Varnish 3.0.1 Backend conditional requests

[Сергей Ямщиков]

Hello. How to enable Backend conditional requests (If-None-Match and
If-Modified-Since) for the Varnish 3.0.1. Now it doesn't send to me
headers (If-None-Match and If-Modified-Since) when max-age=0 or
expired, on first response i set ETag and Last-Modified headers. Subs
and stale_obj from
https://www.varnish-cache.org/trac/wiki/BackendConditionalRequests
throw exception in "*.vcl" file when compile.