STREAM_MATCHED for regexp replace ?

Just what you had in your original post:

 
 when HTTP_REQUEST { 
    set host [HTTP::host] 
 } 
 

You could add a check to see if the host value in the request was null and then use the VIP address or the public IP address if the VIP is an internal IP address:

 
 when HTTP_REQUEST { 
     Check if the client used a host header 
    if {[string length [HTTP::host]}[ 
       set host [HTTP::host] 
    } else { 
        Use the VIP address instead 
       set host [IP::local_addr] 
    } 
 } 
 

Aaron