How to redirect the www.a.com to www.a.com/abc

You can use HTTP::redirect (Click here for the wiki page) to redirect clients that make a request for the root document to the /abc URI:

when HTTP_REQUEST {
   if {[HTTP::uri] eq "/"}{
      HTTP::redirect http://[HTTP::host]/abc
   }
}

If the requested URI isn't /, the client request won't be modified and they will be sent to the pool.

Aaron

(Edited to add [] around HTTP::uri)