LTM Policy for HTTP Host rewrite

Hi jomedusa​ ,

LTM Policies are preferred when it meets all the criteria, but there are some limitations that require an iRule.  In  this case you might need to leverage a stream profile + iRule to handle the Swagger CORS issue.  You can opt for hybird LTM Policy + Irule but I don't like to mix these deployments as it created admin overheard and complexity.  BTW  check out our BIG-IP iRules Assistant Irules Assistant, this is what I used to help create and validate the iRule.  

You will need a HTTP, Stream,  profile ( keep the src/target empty.  If your using a compression profile be aware that stream operations dont work on gzip HTTP bodies.

when HTTP_REQUEST {
    # Preserve what the client asked for
    set client_host [HTTP::host]

    # Rewrite Host header to what backend expects
    HTTP::header replace Host "backend.internal.example.com"

    # Disable compression so STREAM can inspect the body
    HTTP::header remove "Accept-Encoding"

    # Default off; enable selectively on response
    STREAM::disable
}

when HTTP_RESPONSE {
    # Fix CORS allow-origin if backend echoes its own hostname/scheme
    if { [HTTP::header exists "Access-Control-Allow-Origin"] } {
        HTTP::header replace "Access-Control-Allow-Origin" "https://$client_host"
    }

    # Fix Location header on 3xx (or use HTTP profile Redirect Rewrite = Matching)
    if { [HTTP::header exists "Location"] } {
        HTTP::header replace Location \
            [string map "backend.internal.example.com $client_host" [HTTP::header Location]]
    }

    # Body rewrite: swagger.json, HTML, JS that reference the internal host
    switch -glob -- [string tolower [HTTP::header "Content-Type"]] {
        "application/json*" -
        "application/*+json*" -
        "text/html*" -
        "text/javascript*" -
        "application/javascript*" {
            STREAM::expression "@backend.internal.example.com@$client_host@"
            STREAM::enable
        }
    }
}

Here is what our AI Assistant calculated from the iRule above:

1. Objective
   • Preserve the client’s original Host header for later use.
   • Rewrite the Host header to the internal backend hostname so the server sees what it expects.
   • Disable compression on the request so that the response body can be inspected and rewritten.
   • On the response, restore CORS and redirect headers to the client’s host and rewrite in-body references to match the original host.
2. Execution Flow
   • HTTP_REQUEST
   – set client_host to the value of the incoming Host header (HTTP::host).
   – replace the Host header with “backend.internal.example.com” (HTTP::header replace Host).
   – remove the Accept-Encoding header to disable compression (HTTP::header remove "Accept-Encoding").
   – disable the stream filter by default (STREAM::disable).

• HTTP_RESPONSE
  – if “Access-Control-Allow-Origin” exists, replace its value with “https://” (HTTP::header replace).
  – if “Location” exists, map any “backend.internal.example.com” in its value back to the original client_host (string map+HTTP::header replace).
  – inspect the Content-Type header; for JSON, HTML, or JavaScript types, set up a stream expression to rewrite in-body “backend.internal.example.com” to client_host (STREAM::expression) and enable the stream filter (STREAM::enable).

Summary
This iRule fronts an internal backend by swapping the Host header on requests and then, on the way back, transparently restoring client-facing host references in CORS, redirects, and body content for supported MIME types.