Many of the advanced settings in an access control policy govern intrusion detection and prevention configurations that require specific expertise to configure. Advanced settings typically require little or no modification and are not common to every deployment.

Each access control policy uses its default intrusion policy to initially inspect traffic before the system can determine exactly how to inspect that traffic. This is needed because sometimes the system must process the first few packets in a connection, allowing them to pass, before it can decide which access control rule (if any) will handle the traffic. However, so that these packets do not reach their destination uninspected, you can use an intrusion policy—called the default intrusion policy—to inspect them and generate intrusion events. By default, the default intrusion policy uses the default variable set.

A default intrusion policy is especially useful when performing application control and URL filtering, because the system cannot identify applications or filter URLs before a connection is fully established between the client and the server. For example, if a packet matches all the other conditions in an access control rule with an application or URL condition, it and subsequent packets are allowed to pass until the connection is established and application or URL identification is complete, usually 3 to 5 packets.

The system inspects these allowed packets with the default intrusion policy, which can generate events and, if placed inline, block malicious traffic. After the system identifies the access control rule or default action that should handle the connection, the remaining packets in the connection are handled and inspected accordingly.

When you create an access control policy, its default intrusion policy depends on the default action you first chose. Initial default intrusion policies for access control are as follows:

• Balanced Security and Connectivity (a system-provided policy) is the default intrusion policy for an access control policy where you first chose the Intrusion Prevention default action.

• No Rules Active is the default intrusion policy for an access control policy where you first chose the Block all traffic or Network Discovery default action. Although choosing this option disables intrusion inspection on the allowed packets described above, it can improve performance if you are not interested in intrusion data.

  Note	If you are not performing intrusion inspection (for example, in a discovery-only deployment), keep the No Rules Active policy as your default intrusion policy.

If you change your default action after you create the access control policy, the default intrusion policy does not automatically change. To change it manually, use the access control policy’s advanced options.

You can choose a system- or user-created policy.

Note	The network analysis policy associated with the first matching network analysis rule preprocesses traffic for the default intrusion policy. If there are no network analysis rules, or none match, the default network analysis policy is used.

Network analysis policies govern how traffic is decoded and preprocessed so that it can be further evaluated, especially for anomalous traffic that might signal an intrusion attempt. This traffic preprocessing occurs after Security Intelligence blacklisting and traffic decryption, but before intrusion policies inspect packets in detail. By default, the system-provided Balanced Security and Connectivity network analysis policy is the default network analysis policy.

Tip

The system-provided Balanced Security and Connectivity network analysis policy and the Balanced Security and Connectivity intrusion policy work together and can both be updated in intrusion rule updates. However, the network analysis policy governs mostly preprocessing options, whereas the intrusion policy governs mostly intrusion rules.

A simple way to tune preprocessing is to create and use a custom network analysis policy as the default. For advanced users with complex deployments, you can create multiple network analysis policies, each tailored to preprocess traffic differently. Then, you can configure the system to use those policies to govern the preprocessing of traffic using different security zones, networks, or VLANs.

To accomplish this, you add custom network analysis rules to your access control policy. A network analysis rule is simply a set of configurations and conditions that specifies how you preprocess traffic that matches those qualifications. You create and edit network analysis rules in the advanced options in an existing access control policy. Each rule belongs to only one policy.

Each rule has:

• a set of rule conditions that identifies the specific traffic you want to preprocess

• an associated network analysis policy that you want to use to preprocess traffic that meets all the rules’ conditions

When it is time for the system to preprocess traffic, it matches packets to network analysis rules in top-down order by rule number. Traffic that does not match any network analysis rules is preprocessed by the default network analysis policy.