Bedienungsanleitung Netgear DGN2200Bv4 (Seite 61 von 133) (Englisch)

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Security

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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