Application example: Port triggering for Internet Relay Chat
Some application servers, such as FTP and IRC servers, send replies to multiple port
numbers. Using port triggering, you can tell the router to open more incoming ports
when a particular outgoing port starts 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 must
also allow incoming traffic on port 113 to reach the originating computer.” The following
sequence shows the effects of the port triggering rule you define:
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 observing the destination port number of 6667,
your router creates another 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 (for example, port 33333) as the destination port. The IRC server also sends an
“identify” message to your router with destination port 113.
6. When your router receives the incoming message to destination port 33333, it checks
its session table to see if a session is active 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. When your router receives the incoming message to destination port 113, it checks
its session table and finds 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.
User Manual
140
Customize Internet Traffic Rules
for Ports
Orbi 4GX Router Model LBR1020
