The proxy can add extra (or overwrite) headers to requests it receives and passes through to the back-end. These can be used to communicate information to the back-end.
So far I’ve seen a couple used for forcing the use of https in URL scheme:
And wikipedia also mentions:
# a de facto standard:
# Non-standard header used by Microsoft applications and load-balancers:
This what you should add to the VirtualHost on apache: other proxies should have similar functionality
RequestHeader set X-FORWARDED-PROTOCOL https
RequestHeader set X-Forwarded-Ssl on
I think it’s best to set them all, or set one that works and remove the other known ones. To prevent evil clients messing with them.