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Firewall Rules

The purpose of a network firewall is to protect computer and IT resources from malicious sources while allowing internal network users to access the Internet. Today networks use "stateful" firewalls. A stateful firewall monitors the dynamic state of data flow and makes decisions on whether to block or allow data to pass based upon advanced rules.

But in the past all firewalls were "stateless" and made decisions on whether to block or allow data to pass by examining individual packets against the firewall's Access Control List (ACL). This type of firewall was also called a "packet filtering" firewall.

An ACL uses IP address masks to specify what should be permitted and denied. ACL masks are opposite normal IP address masks in that an 0 indicates that the address bits must be an exact match and a 1 in the mask is a "don't care". You can create an ACL masks by subtracting the IP addresses normal mask from

permit ip any
permit ip any
permit ip any
permit ip any
deny ip any any

An ACL then is a list of rules with statements to permit or deny the passage of packets with specific ranges of IP addresses. The rules can be applied to either the inbound or the outbound traffic. At the end of the ACL, by default, there is an implicit deny rule that blocks all traffic for which there is not a rule.

A packet filter firewall alone cannot detect some attacks from the transport layer and application layer, such as TCP SYN flooding and malicious Java applets. The Application Specific Packet Filter (ASPF) specification was proposed to address these issues. An ASPF firewall implements application layer and transport specific packet filtering.

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