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ADC
14.1 - Current Release 13.1 13.0 12.1
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Avoiding HTTP callout recursion

September 14, 2021
Contributed by: 
S C S

Even though the Citrix ADC appliance does not check for the validity of the HTTP callout request, it parses the request once before it sends the request to the HTTP callout agent. This parsing allows the appliance to treat the callout request as any other incoming request, which in turn allows you to configure several useful Citrix ADC features (such as integrated caching) to work on the callout request.

However, during this parsing, the HTTP callout request can select the same policy and therefore invoke itself recursively. The appliance detects the recursive invocation and raises an undefined (UNDEF) condition. However, the recursive invocation results in the policy and HTTP callout select counters being incremented by two counts each instead of one count each.

To prevent a callout from invoking itself, you must identify at least one unique characteristic of the HTTP callout request, and then exclude all requests with this characteristic from being processed by the policy rule that invokes the callout. You can do so by including another default syntax expression in the policy rule. The expression must precede the SYS.HTTP_CALLOUT(<name>) expression so that it is evaluated before the callout expression is evaluated. For example:

<Expression that prevents callout recursion> OR SYS.HTTP_CALLOUT(<name>)
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When you configure a policy rule in this way, when the appliance generates the request and parses it, the compound rule evaluates to FALSE, the callout is not generated a second time, and the select counters are incremented correctly.

One way by which you can assign a unique characteristic to an HTTP callout request is to include a unique custom HTTP header when you configure the callout. Following is an example of an HTTP callout called “myCallout.” The callout generates an HTTP request that checks whether a client’s IP address is present in a database of blacklisted IP addresses. The callout includes a custom header called “Request,” which is set to the value “Callout Request.” A globally bound responder policy, “Pol1,” invokes the HTTP callout but excludes all requests whose Request header is set to this value, thus preventing a second invocation of myCallout. The expression that prevents a second invocation is HTTP.REQ.HEADER(“Request”).EQ("Callout Request").NOT.

Example:

> add policy httpCallout myCallout
 Done

> set policy httpCallout myCallout -IPAddress 10.102.3.95 -port 80 -returnType TEXT -hostExpr "\"10.102.3.95\"" -urlStemExpr "\"/cgi-bin/check_clnt_from_database.pl\"" -headers Request("Callout Request") -parameters cip(CLIENT.IP.SRC) -resultExpr "HTTP.RES.BODY(100)"
Done

> add responder policy Pol1 "HTTP.REQ.HEADER(\"Request\").EQ(\"Callout Request\").NOT && SYS.HTTP_CALLOUT(myCallout).CONTAINS(\"IP Matched\")" RESET
Done

> bind responder global Pol1 100 END -type OVERRIDE
Done
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Note: You can also configure an expression to check whether the request URL includes the stem expression configured for the HTTP callout. To implement the solution, ensure the HTTP callout agent can respond only to HTTP callouts and not to other requests directed through the appliance. If the HTTP callout agent is an application or Web server that serves other client requests, such an expression prevents the appliance from processing those client requests. Instead, use a unique custom header as described earlier.

Avoiding HTTP callout recursion
September 14, 2021
Contributed by: 
S C S
September 14, 2021
Contributed by: 
S C S

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