TODO: Support of Forwarded header

[junkurihara]

In addition to X-Forwarded-For, rpxy should support Forwarded extension header in RFC7239.

c.f. https://www.nginx.com/resources/wiki/start/topics/examples/forwarded/

[xkr47]

Archive version of nginx docs: https://web.archive.org/web/20240511013834/https://www.nginx.com/resources/wiki/start/topics/examples/forwarded/

So, my first thoughts on configuration options we could add:

• downstream
  • trust downstream Forwarded header: y/N
    • only with included secret: _______________ (comma-separated list)
    • OR from given ips: _______________ (comma-separated list)
    • trust any upstreams of above: y/N
• upstream
  • add/extend Forwarded header: Y/n
    • add secret: ______________
    • obfuscate "for" parameter: Y/n
  • remove malformed entries: Y/n (maybe force-yes this when "add/extend" is enabled above?)
  • remove other downstream X-Forwarded-* etc headers when Forwarded header added/present: Y/n

WDYT? Too much?