Nginx as reverse proxy and cache if backend sends a header
http://wiki.nginx.org/HttpCoreModule#Variables gives amongst others the following:
$sent_http_HEADERThe value of the HTTP response header HEADER when converted to lowercase andwith 'dashes' converted to 'underscores', e.g. $sent_http_cache_control, $sent_http_content_type...; $cookie_COOKIEThe value of the cookie COOKIE; if you combine that with an if block you could do something like:
if ($sent_http_your_added_header = "") { proxy_cache your_cache_zone;}if ($cookie_BYPASS = "1") { proxy_bypass $cookie_BYPASS;}Note: you could actually forget about the if and just use $cookie_BYPASS if your BYPASS cookie has either a 1 or 0 value, see http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_cache_bypass
as far cache times goes, as http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_cache_valid explains, nginx responds to “X-Accel-Expires”, “Expires” and "Cache-Control” headers (unless you tell it not to with proxy_ignore_headers directive)