Why is Varnish not caching?

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http://serverfault.com – I am troubleshooting the setup of Varnish 3.x on my Ubuntu server. I'm running Drupal 7 on two sites set up on the box, via named-based vhosts. Before trying to get Varnish to play nice with Drupal I'm trying to just get Varnish to a PNG from cache. Here are the headers I get from a curl -I request of the PNG file: HTTP/1.1 200 OK Server: Apache/2.2.22 (Ubuntu) Last-Modified: Sun, 07 Oct 2012 21:18:59 GMT ETag: "a57c2-3850-4cb7ea73db6c0" Accept-Ranges: bytes Content-Length: 14416 Cache-Control: max-age=1209600 Expires: Thu, 25 Oct 2012 22:55:14 GMT Content-Type: image/png Accept-Ranges: byte (HowTos)