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Without decoding and preprocessing, the system could not appropriately evaluate traffic for intrusions because protocol differences would make pattern matching impossible. As shown in the diagram in Understanding How Policies Examine Traffic For Intrusions, network analysis policies govern these traffic-handling tasks:

• after traffic is filtered by Security Intelligence

• before traffic can be inspected by file or intrusion policies

A network analysis policy governs packet processing in phases. First the system decodes packets through the first three TCP/IP layers, then continues with normalizing, preprocessing, and detecting protocol anomalies.

• The packet decoder converts packet headers and payloads into a format that can be easily used by the preprocessors and later, intrusion rules. Each layer of the TCP/IP stack is decoded in turn, beginning with the data link layer and continuing through the network and transport layers. The packet decoder also detects various anomalous behaviors in packet headers.

• In inline deployments, the inline normalization preprocessor reformats (normalizes) traffic to minimize the chances of attackers evading detection. It prepares packets for examination by other preprocessors and intrusion rules, and helps ensure that the packets the system processes are the same as the packets received by the hosts on your network.

• Various network and transport layers preprocessors detect attacks that exploit IP fragmentation, perform checksum validation, and perform TCP and UDP session preprocessing.

Note that some advanced transport and network preprocessor settings apply globally to all traffic handled by an access control policy. You configure these in the access control policy rather than in a network analysis policy.

• Various application-layer protocol decoders normalize specific types of packet data into formats that the intrusion rules engine can analyze. Normalizing application-layer protocol encodings allows the system to effectively apply the same content-related intrusion rules to packets whose data is represented differently, and to obtain meaningful results. For more information.

• The Modbus and DNP3 SCADA preprocessors detect traffic anomalies and provide data to intrusion rules. Supervisory Control and Data Acquisition (SCADA) protocols monitor, control, and acquire data from industrial, infrastructure, and facility processes such as manufacturing, production, water treatment, electric power distribution, airport and shipping systems, and so on.

• Several preprocessors allow you to detect specific threats, such as Back Orifice, portscans, SYN floods and other rate-based attacks.

Note that you configure the sensitive data preprocessor, which detects sensitive data such as credit card numbers and Social Security numbers in ASCII text, in intrusion policies.

In a newly created access control policy, one default network analysis policy governs preprocessing for all traffic for all intrusion policies invoked by the same parent access control policy. Initially, the system uses the Balanced Security and Connectivity network analysis policy as the default, but you can change it to another system-provided or custom network analysis policy. In a more complex deployment, advanced users can tailor traffic preprocessing options to specific security zones and networks by assigning different custom network analysis policies to preprocess matching traffic. For more information, see Comparing System-Provided with Custom Policies.

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After initial preprocessing, access control rules (when present) evaluate traffic. In most cases, the first access control rule that a packet matches is the rule that handles that traffic; you can monitor, trust, block, or allow matching traffic.

When you allow traffic with an access control rule, the system can inspect the traffic for malware, prohibited files, and intrusions, in that order. Traffic not matching any access control rule is handled by the access control policy’s default action, which can also inspect for intrusions.

Note	All packets, regardless of which network analysis policy preprocesses them, are matched to configured access control rules—and thus are potentially subject to inspection by intrusion policies—in top-down order. For more information, see Limitations of Custom Policies.

The diagram in Understanding How Policies Examine Traffic For Intrusions shows the flow of traffic through a device in an inline, intrusion prevention and AMP deployment, as follows:

• The access control rule allows matching traffic to proceed. The traffic is then inspected for prohibited files and malware by a file policy, and then for intrusions by an intrusion policy.
• In this scenario, the access control policy’s default action allows matching traffic. The traffic is then inspected by an intrusion policy. You can (but do not have to) use a different intrusion policy when you associate intrusion policies with access control rules or the default action.

The example in the diagram does not include any blocking or trusting rules because the system does not inspect blocked or trusted traffic. For more information, see Using Rule Actions to Determine Traffic Handling and Inspection and Setting Default Handling and Inspection for Network Traffic.

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You can use intrusion prevention as the system’s last line of defense before traffic is allowed to proceed to its destination. Intrusion policies govern how the system inspects traffic for security violations and, in inline deployments, can block or alter malicious traffic. The main function of intrusion policies is to manage which intrusion and preprocessor rules are enabled and how they are configured.

Variable Sets

Whenever the system uses an intrusion policy to evaluate traffic, it uses an associated variable set . Most variables in a set represent values commonly used in intrusion rules to identify source and destination IP addresses and ports. You can also use variables in intrusion policies to represent IP addresses in rule suppressions and dynamic rule states.

The system provides a single default variable set, which is comprised of predefined default variables. Most system-provided shared object rules and standard text rules use these predefined default variables to define networks and port numbers. For example, the majority of the rules use the variable $HOME_NET to specify the protected network and the variable $EXTERNAL_NET to specify the unprotected (or outside) network. In addition, specialized rules often use other predefined variables. For example, rules that detect exploits against web servers use the $HTTP_SERVERS and $HTTP_PORTS variables.

Tip

Even if you use system-provided intrusion policies, Cisco strongly recommends you modify key default variables in the default set. When you use variables that accurately reflect your network environment, processing is optimized and the system can monitor relevant systems for suspicious activity. Advanced users can create and use custom variable sets for pairing with one or more custom intrusion policies. For more information, see Optimizing Predefined Default Variables.

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When the system identifies a possible intrusion, it generates an intrusion or preprocessor event (sometimes collectively called intrusion events ). You can view the data to gain a greater understanding of the attacks against your network assets. In an inline deployment, the system can also drop or replace packets that you know to be harmful.

Each intrusion event includes an event header and contains information about the event name and classification; the source and destination IP addresses; ports; the process that generated the event; and the date and time of the event, as well as contextual information about the source of the attack and its target. For packet-based events, the system also logs a copy of the decoded packet header and payload for the packet or packets that triggered the event.

The packet decoder, the preprocessors, and the intrusion rules engine can all cause the system to generate an event. For example:

• If the packet decoder (configured in the network analysis policy) receives an IP packet that is less than 20 bytes, which is the size of an IP datagram without any options or payload, the decoder interprets this as anomalous traffic. If, later, the accompanying decoder rule in the intrusion policy that examines the packet is enabled, the system generates a preprocessor event.
• If the IP defragmentation preprocessor encounters a series of overlapping IP fragments, the preprocessor interprets this as a possible attack and, when the accompanying preprocessor rule is enabled, the system generates a preprocessor event.
• Within the intrusion rules engine, most standard text rules and shared object rules are written so that they generate intrusion events when triggered by packets.

As the device accumulates intrusion events, you can begin your analysis of potential attacks. The system provides you with the tools you need to review intrusion events and evaluate whether they are important in the context of your network environment and your security policies.

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Cisco delivers several pairs of network analysis and intrusion policies with the ASA FirePOWER module. By using system-provided network analysis and intrusion policies, you can take advantage of the experience of the Cisco Vulnerability Research Team (VRT). For these policies, the VRT sets intrusion and preprocessor rule states, as well as provides the initial configurations for preprocessors and other advanced settings. You can use system-provided policies as-is, or you can use them as the base for custom policies.

Tip	Even if you use system-provided network analysis and intrusion policies, you should configure the system’s intrusion variables to accurately reflect your network environment. At a minimum, modify key default variables in the default set; see Optimizing Predefined Default Variables.

As new vulnerabilities become known, the VRT releases intrusion rule updates. These rule updates can modify any system-provided network analysis or intrusion policy, and can provide new and updated intrusion rules and preprocessor rules, modified states for existing rules, and modified default policy settings. Rule updates may also delete rules from system-provided policies and provide new rule categories, as well as modify the default variable set.

If a rule update affects your deployment, the system marks affected intrusion and network analysis policies as out of date, as well as their parent access control policies. You must reapply an updated policy for its changes to take effect.

For your convenience, you can configure rule updates to automatically reapply affected intrusion policies, either alone or in combination with affected access control policies. This allows you to easily and automatically keep your deployment up-to-date to protect against recently discovered exploits and intrusions.

To ensure up-to-date preprocessing settings, you must reapply access control policies, which also reapplies any associated SSL, network analysis, and file policies that are different from those currently running, and can also can update default values for advanced preprocessing and performance options. For more information, see Importing Rule Updates and Local Rule Files.

Cisco delivers the following network analysis and intrusion policies with the ASA FirePOWER module:

No Rules Active intrusion policy

In the No Rules Active intrusion policy, all intrusion rules and advanced settings are disabled. This policy provides a starting point if you want to create your own intrusion policy instead of basing it on the enabled rules in one of the other system-provided policies.

Caution

Cisco uses another policy, Experimental Policy 1, for testing purposes. Do not use it unless instructed to do so by a Cisco representative.

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You may find that the preprocessor options, intrusion rules, and other advanced settings configured in the system-provided network analysis and intrusion policies do not fully address the security needs of your organization.

Building custom policies can improve the performance of the system in your environment and can provide a focused view of the malicious traffic and policy violations occurring on your network. By creating and tuning custom policies you can configure, at a very granular level, how the system processes and inspects the traffic on your network for intrusions.

All custom policies have a base policy, also called a base layer, which defines the default settings for all configurations in the policy. A layer is a building block that you can use to efficiently manage multiple network analysis or intrusion policies; see Using Layers in a Network Analysis or Intrusion Policy Layers.

In most cases, you base custom policies on system-provided policies, but you can use another custom policy. However, all custom policies have a system-provided policy as the eventual base in a policy chain. Because rule updates can modify system-provided policies, importing a rule update may affect you even if you are using a custom policy as your base. If a rule update affects your policies, the module interface marks affected policies as out of date. For more information, see Allowing Rule Updates to Modify a System-Provided Base Policy.

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By default, one network analysis policy preprocesses all unencrypted traffic handled by the access control policy. That means that all packets are decoded and preprocessed according to the same settings, regardless of the intrusion policy (and therefore intrusion rule set) that later examines them.

Initially, the system-provided Balanced Security and Connectivity network analysis policy is the default. A simple way to tune preprocessing is to create and use a custom network analysis policy as the default.

Tuning options available vary by preprocessor, but some of the ways you can tune preprocessors and decoders include:

• You can disable preprocessors that do not apply to the traffic you are monitoring. For example, the HTTP Inspect preprocessor normalizes HTTP traffic. If you are confident that your network does not include any web servers using Microsoft Internet Information Services (IIS), you can disable the preprocessor option that looks for IIS-specific traffic and thereby reduce system processing overhead.

Note

If you disable a preprocessor in a custom network analysis policy, but the system needs to use that preprocessor to later evaluate packets against an enabled intrusion or preprocessor rule, the system automatically enables and uses the preprocessor although the preprocessor remains disabled in the network analysis policy user interface.

• Specify ports, where appropriate, to focus the activity of certain preprocessors. For example, you can identify additional ports to monitor for DNS server responses or encrypted SSL sessions, or ports on which you decode telnet, HTTP, and RPC traffic

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 or networks.

Note

Tailoring preprocessing using custom network analysis policies—especially multiple network analysis policies—is an advanced task. Because preprocessing and intrusion inspection are so closely related, you must be careful to allow the network analysis and intrusion policies examining a single packet to complement each other. For more information, see Limitations of Custom Policies.

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In a newly created access control policy initially configured to perform intrusion prevention, the default action allows all traffic, but first inspects it with the system-provided Balanced Security and Connectivity intrusion policy. Unless you add access control rules or change the default action, all traffic is inspected by that intrusion policy; see the diagram in Comparing System-Provided with Custom Policies.

To customize your intrusion prevention deployment, you can create multiple intrusion policies, each tailored to inspect traffic differently. Then, configure an access control policy with rules that specify which policy inspects which traffic. Access control rules can be simple or complex, matching and inspecting traffic using multiple criteria including security zone, network or geographical location, port, application, requested URL, or user. The scenario in Understanding How Policies Examine Traffic For Intrusions shows a deployment where traffic is inspected by one of two intrusion policies.

The main function of intrusion policies is to manage which intrusion and preprocessor rules are enabled and how they are configured, as follows:

• Within each intrusion policy, you should verify that all rules applicable to your environment are enabled, and improve performance by disabling rules that are not applicable to your environment. In an inline deployment, you can specify which rules should drop or modify malicious packets. For more information, see Setting Rule States.
• You can modify existing rules and write new standard text rules as needed to catch new exploits or to enforce your security policies.

Other customizations you might make to an intrusion policy include:

• The sensitive data preprocessor detects sensitive data such as credit card numbers and Social Security numbers in ASCII text. Note that other preprocessors that detect specific threats (back orifice attacks, several portscan types, and rate-based attacks that attempt to overwhelm your network with excessive traffic) are configured in network analysis policies.
• Global thresholds cause the system to generate events based on how many times traffic matching an intrusion rule originates from or is targeted to a specific address or address range within a specified time period. This helps prevent the system from being overwhelmed with a large number of events. For more information, see Limiting Intrusion Event Logging.
• Suppressing intrusion event notifications and setting thresholds for individual rules or entire intrusion policies can also can prevent the system from being overwhelmed with a large number of events. For more information, see Filtering Intrusion Event Notification Per Policy.
• In addition to intrusion events, you can enable logging to syslog facilities or send event data to an SNMP trap server. Per policy, you can specify intrusion event notification limits, set up intrusion event notification to external logging facilities, and configure external responses to intrusion events. For more information, see Configuring External Alerting for Intrusion Rules.

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Because preprocessing and intrusion inspection are so closely related, you must be careful that your configuration allows the network analysis and intrusion policies processing and examining a single packet to complement each other.

By default, the system uses one network analysis policy to preprocess all traffic. The following diagram shows how a newly created access control policy in an inline, intrusion-prevention deployment initially handles traffic. The preprocessing and intrusion prevention phases are highlighted.

Notice how a default network analysis policy governs the preprocessing of all traffic handled by the access control policy. Initially, the system-provided Balanced Security and Connectivity network analysis policy is the default.

A simple way to tune preprocessing is to create and use a custom network analysis policy as the default, as summarized in Benefits of a Custom Network Analysis Policy. However, if you disable a preprocessor in a custom network analysis policy but the system needs to evaluate preprocessed packets against an enabled intrusion or preprocessor rule, the system automatically enables and uses the preprocessor although it remains disabled in the network analysis policy user interface.

Note	In order to get the performance benefits of disabling a preprocessor, you must make sure that none of your intrusion policies have enabled rules that require that preprocessor.

An additional challenge arises if you use multiple custom network analysis policies. For advanced users with complex deployments, you can tailor preprocessing to specific security zones and networks by assigning custom network analysis policies to preprocess matching traffic. To accomplish this, you add custom network analysis rules to your access control policy. Each rule has an associated network analysis policy that governs the preprocessing of traffic that matches the rule.

Tip	You configure network analysis rules as an advanced setting in an access control policy. Unlike other types of rules in the ASA FirePOWER module, network analysis rules invoke—rather than being contained by—network analysis policies.

The system matches packets to any configured network analysis rules in top-down order by rule number. Traffic that does not match any network analysis rule is preprocessed by the default network analysis policy. While this allows you a great deal of flexibility in preprocessing traffic, keep in mind that all packets, regardless of which network analysis policy preprocessed them, are subsequently matched to access control rules—and thus to potential inspection by intrusion policies—in their own process. In other words, preprocessing a packet with a particular network analysis policy does not guarantee that the packet will be examined with any particular intrusion policy. You must carefully configure your access control policy so it invokes the correct network analysis and intrusion policies to evaluate a particular packet.

The following diagram shows in focused detail how the network analysis policy (preprocessing) selection phase occurs before and separately from the intrusion prevention (rules) phase. For simplicity, the diagram eliminates the file/malware inspection phases. It also highlights the default network analysis and default-action intrusion policies.

In this scenario, an access control policy is configured with two network analysis rules and a default network analysis policy:

• Network Analysis Rule A preprocesses matching traffic with Network Analysis Policy A. Later, you want this traffic to be inspected by Intrusion Policy A.
• Network Analysis Rule B preprocesses matching traffic with Network Analysis Policy B. Later, you want this traffic to be inspected by Intrusion Policy B.
• All remaining traffic is preprocessed with the default network analysis policy. Later, you want this traffic to be inspected by the intrusion policy associated with the access control policy’s default action.

After the system preprocesses traffic, it can examine the traffic for intrusions. The diagram shows an access control policy with two access control rules and a default action:

• Access Control Rule A allows matching traffic. The traffic is then inspected by Intrusion Policy A.
• Access Control Rule B allows matching traffic. The traffic is then inspected by Intrusion Policy B.
• The access control policy’s default action allows matching traffic. The traffic is then inspected by the default action’s intrusion policy.

Each packet’s handling is governed by a network analysis policy and intrusion policy pair, but the system does not coordinate the pair for you. Consider a scenario where you misconfigure your access control policy so that Network Analysis Rule A and Access Control Rule A do not process the same traffic. For example, you could intend the paired policies to govern the handling of traffic on a particular security zone, but you mistakenly use different zones in the two rules’ conditions. This could cause traffic to be incorrectly preprocessed. For this reason, tailoring preprocessing using network analysis rules and custom policies is an advanced task.

Note that for a single connection, although the system selects a network analysis policy before an access control rule, some preprocessing (notably application layer preprocessing) occurs after access control rule selection. This does not affect how you configure preprocessing in custom network analysis policies.