Varnish How to redirect non-www URLs to www

How to redirect non-www URLs to www in Varnish

If a website's canonical URL has www, it is desirable, as a good SEO practice, to redirect the non-www URLs to www.  That is, if the canonical URL is www.briansnelson.com, example.com should be redirected to www.briansnelson.com.  How to do this when Varnish is listening on port 80 as a reverse HTTP proxy.