ZyXEL Communications 792H manual Stateful Inspection and the Prestige

Prestige 792H G.SHDSL Router

4.Based on the obtained state information, a firewall rule creates a temporary access list entry that is inserted at the beginning of the WAN interface's inbound extended access list. This temporary access list entry is designed to permit inbound packets of the same connection as the outbound packet just inspected.

5.The outbound packet is forwarded out through the interface.

6.Later, an inbound packet reaches the interface. This packet is part of the connection previously established with the outbound packet. The inbound packet is evaluated against the inbound access list, and is permitted because of the temporary access list entry previously created.

7.The packet is inspected by a firewall rule, and the connection's state table entry is updated as necessary. Based on the updated state information, the inbound extended access list temporary entries might be modified, in order to permit only packets that are valid for the current state of the connection.

8.Any additional inbound or outbound packets that belong to the connection are inspected to update the state table entry and to modify the temporary inbound access list entries as required, and are forwarded through the interface.

9.When the connection terminates or times out, the connection's state table entry is deleted and the connection's temporary inbound access list entries are deleted.

8.5.2 Stateful Inspection and the Prestige

Additional rules may be defined to extend or override the default rules. For example, a rule may be created which will:

i.Block all traffic of a certain type, such as IRC (Internet Relay Chat), from the LAN to the Internet.

ii.Allow certain types of traffic from the Internet to specific hosts on the LAN.

iii.Allow access to a Web server to everyone but competitors.

iv.Restrict use of certain protocols, such as Telnet, to authorized users on the LAN.

These custom rules work by evaluating the network traffic’s Source IP address, Destination IP address, IP protocol type, and comparing these to rules set by the administrator.

The ability to define firewall rules is a very powerful tool. Using custom rules, it is possible to disable all firewall protection or block all access to the Internet. Use extreme caution when creating or deleting firewall rules. Test changes after creating them to make sure they work correctly.

Firewalls 8-9