You cannot modify XHR requests coming back from the ASP.NET. Doing so would be to attempt a Man In The Middle Attack (which your proxy is acting as), but Microsoft has good reason to prevent.

Here's a dummy message to explore the syntax ASP.NET uses in the response:


    1|#||2|52|updatePanel|ctl00_mainContentPlaceHolder_firstUpdatePanel|
        <p> New Content For First Update Panel </p>

The header starts with `1|#| |` and then the number of updates in the rest of the message (2)  
Then each update section follows the pattern:  

    |char_len|update_type|id_of_field_to_update|   
    New contents to insert into field

The `len` in each section must exactly equal the number of characters to follow.  So finding and replacing content in these messages is extremely fickle.

The best recommendation is to simply return a relative URL that is server agnostic so the client can be redirected relative to their current domain.


When going through a proxy server (A), any self-referential links sent from the apps server (B) need to be re-written to use the proxy as a host instead.

Here's an example:

Response from (B) contains: <code>&lt;a href=&quot;http://<b>apps</b>/path&quot;&gt;path&lt;/a&gt;</code>  
Proxy (A) needs to rewrite as: <code>&lt;a href=&quot;http://<b>proxy</b>/path&quot;&gt;path&lt;/a&gt;</code>

Normally, this is done by creating an outbound rule that inspects html responses for tags that contain urls, looks for references to to the apps server, and rewrites them.  

**Here's a normal rule** <sup>[*GUI version*][1]</sup>:  



    <outboundRules>
        <rule name="Outbound Links" preCondition="IsHTML" enabled="true">
            <match filterByTags="A, Form, IFrame, Img, Input, Link, Script" pattern="(https?:\/\/proxy|^)\/(.*)" />
            <conditions logicalGrouping="MatchAll" trackAllCaptures="true" />
            <action type="Rewrite" value="http://apps/{R:2}" />
        </rule>

**Where `IsHTML` is defined as**:



    <preConditions>
        <preCondition name="IsHTML">
            <add input="{RESPONSE_CONTENT_TYPE}" pattern="text\/html" />
        </preCondition>

**The problem** is that some of the page content is returned via an XHR request.  Minimally, this fails the HTML precondition.

[![Content Type - text/html vs XHR][2]][2]

 but I can expand the rule to also include content types of `xhr`

However, URL Rewrite still has trouble **parsing** the returned text into tags because it is not valid HTML.  

Here's an example of what the response looks like:



    |6383|updatePanel|ctl00_mainContentPlaceHolder_contentUpdatePanel|
    
    <div id="ctl00_mainContentPlaceHolder_resultsPanel">
       <a href="http://apps/path">path</a>   
    </div>

    ...
    
    |0|hiddenField|__EVENTTARGET||0|hiddenField|__EVENTARGUMENT||0|hiddenField|

However, when I do this, I get the error:

> **Sys.WebForms.PageRequestManagerParserErrorException**:  
The message received from the server could not be parsed.

  [1]: http://i.stack.imgur.com/Eaewx.png
  [2]: http://i.stack.imgur.com/Wd0n8.png