Configuring advanced policy expression: getting started

Advanced policies evaluate data based on information that you supply in Advanced policy expressions. An Advanced policy expression analyzes data elements (for example, HTTP headers, source IP addresses, the Citrix ADC system time, and POST body data). In addition to configuring an Advanced policy expression in a policy, in some Citrix ADC features, you configure Advanced policy expression outside of the context of a policy.

To create an Advanced policy expression, you select a prefix that identifies a piece of data that you want to analyze, and then you specify an operation to perform on the data. For example, an operation can match a piece of data with a text string that you specify, or it can transform a text string into an HTTP header. Other operations match a returned string with a set of strings or a string pattern. You configure compound expressions by specifying Boolean and arithmetic operators, and by using parentheses to control the order of evaluation.

Advanced policy expression can also contain classic expressions. You can assign a name to a frequently used expression to avoid having to build the expression repeatedly.

Policies and a few other entities include rules that the Citrix ADC uses to evaluate a packet in the traffic flowing through it, to extract data from the Citrix ADC system itself, to send a request (a “callout”) to an external application, or to analyze another piece of data. A rule takes the form of a logical expression that is compared against traffic and ultimately returns values of TRUE or FALSE.

The elements of the rule can themselves return TRUE or FALSE, string, or numeric values.

Before configuring an Advanced policy expression, you need to understand the characteristics of the data that the policy or other entity is to evaluate. For example, when working with the Integrated Caching feature, a policy determines what data can be stored in the cache. With Integrated Caching, you need to know the URLs, headers, and other data in the HTTP requests and responses that the Citrix ADC receives. With this knowledge, you can configure policies that match the actual data and enable the Citrix ADC to manage caching for HTTP traffic. This information helps you determine the type of expression that you need to configure in the policy.