I have installed Varnish in front of a Drupal multi-site installation, but I have seen that the Age http header reaches only till 120, and the age becomes 0 again. Then Varnish shows a MISS for that particular file ( using curl -I http://example.com )

It works fine if I add following lines to .htaccess of each site


Header set Cache-Control "max-age=43200, public"

How do I figure out what is setting the max-age to 120?