Varnish wildcard purge

Well, I have an answer - and surprisingly the problem wasn't even that varnish wasn't passing on the HTTP/500 response content.

The HTTP/503 error was actually being thrown up when Varnish was timing out while connecting to the backend server, or waiting for bytes to be sent.

Basically, if the connection to the backend was successful, but the backend doesn't send the first byte without a certain timeout period, or there is a delay between sending bytes, Varnish will timeout and return a HTTP/503 error.

The solution is to update the timeout values (and of course, fix up the slow backend server) such as below (as detailed at: http://varnish-cache.org/wiki/VCL):

backend www {
   .host = "www.example.com";
   .port = "http";
   .connect_timeout = 1s;
   .first_byte_timeout = 5s;
   .between_bytes_timeout = 2s;
}

With few lines of scripting you will be able to do that.

You need to setup a shell script which its purpose will be to periodically dig +short your domain name and if IP has changed, will reload varnish.