Altering the backend response

Here we override the TTL of a object coming from the backend if it matches certain criteria:

sub vcl_backend_response {
   if (bereq.url ~ "\.(png|gif|jpg)$") {
     unset beresp.http.set-cookie;
     set beresp.ttl = 1h;
  }
}

We also remove any Set-Cookie headers in order to avoid creation of a hit-for-miss object. See VCL Actions.