IP to FQDN

Do you want to check the Host header on all requests through the VIP and if it's one particular IP address, replace the value with a hardcoded value? If so, you can add an HTTP profile to the VIP and use a rule like this:

 
 when HTTP_REQUEST { 
  
     Check if Host header matches 1.1.1.1. 
     Use catch to handle non-IP Host values. 
     If IP::addr matches 1.1.1.1, $result will be 1 
    if {not ([catch {IP::addr [HTTP::host] equals 1.1.1.1} result]) && $result==1}{ 
       HTTP::header replace Host "www.example.com" 
    } 
 } 
 

If that's not an accurate assumption of your scenario, can you clarify what you're trying to do?

Aaron