iRule to change HTTP GET into POST

The trick is that you have to do this in the TCP events (CLIENT_ACCEPTED and CLIENT_DATA) as the HTTP::method command is read-only. Here is a somewhat simple example of how this looks:

when CLIENT_ACCEPTED {
    TCP::collect
}
when CLIENT_DATA {
     separate uri and query string
    set uri [findstr [TCP::payload] "GET " 4 " HTTP/1."]
    set uri_no_query [findstr $uri "" 0 "?"]
    set query [findstr $uri "?" 1]

     replace TCP payload
    regsub -all -nocase "GET" [TCP::payload] "POST" newdata
    set newdata1 [string map "$uri $uri_no_query" $newdata]
    regsub -all -nocase "Accept: " $newdata1 "Content-Type: application/x-www-form-urlencoded\r\nContent-Length: [string length $query]\r\nAccept: " newdata2

    TCP::payload replace 0 [TCP::payload length] ${newdata2}$query
    TCP::release
}

I first separate the URI from the query string, then perform a series of replace functions to 1) change GET to POST, change the original URI to the URI without query string, and 3) inserts the Content-Type and Content-Length headers before the Accept.

This is just an example, so it doesn't account for the following conditions:

1. Whether or not the request is originally a GET or POST
2. Whether or not Content-Type and Content-Length headers are already present