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Page 51 / 185 Scroll up to view Page 46 - 50
Security Settings
51
N600 Wireless Dual Band Gigabit ADSL2+ Modem Router DGND3700
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 your router’s public IP address. This 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 chosen by the router, such as 33333.
This is necessary because two computers could independently be using the same
session number.
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 there is an active session for port number 33333. Finding an active session, the
router then modifies the message to restore the original address information 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
. Your computer’s IP address.
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 back 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 back to multiple port numbers. Using the port
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Security Settings
52
N600 Wireless Dual Band Gigabit ADSL2+ Modem Router DGND3700
triggering function of your router, you can tell the router to open additional 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 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 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, let’s 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 there is an active session for port number 33333.
Finding an active session, the 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 router checks its session
table and learns that there is an active session for port 113, associated with your computer.
The 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 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 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 user groups or newsgroups.
Note:
Only one computer at a time can use the triggered application.
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Page 53 / 185
Security Settings
53
N600 Wireless Dual Band Gigabit ADSL2+ Modem Router DGND3700
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 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 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
192.168.1.123.” 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 should be 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.
You usually can determine this information by contacting the publisher of the application or
the relevant user groups and newsgroups.
How Port Forwarding Differs from Port Triggering
The following points summarize the differences between port forwarding and port triggering:
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Security Settings
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N600 Wireless Dual Band Gigabit ADSL2+ Modem Router DGND3700
•
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 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.
Configure Port Forwarding to Local Servers
Using the port forwarding feature, you can 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 screen to configure the 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 starting, you need to 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.
Tip:
To ensure that your server computer always has the same IP address,
use the reserved IP address feature of your product.
To configure port forwarding:
1.
Select
Content Filtering > Port Forwarding/Port Triggering
to display the following
screen:
Figure 22.
Setting up port forwarding
2.
Select the
Port Forwarding
radio button as the service type.
3.
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 55.
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Security Settings
55
N600 Wireless Dual Band Gigabit ADSL2+ Modem Router DGND3700
4.
In the corresponding Server IP Address field, enter the last digit of the IP address of your
local computer that will provide this service.
5.
Click
Add
. The service appears in the list in the screen.
Add a Custom Service
To define a service, game, or application that does not appear in the Service Name list, you
have to first determine which port number or range of numbers is used by the application.
You can usually determine this information by contacting the publisher of the application or
user groups or newsgroups.
When you have the port number information, follow these steps:
1.
Select
Content Filtering > Port Forwarding/Port Triggering
.
2.
Select the
Port Forwarding
radio button as the service type.
3.
Click the
Add Custom Service
button to display the following screen:
Figure 23.
Set up custom services
4.
In the
Service Name
field, enter a descriptive name.
5.
In the
Protocol
field, select the protocol. If you are unsure, select
TCP/UDP
.
6.
In the
Starting Port
field, enter the beginning port number.
•
If the application uses a single port, enter the same port number in the
Ending Port
field.
•
If the application uses a range of ports, enter the ending port number of the range in
the
Ending Port
field.
7.
In the
Server 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.
Edit or Delete a Port Forwarding Entry
To edit or delete a port forwarding entry:
1.
In the table, select the button next to the service name.
2.
Click
Edit Service
or
Delete Service
.
Application Example: Making 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.
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