ios http请求出现502-bad-gateway错误

当我试着在Ruby China社区的iPhone客户端基础上，兼容v2ex社区的Project Babel api接口。 请求遇到502-bad-gateway错误，一开始以为是使用AFNetworking的问题，我通过NSString同步获取，发现也没获取到，所以我推测应该是跟后台基于nginx反向代理有关。

header输出结果比较：

1、使用AFNetworking框架底层设置的User-Agent默认值 整个请求头如下:

{
    Accept = "application/json";
    "Accept-Language" = "en;q=1, fr;q=0.9, de;q=0.8, zh-Hans;q=0.7, zh-Hant;q=0.6, ja;q=0.5";
    "User-Agent" = "JLV2EX/1.0 (iPhone Simulator; iOS 7.0.3; Scale/2.00)";
}

请求报错:502 bad gateway :(

2、通过手机浏览器打开http://whatsmyuseragent.com/来获取到User-Agent，再赋给header的User-Agent 整个请求头，

{
    Accept = "application/json";
    "Accept-Language" = "en;q=1, fr;q=0.9, de;q=0.8, zh-Hans;q=0.7, zh-Hant;q=0.6, ja;q=0.5";
    "User-Agent" = "Mozilla/5.0 (iPhone; CPU iPhone OS 7_0_3 like Mac OS X) AppleWebKit/537.51.1 (KHTML, like Gecko) Version/7.0 Mobile/11B508 Safari/9537.53";
}

可以顺利获取信息 :)

网上google了一通，找到如下解决方案。特此记录。参考SO帖子： http://stackoverflow.com/questions/8487581/uiwebview-ios5-changing-user-agent/8666438#8666438

//RCAPIClient继承于AFHTTPClient，做如下修改：
- (id)initWithBaseURL:(NSURL *)url
{
    self = [super initWithBaseURL:url];
    if (self) {
        self.parameterEncoding = AFJSONParameterEncoding;
        
        // 502-bad-gateway error, set user agent from http://whatsmyuseragent.com/
        // http://stackoverflow.com/questions/8487581/uiwebview-ios5-changing-user-agent/8666438#8666438
        if (ForumBaseAPIType_V2EX == FORUM_BASE_API_TYPE) {
            NSString* testUserAgent = @"Mozilla/5.0 (iPhone; CPU iPhone OS 7_0_3 like Mac OS X) AppleWebKit/537.51.1 (KHTML, like Gecko) Version/7.0 Mobile/11B508 Safari/9537.53";
            [self setDefaultHeader:@"User-Agent" value:testUserAgent];
        }
    }
    return self;
}

代码见：https://github.com/jimneylee/JLRubyChina-iPhone/blob/master/JLRubyChina/Common/RCAPIClient.m的initWithBaseURL方法