Using Irules to mimic apache Virtual hosts

I'm guessing you don't want to redirect the client. If you did, the client would see the change, try to resolve www.apacheserver.com to an IP address and then make a new request to that IP. To rewrite the host header and URI you could use an iRule like this:

 
 when HTTP_REQUEST { 
  
    log local0. "[IP::client_addr][TCP::client_port]: New request to [HTTP::host][HTTP::uri]" 
  
    switch [string tolower [HTTP::host]] { 
  
       "www.site1.com" { 
  
           Rewrite host to www.apacheserver.com 
  HTTP::header replace Host "www.apacheserver.com" 
  
           Prepend /site1 to the URI 
          HTTP::uri "/site1[HTTP::uri]" 
       } 
       "www.site2.com" { 
  
           Rewrite host to www.apacheserver.com 
  HTTP::header replace Host "www.apacheserver.com" 
  
           Prepend /site2 to the URI 
          HTTP::uri "/site2[HTTP::uri]" 
       } 
       "www.site3.com" { 
  
           Rewrite host to www.apacheserver.com 
  HTTP::header replace Host "www.apacheserver.com" 
  
           Prepend /site3 to the URI 
          HTTP::uri "/site3[HTTP::uri]" 
       } 
       default { 
  
           Take some default action? 
       } 
    } 
 } 
 when HTTP_REQUEST priority 501 { 
  
     This event is added for debug only.  It bypassed the variable caching for HTTP::host and HTTP::uri 
    log local0. "[IP::client_addr][TCP::client_port]: Updated request to [HTTP::host][HTTP::uri]" 
 } 
 

If the application uses absolute references to www.apacheserver.com in response headers and/or content you could rewrite them using 'HTTP::header replace' and/or a stream profile and STREAM::expression based iRule, respectively.

Aaron