6.2. ALGs
6.2.1. Overview
To complement low-level packet filtering, which only inspects packet headers in protocols such as
IP, TCP, UDP, and ICMP, NetDefend Firewalls provide Application Layer Gateways (ALGs) which
provide filtering at the higher application OSI level.
An ALG object acts as a mediator in accessing commonly used Internet applications outside the
protected network, for example web access, file transfer and multimedia transfer. ALGs provide
higher security than packet filtering since they are capable of scrutinizing all traffic for a specific
protocol and perform checks at the higher levels of the TCP/IP stack.
ALGs exist for the following protocols in NetDefendOS:
• HTTP
• FTP
• TFTP
• SMTP
• POP3
• SIP
• H.323
• TLS
Deploying an ALG
Once a new ALG object is defined by the administrator, it is brought into use by first associating it
with a Service object and then associating that service with an IP rule in the NetDefendOS IP rule
set.
Figure 6.1. Deploying an ALG
6.2. ALGs Chapter 6. Security Mechanisms
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