On Networks N300RM User Manual - Manual (Page 52)

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52
N300 WiFi ADSL2+ Modem Router (N300RM)
Port Triggering to Open Incoming Ports
In the preceding example, requests are sent to a remote computer by your modem router
from a service port with a particular number. Replies from the remote computer to your
modem router are directed to that port. If the remote server sends a reply to a different port,
your modem router does not recognize it and discards it. However, some application servers
(such as FTP and IRC servers) send replies to multiple ports. Using the port triggering
function of your modem router, you can tell the modem 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 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 (as in the previous example, say 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