How can I return a 500 response for all requests to a specific file at the Varnish level?

Background:

Our network structure brings all traffic into a Varnish installation, which then ports traffic to one of 5 different web servers, based on rules that a previous administrator setup. I don't have much experience with Varnish.

Last night we were being bombarded by requests to a specific file. This file is one that we limit to a specific set of servers, and it has direct link to our master database, due to reasons. Obviously, this wasn't optimal, and our site was hit pretty hard because of it. What I attempted to do, and failed, was to write a block of code in the Varnish VCL that would return a 500 response for every request to that file, which I could then comment out after the attack period ended.

Question:

What would that syntax be? I've done my googling, but at this point I think it's the fact that I don't know enough about Varnish to be able to word my search properly, so I'm not finding the information that I need.

Solution

You can define your own vcl_recv, prior to any other vcl_recv in your configuration, reload Varnish, and you should get the behaviour you're looking for.

sub vcl_recv {
    if (req.url ~ "^/path/to/file(\?.*)?$") {
        return (synth(500, "Internal Server Error"));
    }
}