HttpClient/HttpURLConnection + HttpDns最佳实践

在Android端如果OkHttp作为网络请求框架,由于其提供了自定义DNS服务接口,可以很优雅地结合HttpDns,相关实现可参考:HttpDns+OkHttp最佳实践
如果您使用HttpClientHttpURLConnection发起网络请求,尽管无法直接自定义Dns服务,但是由于HttpClientHttpURLConnection也通过InetAddress进行域名解析,通过修改InetAddress的DNS缓存,同样可以比通用方案更为优雅地使用HttpDns。

InetAddress在虚拟机层面提供了域名解析能力,通过调用InetAddress.getByName(String host)即可获取域名对应的IP。调用InetAddress.getByName(String host)时,InetAddress会首先检查本地是否保存有对应域名的ip缓存,如果有且未过期则直接返回;如果没有则调用系统DNS服务(Android的DNS也是采用NetBSD-derived resolver library来实现)获取相应域名的IP,并在写入本地缓存后返回该IP。

核心代码位于java.net.InetAddress.lookupHostByName(String host, int netId)

public class InetAddress implements Serializable {
  ...
      /**
     * Resolves a hostname to its IP addresses using a cache.
     *
     * @param host the hostname to resolve.
     * @param netId the network to perform resolution upon.
     * @return the IP addresses of the host.
     */
    private static InetAddress[] lookupHostByName(String host, int netId)
            throws UnknownHostException {
        BlockGuard.getThreadPolicy().onNetwork();
        // Do we have a result cached?
        Object cachedResult = addressCache.get(host, netId);
        if (cachedResult != null) {
            if (cachedResult instanceof InetAddress[]) {
                // A cached positive result.
                return (InetAddress[]) cachedResult;
            } else {
                // A cached negative result.
                throw new UnknownHostException((String) cachedResult);
            }
        }
        try {
            StructAddrinfo hints = new StructAddrinfo();
            hints.ai_flags = AI_ADDRCONFIG;
            hints.ai_family = AF_UNSPEC;
            // If we don't specify a socket type, every address will appear twice, once
            // for SOCK_STREAM and one for SOCK_DGRAM. Since we do not return the family
            // anyway, just pick one.
            hints.ai_socktype = SOCK_STREAM;
            InetAddress[] addresses = Libcore.os.android_getaddrinfo(host, hints, netId);
            // TODO: should getaddrinfo set the hostname of the InetAddresses it returns?
            for (InetAddress address : addresses) {
                address.hostName = host;
            }
            addressCache.put(host, netId, addresses);
            return addresses;
        } catch (GaiException gaiException) {
          ...
        }
    }
}

其中addressCacheInetAddress的本地缓存:

private static final AddressCache addressCache = new AddressCache();

结合InetAddress的解析策略,我们可以通过如下方法实现自定义DNS服务:

  • 通过HttpDns SDK获取目标域名的ip
  • 利用反射的方式获取到InetAddress.addressCache对象
  • 利用反射方式调用addressCache.put()方法,域名和ip的对应关系写入InetAddress缓存

具体实现可参考以下代码:

public class CustomDns {

    public static void writeSystemDnsCache(String hostName, String ip) {
        try {
            Class inetAddressClass = InetAddress.class;
            Field field = inetAddressClass.getDeclaredField("addressCache");
            field.setAccessible(true);
            Object object = field.get(inetAddressClass);
            Class cacheClass = object.getClass();
            Method putMethod;
            if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
                //put方法在api21及以上为put(String host, int netId, InetAddress[] address)
                putMethod = cacheClass.getDeclaredMethod("put", String.class, int.class, InetAddress[].class);
            } else {
                //put方法在api20及以下为put(String host, InetAddress[] address)
                putMethod = cacheClass.getDeclaredMethod("put", String.class, InetAddress[].class);
            }
            putMethod.setAccessible(true);
            String[] ipStr = ip.split("\\.");
            byte[] ipBuf = new byte[4];
            for(int i = 0; i < 4; i++) {
                ipBuf[i] = (byte) (Integer.parseInt(ipStr[i]) & 0xff);
            }
            if(Build.VERSION.SDK_INT  >= Build.VERSION_CODES.LOLLIPOP) {
                putMethod.invoke(object, hostName, 0, new InetAddress[] {InetAddress.getByAddress(ipBuf)});
            } else {
                putMethod.invoke(object, hostName, new InetAddress[] {InetAddress.getByAddress(ipBuf)});
            }

        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        } catch (UnknownHostException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        }
    }
}

和通用方案相比,使用该方法具有下列优势:

  • 实现简单
  • 通用性强,该方案在HTTPS,SNI以及设置Cookie等场景均适用。规避了证书校验,域名检查等环节
  • 全局生效,InetAddress.addressCache为全局单例,该方案对所有使用InetAddress作为域名解析服务的请求全部生效

<font color="red">另外使用该方案请务必注意以下几点:</font>

  • AddressCache的默认TTL为2S,且默认最多可以保存16条缓存记录:

    class AddressCache {
    ...
       /**
        * When the cache contains more entries than this, we start dropping the oldest ones.
        * This should be a power of two to avoid wasted space in our custom map.
        */
       private static final int MAX_ENTRIES = 16;
    
       // The TTL for the Java-level cache is short, just 2s.
       private static final long TTL_NANOS = 2 * 1000000000L;
       }
    }
    

    Android虚拟机下反射规则与JVM存在差异,无法直接修改final变量的值。所以使用该方法请务必注意IP过期时间及缓存数量。另外针对该问题可尝试另一种解决方案:重写AddressCache类,并通过ClassLoader优先加载,覆盖系统类。

  • AddressCache.put方法在 API 21进行了改动,增加了netId参数,为保证兼容性需要针对不同版本区别处理。具体方案参考上文代码

  • 该方式可以解决HTTPS,SNI以及设置cookie等场景,但不适用于WebView场景。Android Webview使用ChromiumWebkit作为内核(Android 4.4开始,Webview内核由Chromium替代Webkit)。上述两者均绕开InetAddress而直接使用系统DNS服务,所以该方案对此场景无效。

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