AFNetworking 原作者都无法解决的问题: 如何使用ip直接访问https网站?

背景

最近App似乎有报异常是DNS无法解析,尝试解决此问题.搜集到的资料很少,甚至连AFN原作者都判定这可能是一个无解的问题,参见: https://github.com/AFNetworking/AFNetworking/issues/2954,不过最终还是靠着stackoverflow上的一丁点提示,顺利找到并汇集成了一个可用的解决方案.大喜,与君共享!

问题描述

通过IP直接访问网站,可以解决DNS劫持问题.DNS劫持,可以通过修改电脑的host文件模拟.如果是HTTP请求,使用ip地址直接访问接口,配合header中Host字段带上原来的域名信息即可;如果是 https请求,会很麻烦,需要 Overriding TLS Chain Validation Correctly;curl 中有一个 -resolve 方法可以实现使用指定ip访问https网站,iOS中集成curl库应该也可以,不过改动太大,未验证;对于服务器IP经常变的情况,可能需要使用httpDNS服务,参见:https://www.dnspod.cn/httpdns.

解决方案讨论

1. 最直接的方式是允许无效的SSL证书,生产环境不建议使用;

2.一个需要部分重写AFN源码的方法.

• 在Info.plist中添加NSAppTransportSecurity类型Dictionary,在NSAppTransportSecurity下添加NSAllowsArbitraryLoads类型Boolean,值设为YES.这些本来是用来解决iOS9下,允许HTTP请求访问网络的,当然作用不止这些.具体原因感兴趣的自行google.

• 给 AFURLConnectionOperation 类添加新属性:

/** 可信任的域名,用于支持通过ip访问此域名下的https链接.
 Trusted domain, this domain for support via IP access HTTPS links.
 */
@property(nonatomic, strong) NSMutableArray * trustHostnames;

• 给 AFURLConnectionOperation 实现的代理方法: - (void)connection:(NSURLConnection *)connection
  willSendRequestForAuthenticationChallenge:(NSURLAuthenticationChallenge *)challenge 添加添加可信任的域名的相关逻辑代码:

- (void)connection:(NSURLConnection *)connection
willSendRequestForAuthenticationChallenge:(NSURLAuthenticationChallenge *)challenge
{
    if (self.authenticationChallenge) {
        self.authenticationChallenge(connection, challenge);
        return;
    }
    
    if ([challenge.protectionSpace.authenticationMethod isEqualToString:NSURLAuthenticationMethodServerTrust]) {
        SecTrustRef serverTrust = challenge.protectionSpace.serverTrust;
        
        /* 添加可信任的域名,以支持:直接使用ip访问特定https服务器.
         Add trusted domain name to support: direct use of IP access specific HTTPS server.*/
        for (NSString * trustHostname  in [self trustHostnames]) {
            serverTrust = AFChangeHostForTrust(serverTrust, trustHostname);
        }

    ....

• 参考Apple官方文档,实现自定义的添加可信域名的函数: AFChangeHostForTrust

static inline SecTrustRef AFChangeHostForTrust(SecTrustRef trust, NSString * trustHostname)
{
    if ( ! trustHostname || [trustHostname isEqualToString:@""]) {
        return trust;
    }
    
    CFMutableArrayRef newTrustPolicies = CFArrayCreateMutable(
                                                              kCFAllocatorDefault, 0, &kCFTypeArrayCallBacks);
    
    SecPolicyRef sslPolicy = SecPolicyCreateSSL(true, (CFStringRef)trustHostname);
    
    CFArrayAppendValue(newTrustPolicies, sslPolicy);
    
    
#ifdef MAC_BACKWARDS_COMPATIBILITY
    /* This technique works in OS X (v10.5 and later) */
    
    SecTrustSetPolicies(trust, newTrustPolicies);
    CFRelease(oldTrustPolicies);
    
    return trust;
#else
    /* This technique works in iOS 2 and later, or
     OS X v10.7 and later */
    
    CFMutableArrayRef certificates = CFArrayCreateMutable(
                                                          kCFAllocatorDefault, 0, &kCFTypeArrayCallBacks);
    
    /* Copy the certificates from the original trust object */
    CFIndex count = SecTrustGetCertificateCount(trust);
    CFIndex i=0;
    for (i = 0; i < count; i++) {
        SecCertificateRef item = SecTrustGetCertificateAtIndex(trust, i);
        CFArrayAppendValue(certificates, item);
    }
    
    /* Create a new trust object */
    SecTrustRef newtrust = NULL;
    if (SecTrustCreateWithCertificates(certificates, newTrustPolicies, &newtrust) != errSecSuccess) {
        /* Probably a good spot to log something. */
        
        return NULL;
    }
    
    return newtrust;
#endif
}

• 使用AOP方法,重写 AFURLConnectionOperation 的trustHostnames属性:

/* 使用AOP方式,指定可信任的域名, 以支持:直接使用ip访问特定https服务器.*/
    [AFURLConnectionOperation aspect_hookSelector:@selector(trustHostnames) withOptions:AspectPositionInstead usingBlock: ^(id<AspectInfo> info){
        __autoreleasing NSArray * trustHostnames = @[@"www.example.com"];
        
        NSInvocation *invocation = info.originalInvocation;
        [invocation setReturnValue:&trustHostnames];
    }error:NULL];

此处用到的是一个 iOS AOP库,不熟悉的点这里: http://www.ios122.com/2015/08/aspects/.