Add support for proxy headers

[dunglas]

This PR introduces a new USE_FORWARDED_HEADERS. When set to 1, the hub will read the X-Forwarded-* or the Forwarded headers provided by intermediate proxy servers.

Closes #114 .

[stof]

Many proxies are using either the X-Forwarded-* headers or the Forwarded header, but not both in parallel. So the config should allow trusting only one of them.
Same is true for X-Real-IP btw.

[stof]

Btw, this is exactly the reason why symfony/http-foundation requires to set which headers get trusted.

[dunglas]

It’s how the Gorilla handler works. You don’t have to set both in parallel, only one between X-Forwarded-For, Forwarded and X-Real-IP is enough.

However, headers must be set for both the IP, the host and the protocol.

[stof]

the thing is, if you set one of them in your proxy and you don't clear the other one (because your proxy just does not support it, which might be out of your control), an attacker could use the other header to inject a fake value (which unsupported proxy header allows such attack depends on the header priority in the Gorilla handler)

[dunglas]

Indeed. It’s why we’re documenting that both headers have to be set. Having a more flexible configuration could be nice, but it must be added to Gorilla first. Currently Gorilla doesn’t support this.