Varnish cache | Magento Hosting by Sonassi

Varnish cache is disabled by default for all content. You can enable Varnish on your domain by adding the following parameter to the top of your ___general/example.com.conf (or example.com.location.semi-static.conf, example.com.location.static.conf as applicable).

Caching works on the premise of cache everything until told otherwise, you can fully control Varnish using configuration directives.

Eg.

set $magestack_cacheable true;
set $magestack_cache_lifetime 7d;

It can be confirmed by checking the HTTP debug response headers MageStack-Cache

Also see,

Controlling cacheable content
Purging Varnish

! Any 3rd party admin extensions (with custom routers) or a change of admin URL must be explicitly set in the Varnish exclusion configuration file