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Cisco Security Appliance Command Line Configuration Guide
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Chapter 21 Applying Application Layer Protocol Inspection
Application Inspection Engines
Overview
The Adaptive Security Algorithm, used by the security appliance for stateful application inspection,
ensures the secure use of applications and services. Some applications require special handling by the
security appliance and specific application inspection engines are provided for this purpose.
Applications that require special application inspection engines are those that embed IP addressing
information in the user data packet or open secondary channels on dynamically assigned ports.
Application inspection engines work with NAT to help identify the location of embedded addressing
information. This allows NAT to translate these embedded addresses and to update any checksum or
other fields that are affected by the translation.
Each application inspection engine also monitors sessions to determine the port numbers for secondary
channels. Many protocols open secondary TCP or UDP ports to improve performance. The initial session
on a well-known port is used to negotiate dynamically assigned port numbers. The application inspection
engine monitors these sessions, identifies the dynamic port assignments, and permits data exchange on
these ports for the duration of the specific session.
How Inspection Engines Work
As illustrated in Figure 21-1, the security appliance uses three databases for its basic operation:
• Access lists —Used for authentication and authorization of connections based on specific networks,
hosts, and services (TCP/UDP port numbers).
• Inspections—Contains a static, predefined set of application-level inspection functions.
• Connections (XLATE and CONN tables)—Maintains state and other information about each
established connection. This information is used by the Adaptive Security Algorithm and
cut-through proxy to efficiently forward traffic within established sessions.
Figure 21-1 Basic Adaptive Security Algorithm Operations
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