It's a header that has exactly the same functionality as
X-Forwarded forÂ
Since Before 2014, there was no standard HTTP header to insert proxies client IP. The most common nonstandard HTTP header was
X-Forwarded-For.Â
Some other proxies / reverse proxies use
True-Client-IP (like Akamai, Cloudflare, etc)Â
Look at RFC 7239