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场景：
客户网站或者移动APP在请求到达后端服务之前，会经过层层代理的转发。
典型的是CDN（反向代理+Squid缓存），如果是带有安全或者WAF功能的则结构要更为复杂些。
一般的解决方案：
proxy_set_header Host $host; proxy_set_header X-real-ip
$remote_addr; proxy_set_header X-Forwarded-For
$proxy_add_x_forwarded_for;
后端服务获取客户端真实IP的方法：
request.getAttribute("X-real-ip")
现象：
后端服务获取到的IP并不是客户端真实IP，而是某一级代理的IP。
分析：
从CDN开始，每经过一个代理做一次转发，x_forwarded_for就会在后面追加一个代理IP。请求到达nginx时，x_forwarded_for已经变成一个以逗号分隔的ip串，并且以转发顺序排序。
nginx的内置变量remote_addr仅能代表nginx的上一层代理的IP，现有的nginx配置将该值赋给X_Real_Ip，那么后端获取到的X_Real_Ip也是nginx上一层代理的IP，而不是客户端真实IP。
其他解决方法：
如果采用阿里腾讯华为百度西数等云的cdn控制台可以进行参数设置IP寻址方式，但美中不足的是不能设置user-agent请求方式（无法判断），所以只能在客户端编码的形式进行获取真实IP。
