squid 3.1开始，forwarded_for 配置项更加完善。
forwarded_for transparent
则squid自己不会添加 X-Forwarded-For ，然后如果客户端请求squid时自己带了这个Header，那squid对其也不做任何改变。
引用官方说明：
http://www.squid-cache.org/Doc/config/forwarded_for/
Squid configuration directive forwarded_for
Available in: 3.HEAD 3.4 3.3 3.2 2.7 3.1 3.0 2.6
History:
Changes in 2.7 forwarded_for
Has several new modes, allowing one to finetune how/if the requesting client IP should be forwarded in X-Forwarded-For
Changes in 3.1 forwarded_for
New setting options. transparent, truncate, delete.
If set to "transparent", Squid will not alter the
X-Forwarded-For header in any way.
If set to "delete", Squid will delete the entire
X-Forwarded-For header.
If set to "truncate", Squid will remove all existing
X-Forwarded-For entries, and place the client IP as the sole entry.
Configuration Details:
Option Name: forwarded_for
Replaces:
Requires:
Default Value: forwarded_for on
Suggested Config:
If set to "on", Squid will append your client's IP address
in the HTTP requests it forwards. By default it looks like:
X-Forwarded-For: 192.1.2.3
If set to "off", it will appear as
X-Forwarded-For: unknown
If set to "transparent", Squid will not alter the
X-Forwarded-For header in any way.
If set to "delete", Squid will delete the entire
X-Forwarded-For header.
If set to "truncate", Squid will remove all existing
X-Forwarded-For entries, and place the client IP as the sole entry.