Chapter 14 Firewalls
6Later, 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.
7The 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.
8Any 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.
9When 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.
14.5.2Stateful Inspection on Your ZyXEL DeviceAdditional rules may be defined to extend or override the default rules. For example, a rule may be created which will:
•Block all traffic of a certain type, such as IRC (Internet Relay Chat), from the LAN to the Internet.
•Allow certain types of traffic from the Internet to specific hosts on the LAN.
•Allow access to a Web server to everyone but competitors.
•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.
Below is a brief technical description of how these connections are tracked. Connections may either be defined by the upper protocols (for instance, TCP), or by the ZyXEL Device itself (as with the "virtual connections" created for UDP and ICMP).
14.5.3TCP SecurityThe ZyXEL Device uses state information embedded in TCP packets. The first packet of any new connection has its SYN flag set and its ACK flag cleared; these are "initiation" packets. All packets that do not have this flag structure are called "subsequent" packets, since they represent data that occurs later in the TCP stream.
If an initiation packet originates on the WAN, this means that someone is trying to make a connection from the Internet into the LAN. Except in a few special cases (see "Upper Layer Protocols" shown next), these packets are dropped and logged.
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