How to inspect POST data on HTTP_REQUEST

HTTP::payload should normally return any payload bytes received in the first packet containing the HTTP headers.

Regardless to guarantee collection of the full HTTP payload, you need to call HTTP::collect. Make sure to limit collect to less than 4Mb to avoid a TMM crash:

 From http://devcentral.f5.com/wiki/iRules.http__collect.ashx
when HTTP_REQUEST {

  if {[HTTP::method] eq "POST"}{
     Trigger collection for up to 1MB of data
    if {[HTTP::header exists "Content-Length"] && [HTTP::header "Content-Length"] <= 1048576}{
      set content_length [HTTP::header "Content-Length"]
    } else {
        set content_length 1048576
    }
     Check if $content_length is not set to 0
    if { content_length > 0} {
      HTTP::collect $content_length
    }
  }
}
when HTTP_REQUEST_DATA {
   do stuff with the payload
  set payload [HTTP::payload]
}

Aaron