how to do HTTP header replace with iRule

There is no Host header in HTTP responses. What is the customer trying to achieve? Do they want to change the hostname displayed in the browser address bar? If so, you'd want to send the client an HTTP redirect instead of rewriting a header in the response. Other possibilities are:

- The customer wants to rewrite a redirect in a 30x response.

In that case you could use an iRule like this: http://devcentral.f5.com/wiki/default.aspx/iRules/RewriteHTTPRedirectHostname.html

- The customer wants to rewrite the host header in the request without the client seeing the change. In that case, you could use a very similar iRule to what you posted:

 
 when HTTP_REQUEST { 
  
    if { [HTTP::header host] eq "www.abc.com" } { 
       HTTP::header replace Host "www.xyz.com" 
    } 
 }  
 

Aaron