Is there a way to configure varnish or any other caching reverse proxy, to serve pages from its cache when the back-end fails? At the moment, if the back-end goes down a
503 Service Unavailable error would be returned to the browser. I would prefer it if visitors got to see a cached version than an error page while the back-end is being fixed.
[varnish (public ip)] <===> [router] <===> [web server (private ip)]
PS: I have only one back-end web server.