HTTP POST redirect preserving POST data

Problem this snippet solves:

Use Javascript in an iRule to redirect HTTP POST requests to HTTPS. When an HTTP 30x redirect it sent to a client that has sent a POST request, the user-agent transparently issues a new GET request. As a result, the original POST request payload is lost.

The idea behind this iRule is when client sends a POST to an HTTP virtual server, LTM replies with an HTML page which contains a form. The form contains post-data that the client just sent. The form will be auto-submitted by Javascript via HTTPS.

Here is how this iRule works:

1. if method = post, it issues HTTP::collect, which will invoke HTTP_REQUEST_DATA
2. then it scans POST-data and prepares new content to respond to client. the new content will include
   • form with all INPUT field names retrieved from POST-data
   • the "action" parameter in the form points to external server
   • all input fields are set as hidden (so client won't see the data during the process)
   • Javascript that submits the form automatically
3. then iRule replies to client with HTTP::respond command with content prepared in step 2

Code :

when RULE_INIT {
    set static::ext_url "https://10.10.71.3/test.post"
}
when HTTP_REQUEST {

   # Check if request was a POST
   if { [string tolower [HTTP::method]] eq "post" } {

      # Check if there is a Content-Length header
      if { [HTTP::header exists "Content-Length"] } {

         if { [HTTP::header "Content-Length"] > 1048000 }{

    # Content-Length over 1Mb so collect 1Mb
            set content_length 1048000

 } else {

    # Content-Length under 1Mb so collect actual length
            set content_length [HTTP::header "Content-Length"]
         }
      } else {

 # Response did not have Content-Length header, so use default of 1Mb
         set content_length 1048000
       }
       # Don't collect content if Content-Length header value was 0
       if { $content_length > 0 } {
          HTTP::collect $content_length
       }
    }    
}
when HTTP_REQUEST_DATA {
    set content "< script type=text/javascript language=javascript> \
        function s(){ document.f.submit(); }  \
        
"
        
    foreach p [split [HTTP::payload] &] {
        set name  [URI::decode [getfield $p = 1]]
        set value [URI::decode [getfield $p = 2]]
        set content "${content}"
    }
    set content "${content}
"

    HTTP::respond 200 content $content
}