your
computer’s IP address and forwards the message to your computer.
8.
When you finish your chat session, your modem router eventually senses a period of
inactivity in the communications. The modem router then removes the session information
from its session table, and incoming traffic is no longer accepted on port numbers 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 will trigger the opening of the
inbound ports. You can usually determine this information by contacting the publisher of the
application or the relevant user groups or news groups.
Only one computer at a time can use the triggered application.
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N300 Wireless ADSL2+ Modem Router DGN2200v4
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 might need to allow a client computer on the
Internet to initiate a connection to a server computer on your network. Normally, your modem
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 remote computer’s
browser needs to access a web server running on a computer in your local network. Using
port forwarding, you can tell the modem 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
192.168.0.123.” 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 modem 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
modem 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
modem router.
2.
Your modem 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 should be forwarded to local IP address 192.168.0.123. Therefore,
your modem router modifies the destination information in the request message:
The destination address is replaced with 192.168.0.123.
Your modem router then sends this request message to your local network.
3.
Your web server at 192.168.0.123 receives the request and composes a return message
with the requested web page data. Your web server then sends this reply message to your
modem router.
4.
Your modem 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.
You usually can determine this information by contacting the publisher of the application or
the relevant user groups or news groups.
Page 63 / 133
Security
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N300 Wireless ADSL2+ Modem Router DGN2200v4
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.
•
With port triggering, the modem router does not need to know the computer’s IP address
in advance. The IP address is captured automatically.
•
Port forwarding requires that you specify the computer’s IP address during configuration,
and the IP address can never change.
•
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 does not need to be triggered.
Set Up Port Forwarding to Local Servers
The port forwarding feature lets you allow certain types of incoming traffic to reach servers on
your local network. For example, you might want to make a local web server, FTP server, or
game server visible and available to the Internet.
Use the Port Forwarding/Port Triggering screen to configure the modem router to forward
specific incoming protocols to computers on your local network. In addition to servers for
specific applications, you can also specify a default DMZ server to which all other incoming
protocols are forwarded.
Before you start, determine which type of service, application, or game you want to provide,
and the local IP address of the computer that will provide the service. The server computer
has to always have the same IP address.
To ensure that your server computer always has the same IP address, use the reserved IP
address feature of your product. See
Address Reservation
on page
42.
To forward specific incoming protocols:
1.
Select
Advanced > Advanced Setup > Port Forwarding/Port Triggering
.
Page 64 / 133
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N300 Wireless ADSL2+ Modem Router DGN2200v4
2.
From the Service Name list, select the service or game that you will host on your network.
If the service does not appear in the list, see
Add a Custom Service
on page
64.
3.
In the Action list, select when you want to allow or block this port forwarding rule.
4.
In the Server IP Address field, enter IP address of your local computer that will receive the
inbound traffic covered by this rule.
5.
In the WAN Servers field, fill in the IP addresses covered by this rule.
6.
In the Log list, select
Never
or
Always
to specify when to log packets covered by this rule.
7.
Click
Add
.
The service appears in the list on the Port Forwarding screen.
Add a Custom Service
To define a service, game, or application that does not appear in the Service Name list, first
determine which port number or range of numbers the application uses. You can usually
determine this information by contacting the publisher of the application or user groups or
news groups. When you have the port number information, follow these steps.
To add a custom service:
1.
Select
Advanced > Advanced Setup > Port Forwarding/Port Triggering
.
2.
Select the
Port Forwarding
radio button as the service type.
3.
Click the
Add Custom Service
button.
4.
In the Name field, enter a descriptive name.
5.
In the Service Type list, select the protocol. If you are unsure, select
TCP/UDP
.
6.
In the External and Internal Starting Port fields, enter the beginning port number.
•
If the service uses only one port, enter the port number in the Ending Port field.
•
If the service uses a range of ports, enter the end port number in the Ending Port field.
7.
In the Internal IP Address field, enter the IP address of your local computer that will provide
this service.
8.
Click
Apply
.
The service appears in the list in the Port Forwarding/Port Triggering screen.
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N300 Wireless ADSL2+ Modem Router DGN2200v4
Edit or Delete a Port Forwarding Entry
To edit or delete a port forwarding entry:
1.
In the table, select the radio button next to the service name.
2.
Click
Edit Service
or
Delete Service
.
Application Example: Make a Local Web Server Public
If you host a web server on your local network, you can use port forwarding to allow web
requests from anyone on the Internet to reach your web server.
To make a local web server public:
1.
Assign your web server either a fixed IP address or a dynamic IP address using DHCP
address reservation.
In this example, your modem router always gives your web server an IP address of
192.168.0.33.
2.
In the Port Forwarding/Port Triggering screen, configure the modem router to forward the
HTTP service to the local address of your web server at
192.168.0.33
.
HTTP (port 80) is the standard protocol for web servers.
3.
(Optional) Register a host name with a Dynamic DNS service, and configure your modem
router to use the name.
To access your web server from the Internet, a remote user has to know the IP address
that your ISP assigned. However, if you use a Dynamic DNS service, the remote user can
reach your server by a user-friendly Internet name, such as mynetgear.dyndns.org.
Set Up Port Triggering
Port triggering is a dynamic extension of port forwarding that is useful in these cases:
•
More than one local computer needs port forwarding for the same application (but not
simultaneously).
•
An application needs to open incoming ports that are different from the outgoing port.
When port triggering is enabled, the modem router monitors outbound traffic looking for a
specified outbound “trigger” port. When the modem router detects outbound traffic on that
port, it remembers the IP address of the local computer that sent the data. The modem router
then temporarily opens the specified incoming port or ports and forwards incoming traffic on
the triggered ports to the triggering computer.
Port forwarding creates a static mapping of a port number or range to a single local computer.
Port triggering can dynamically open ports to any computer that needs them and can close
the ports when they are no longer needed.

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