Get Varnish to handle JSONP requests - working example

varnish-jsonp.vcl

backend jsonp_varnish { 
  .host = "localhost"; 
  .port = "80"; 
}

#------------- begin jsonp functionality -------------# 
# How the jsonp_template_backend dispatches to the ESI generating code: 
sub jsonp_throw_error_recv { 
  if (req.url == "/JSONP-ESI-TEMPLATE") { 
    error 760; 
  } 
}

sub jsonp_prepare_request_recv { 
  # If URL includes callback=, rewrite to an ESI template

  if (req.url ~ "callback=\S") { 
    set req.http.X-Callback = regsub( 
    req.url, ".*[\?&]callback=([\.A-Za-z0-9_\[\]]+).*", "\1" 
  ); 
  set req.http.X-ESI-Url = regsub(req.url, "&?callback=[\.A-Za-z0-9_\[\]]+", ""); 
  # Remove a trailing ? 
  set req.http.X-ESI-Url = regsub(req.http.X-ESI-Url, "\?$", ""); 
  # Fix any accidental ?& 
  set req.http.X-ESI-Url = regsub(req.http.X-ESI-Url, "\?&", "?");

  set req.url = "/JSONP-ESI-TEMPLATE"; 

  # We go BACK to varnish to get it to generate an ESI template that 
  # generates a JSON-P response. 
  set req.backend = jsonp_varnish; 
    return (pass); # NEVER cache template, since it varies on X-Callback/ESI-Url 
  } 
}

sub jsonp_begin_esi_fetch { 
  # X-ESI: 1 from backend triggers ESI processing 
  if (beresp.http.X-ESI) { 
    remove beresp.http.X-ESI; 
    set beresp.do_esi = true; 
  }
}

sub jsonp_clean_fetch { 
  # X-JSONP-Server means we need to clean up the response a bit 
  if (beresp.http.X-JSONP-Server) { 
    remove beresp.http.X-JSONP-Server; 
    remove beresp.http.Via; # Gets added again later on, but a bit less messy 
    remove beresp.http.Retry-After; 
    set beresp.http.Server = "JSONP-Server"; 
  } 
}

# We're using a custom error here because it's the only way I could find to get 
# varnish to compose a custom response. 
sub vcl_error { 
  if (obj.status == 760) { 
    set obj.http.Content-Type = "application/javascript"; 
    set obj.http.X-ESI = "1"; 
    set obj.http.X-JSONP-Server = "1"; 
    set obj.status = 200; 
    set obj.response = "OK";

synthetic 
"<esi:include />"  + # Blank directive, needed to avoid this error: ESI_xmlerror: No ESI processing, first char not '<' 
req.http.X-Callback + 
{"(<esi:include src="http://your.domain.com"} + req.http.X-ESI-Url + {"" />);"};

    return(deliver);
  } 
} 
#------------- end jsonp functionality -------------#

sub vcl_recv {
  call jsonp_throw_error_recv; 
  call jsonp_prepare_request_recv;
}

sub vcl_fetch {
  call jsonp_begin_esi_fetch;
  call jsonp_clean_fetch;

  # Add this if You use gzip compression for Your json assets 
  if ( beresp.http.content-type ~ "application/javascript" ) { 
    set beresp.do_gzip = true;
  }
}

It was a great help! Thx! :)