Sending custom response back from varnish through VCL

Is there any way to send back custom responses from varnish itself?

if (req.url ~ "^/hello") {
  return "hello world";
}

Solution

You would do this with a synthetic response. For example:

sub vcl_recv {
  if (req.url ~ "^/hello") {
    error 700 "OK";
  }
}

sub vcl_error {
  if (obj.status == 700) {
    set obj.http.Content-Type = "text/html; charset=utf-8";
    set obj.status = 200;
    synthetic {"
     <html>
       <head>
          <title>Hello!</title>
       </head>
       <body>
          <h1>Hello world!</h1>
       </body>
     </html>
    "};
  }
}