Configuration and Management
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1/1551-CRH 102 167 Uen Rev C 2007-04-19
2.7.2
Application Level Gateways
From a security perspective, certain Internet applications, for example FTP
applications that open additional ports upon transfer, are especially
problematic to handle. An Application Level Gateway (ALG) provides a
translation and transportation service for such a specific application.
Incoming data packets are checked against existing NAT and packet
filtering rules, IP addresses are evaluated and a detailed packet analysis is
performed. If necessary, the contents of a packet are modified and if a
secondary port is required, the ALG will open one. The Ericsson W20/21
includes ALG support for the following applications:
Table 4
ALG Supported Applications
Application Protocol
Port
number
File Transfer Protocol (FTP)
TCP
21
Trivial File Transfer Protocol (TFTP)
UDP
69
The ALG for each application does not require additional configuration. The
supported ALGs can be enabled and disabled individually. To disable an
ALG, clear the corresponding check box on the
NAT
page and click
Apply
.
2.7.3 Port
Forwarding
Port forwarding is used to allow an external user to access a service
residing on a server connected to the LAN (to cross NAT-border). It
enables access to servers on the LAN from Internet (e.g. Web server) and
also enables applications to work from the LAN (e.g. games, voice and
chat) to the Internet.
When a computer on the Internet sends data to the public IP address of
Ericsson W20/21, it needs to know what to do with the data. Port
Forwarding tells Ericsson W20/21 which computer on the local area
network to send the data to.
Note:
Port forwarding requires a public IP address of the Ericsson
W20/21. The Ericsson W20/21 IP address is displayed on the
Internet
page. A private IP address usually begins with
10
,
172
, or
192
. In this case, no incoming access from the Internet is allowed.
For more information on public and private IP addresses, contact
your service provider.