ZyAIR
With stateful inspection, fields of the packets are compared to packets that are already known to be trusted. For example, if you access some outside service, the proxy server remembers things about your original request, like the port number and source and destination addresses. This remembering is called saving the state. When the outside system responds to your request, the firewall compares the received packets with the saved state to determine if they are allowed in. The ZyAIR uses stateful packet inspection to protect the private LAN from hackers and vandals on the Internet. By default, the ZyAIR’s stateful inspection allows all communications to the Internet that originate from the LAN, and blocks all traffic to the LAN that originates from the Internet. In summary, stateful inspection:
•Allows all sessions originating from the LAN (local network) to the WAN (Internet).
•Denies all sessions originating from the WAN to the LAN.
Figure 78 Stateful Inspection
The previous figure shows the ZyAIR’s default firewall rules in action as well as demonstrates how stateful inspection works. User A can initiate a Telnet session from within the LAN and responses to this request are allowed. However other Telnet traffic initiated from the WAN is blocked.
14.5.1 Stateful Inspection ProcessIn this example, the following sequence of events occurs when a TCP packet leaves the LAN network through the firewall's WAN interface. The TCP packet is the first in a session, and the packet's application layer protocol is configured for a firewall rule inspection:
1The packet travels from the firewall's LAN to the WAN.
2The packet is evaluated against the interface's existing outbound access list, and the packet is permitted (a denied packet would simply be dropped at this point).
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