Security Settings
47
N150 Wireless ADSL2+ Modem Router DGN1000
Open Inbound Ports (Port Forwarding)
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.By using the
inbound rule 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
d
estination 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. WIth inbound rules, 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 inbound rule you have
defined:
1. Y
ou open an IRC client program to start a chat session on your computer.
2. Y
our 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. Y
our 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. Notin
g your inbound 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 se
rver sends a return message to your 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 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 inbound rules, you need to know which in
bound 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 relevent user groups or newsgroups.
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