Citrix ADC

Invoking an HTTP Callout

After you configure an HTTP callout, you invoke the callout by including the SYS.HTTP_CALLOUT(<name>) expression in a default syntax policy rule. In this expression, <name> is the name of the HTTP callout that you want to invoke.

You can use default syntax expression operators with the callout expression to process the response and then perform an appropriate action. The return type of the response from the HTTP callout agent determines the set of operators that you can use on the response. If the part of the response that you want to analyze is text, you can use a text operator to analyze the response. For example, you can use the CONTAINS(<string>) operator to check whether the specified portion of the response contains a particular string, as in the following example:

SYS.HTTP_CALLOUT(mycallout).contains("Good IP address")

If you use the preceding expression in a responder policy, you can configure an appropriate responder action.

Similarly, if the part of the response that you want to evaluate is a number, you can use a numeric operator such as GT(int). If the response contains a Boolean value, you can use a Boolean operator.

Note: An HTTP callout can invoke itself recursively. HTTP callout recursion can be avoided by combining the HTTP callout expression with a default syntax expression that prevents recursion. For information about how you can avoid HTTP callout recursion, see Avoiding HTTP Callout Recursion.

You can also cascade HTTP callouts by configuring policies that each invoke a callout after evaluating previously generated callouts. In this scenario, after one policy invokes a callout, when the Citrix ADC appliance is parsing the callout before sending the callout to the callout server, a second set of policies can evaluate the callout and invoke additional callouts, which can in turn be evaluated by a third set of policies, and so on. Such an implementation is described in the following example.

First, you could configure an HTTP callout called myCallout1, and then configure a responder policy, Pol1, to invoke myCallout1. Then, you could configure a second HTTP callout, myCallout2, and a responder policy, Pol2. You configure Pol2 to evaluate myCallout1 and invoke myCallout2. You bind both responder policies globally.

To avoid HTTP callout recursion, myCallout1 is configured with a unique custom HTTP header called “Request1.” Pol1 is configured to avoid HTTP callout recursion by using the default syntax expression,

HTTP.REQ.HEADER("Request1").EQ("Callout Request").NOT.

Pol2 uses the same default syntax expression, but excludes the .NOT operator so that the policy evaluates myCallout1 when the Citrix ADC appliance is parsing it. Note that myCallout2 identifies its own unique header called “Request2,” and Pol2 includes a default syntax expression to prevent myCallout2 from invoking itself recursively.


> add policy httpCallout myCallout1


> set policy httpCallout myCallout1 -IPAddress -port 80 -returnType TEXT -hostExpr
 """" -urlStemExpr "\"/cgi-bin/\"" -headers Request1
("Callout Request") -parameters cip(CLIENT.IP.SRC) -resultExpr "HTTP.RES.BODY(100)"


> add responder policy Pol1 "HTTP.REQ.HEADER(\"Request1\").EQ(\"Callout Request\").NOT &&


> bind responder global Pol1 100 END -type OVERRIDE


> add policy httpCallout myCallout2


> set policy httpCallout myCallout2 -IPAddress -port 80 -returnType TEXT -hostExpr
"\"\"" -urlStemExpr "\"/cgi-bin/\"" -headers Request2
("Callout Request") -parameters cip(CLIENT.IP.SRC) -resultExpr "HTTP.RES.BODY(200)"


> add responder policy Pol2 "HTTP.REQ.HEADER(\"Request2\").EQ(\"Callout Request\").NOT &&
 HTTP.REQ.HEADER(\"Request1\").EQ(\"Callout Request\") && SYS.HTTP_CALLOUT(myCallout2).CONTAINS


> bind responder global Pol2 110 END -type OVERRIDE

Invoking an HTTP Callout