CHAPTER 11

Advanced Policies: Sending HTTP

Service Callouts to Applications

You can use HTTP callouts to obtain information from external applications. For example, if a server makes a request, you can use an HTTP callout to determine if this server is on a “deny access” list. The callout policy sends an HTTP request to an external application. An agent that you deploy in front of the application formats the request for the application. When the application returns a response, the agent formats the response for the NetScaler, and the callout policy extracts data of interest from the response. For example, you can configure a callout that sends a client's source domain to a server that looks up bad domains from a list. When the server sends a response, the callout can extract just the “allowed” or “denied” portion of the response.

You configure an HTTP callout as a specialized type of policy. After configuring an HTTP callout policy, you can invoke it from policies and other functions (for example, actions) that use advanced expressions. The expression prefix SYS.

HTTP_CALLOUT invokes a callout policy. For example, you can invoke an HTTP callout policy from a Rewrite action and insert the value that is returned by the callout in an HTTP response header.

Note that you cannot invoke an HTTP callout in a DELETE Rewrite action.

In This Chapter

About Calling Out to an External Application

About HTTP Callout Policies

Configuring an HTTP Callout Policy

Invoking an HTTP Callout Policy

Note: To deploy an HTTP callout, in addition to configuring the policy as described in this chapter, you must also create an agent that formats the callout request for the receiving application and formats the application's response to the NetScaler. For details on the callout agent, see the chapter on HTTP callouts in the Citrix NetScaler AppExpert Guide.

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