[1] 要求に CF-Connecting-IP: ヘッダーが既に含まれている場合、 CloudFlare の逆串が 403 応答を返すようです。
CF-Connecting-IP:
403
[404] How does CloudFlare handle HTTP Request headers? – CloudFlare Support ( (2014-05-09 04:16:11 +09:00 版)) https://support.cloudflare.com/hc/en-us/articles/200170986-How-does-CloudFlare-handle-HTTP-Request-headers-