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5-2
Cisco PIX Firewall and VPN Configuration Guide
78-15033-01
Chapter 5 Configuring Application Inspection (Fixup)
How Application Inspection Works
Figure 5-1 Basic ASA Operations
67564
1
7
6
5
2
3 4
Client
ACL
XLATE
CONN
Inspection
Server
Security
appliance
In Figure 5-1, operations are numbered in the order they occur, and are described as follows:
1. A TCP SYN packet arrives at the PIX Firewall to establish a new connection.
2. The PIX Firewall checks the access control list (ACL) database to determine if the connection is
permitted.
3. The PIX Firewall creates a new entry in the connection database (XLATE and CONN tables).
4. The PIX Firewall checks the Inspections database to determine if the connection requires
application-level inspection.
5. After the application inspection function completes any required operations for the packet, the
PIX
Firewall forwards the packet to the destination system.
6. The destination system responds to the initial request.
7. The PIX Firewall receives the reply packet, looks up the connection in the connection database, and
forwards the packet because it belongs to an established session.
The default configuration of the PIX Firewall includes a set of application inspection entries that
associate supported protocols with specific TCP or UDP port numbers and that identify any special
handling required. The inspection function does not support NAT or PAT for certain applications because
of the constraints imposed by the applications. You can change the port assignments for some
applications, while other applications have fixed port assignments that you cannot change.
Table 5-1
summarizes this information about the application inspection functions provided with PIX Firewall
Version 6.2 and higher.
Ta b l e 5-1 Application Inspection Functions
Application PAT? NAT (1-1)? Configure? Default Port Standards Limitations/Comments
CTIQBE Yes Yes Yes TCP/2748 Introduced with PIX Firewall
Version 6.3
CU-SeeMe No No No UDP/7648 None.
DNS
1
Yes Yes No UDP/53 RFC 1123 Only forward NAT. No PTR
records are changed.
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