Is it possible to return a JSON response by varnish if backend is down?

Question

A JSON response something like as shown below should be returned if the underlying backend is down or something.

{
    "msg" :{
        "status": 2
    }
}

Is this possible with varnish ?

Answer 1

How about this:

sub vcl_backend_error {
    set beresp.http.Content-Type = "application/json; charset=utf-8";
    synthetic( {"{ "msg" :{ "status": 2 } }"} );
    return (deliver);
}