Asp.net获取客户端真实IP,非代理地址

public string GetUserIP()

   {

       string _userIP;

       if(Request.ServerVariables["HTTP_VIA"] == null)

         {

             userIP= Request.UserHostAddress;

         }

       else

        {

           userIP= Request.ServerVariables["HTTP_X_FORWARDED_FOR"];

        }

       return userIP;

    }