Irule Vs rewrite - FQDN hostname

Mr__T So a few things stand out to me. First is that you don't want a redirect you would like a rewrite but in your rewrite it looks like you want to add on a port at the end of the host which shouldn't be necessary because your pool member should be listening on that port already. The last is whenever they hit just "/" as the URI do you want to redirect them to a specific path? So as an example of how I believe this works.

User requests https://appname.domain.com/
F5 responds with a redirect to https://appname.domain.com/something/something/something/etc/etc/etc.html
User now requests https://appname.domain.com/something/something/something/something/etc/etc/etc.html
F5 rewrites appname.domain.com to longappname.com
F5 sends request to server on https://longappname.com/something/something/something/etc/etc/etc.html
Response comes back from server and F5 rewrites longappname.com to appname.domain.com
F5 sends response back to client for https://appname.domain.com/something/something/something/etc/etc/etc.html

If for some reason I did not understand this correctly can you provide additional information as to how it has to function?

HTTP::redirect will effectively redirect the client to the new location, and will change the url in the browser. What you need is to replace the host header in the pool side of the connection as described here https://clouddocs.f5.com/api/irules/HTTP__host.html

when HTTP_REQUEST {

   # Check if requested host doesn't start with www.example.com
   if {not ([string tolower [HTTP::host]] starts_with "www.example.com")}{

      # Replace the host header value with newhost.example.com
      HTTP::header replace Host "newhost.example.com"
   }
}

And to replace the path with another path without redirection, this is described here https://my.f5.com/manage/s/article/K02027845 :

when HTTP_REQUEST {
    if { [HTTP::uri] starts_with "/sometext" } {
        set uri [string map -nocase {"/sometext" "/newtext"} [HTTP::uri]]
        HTTP::uri $uri
    }
}