Vertx DNS解析源码

最近在生产环境碰到过多次域名解析失败的问题，有时候还是客户windowns环境报障，是时候深入了解下Vertx内部的域名解析机制了。

1、Vertx使用DNS方法
import java.util.Arrays;

import io.vertx.core.Vertx;
import io.vertx.core.VertxOptions;
import io.vertx.core.dns.AddressResolverOptions;
import io.vertx.core.http.HttpClient;
import io.vertx.core.http.HttpClientOptions;
import io.vertx.core.http.HttpClientRequest;

public class DemoMain {
public static void main(String[] args) throws Exception {
AddressResolverOptions addressResolverOptions = new AddressResolverOptions();
addressResolverOptions.setNdots(1);
addressResolverOptions.setServers(Arrays.asList("8.8.8.8"));
addressResolverOptions.setSearchDomains(Arrays.asList(".com"));

VertxOptions vertxOptions = new VertxOptions();
vertxOptions.setAddressResolverOptions(addressResolverOptions);
Vertx vertx = Vertx.vertx(vertxOptions);
HttpClientOptions clientOp = new HttpClientOptions();
clientOp.setSsl(true);
clientOp.setTrustAll(true);
clientOp.setVerifyHost(false);
HttpClient httpClient = vertx.createHttpClient(clientOp);
HttpClientRequest req = httpClient.get(443, "www.baidu.com", "/index.html", resp -> {
  System.out.println(resp.statusCode());
  vertx.close();
});
req.end();

}
}
AddressResolverOptions 有几个重要属性
servers: 8.8.8.8,8.8.4.4 #对应Linux /etc/resolv.conf的nameserver,DNS服务器地址，支持配置多个，以逗号隔开
ndots: 1 #对应linux /etc/resolv.conf里面的options: ndots, 作用就是如果给的域名里面包含的点的个数少于该阈值，那么DNS解析的时候就会默认加上searchDomains的值，这个必须和searchDomains搭配使用，Linux默认为1，华为公有云PAAS（包含容器）默认是4
searchDomains: a,b,c #对应linux /etc/resolv.conf里面的search，和ndots搭配使用，如果当前域名的点个数少于设置值，解析时就会把这些值添加到域名后面一起解析，比如ndots设置的为4，当前域名为servicecomb.cn-north-1.myhwclouds.com，只有三个点，那么解析的时候就会自动加上servicecomb.cn-north-1.myhwclouds.com.a去解析，没解析出来在用servicecomb.cn-north-1.myhwclouds.com.b，直到能最后解析出来
optResourceEnabled: true #optional record is automatically included in DNS queries
cacheMinTimeToLive: 0 #最小缓存时间
cacheMaxTimeToLive: 10000 #最大缓存时间
cacheNegativeTimeToLive: 0 #DNS解析失败后，下次重试的等待时间
queryTimeout: 5000 #查询超时时间
maxQueries: 4 #查询次数
rdFlag: true #设置DNS递归查询
rotateServers: true #设置是否支持轮询，如果有多个域名服务器，轮训可以加快域名解析速度

2、代码解析过程
使用Vertx vertx = Vertx.vertx(vertxOptions)，在VertxImpl构造方法里面会初始化this.addressResolver = new AddressResolver(this, options.getAddressResolverOptions());
2.1 io.vertx.core.impl.AddressResolver里面有个static块，会读取/etc/resolv.conf文件，解析得到ndots和rotate默认值，在构造方法内初始化
public AddressResolver(Vertx vertx, AddressResolverOptions options) {
this.provider = ResolverProvider.factory(vertx, options);
this.resolverGroup = provider.resolver(options);
this.vertx = vertx;
}

ResolverProvider获取provider的时候，默认得到DnsResolverProvider，这个是Vertx提供的默认域名解析。

2.2 DnsResolverProvider代码解析

public DnsResolverProvider(VertxImpl vertx, AddressResolverOptions options) {
List dnsServers = options.getServers();
List serverList = new ArrayList<>();
// 看是否有配置dns服务地址，如果配置了则使用配置的地址
if (dnsServers != null && dnsServers.size() > 0) {
for (String dnsServer : dnsServers) {
// 解析域名地址和端口，默认是53
int sep = dnsServer.indexOf(':');
String ipAddress;
int port;
if (sep != -1) {
ipAddress = dnsServer.substring(0, sep);
port = Integer.parseInt(dnsServer.substring(sep + 1));
} else {
ipAddress = dnsServer;
port = 53;
}
try {
// 检查地址是否正确
serverList.add(new InetSocketAddress(InetAddress.getByAddress(NetUtil.createByteArrayFromIpAddressString(ipAddress)), port));
} catch (UnknownHostException e) {
throw new VertxException(e);
}
}
} else {
// 如果没有配置域名地址，则需要读取服务器上的地址
// 这里直接使用了Netty提供的获取服务器上默认域名地址，这里最终会调用DefaultDnsServerAddressStreamProvider，详细参考2.3
DnsServerAddressStream stream = DnsServerAddresses.defaultAddresses().stream();
Set all = new HashSet<>();
while (true) {
InetSocketAddress address = stream.next();
if (all.contains(address)) {
break;
}
serverList.add(address);
all.add(address);
}
}
DnsServerAddresses nameServerAddresses = options.isRotateServers() ? DnsServerAddresses.rotational(serverList) : DnsServerAddresses.sequential(serverList);
DnsServerAddressStreamProvider nameServerAddressProvider = hostname -> nameServerAddresses.stream();

// 解析host文件，不需要通过域名服务器解析，比如/etc/hosts,支持配置文件路径和直接设置。如果都为空，则读取系统的默认配置，windows读取C:\Windows\System32\drivers\etc\hosts，Linux读取/etc/hosts
HostsFileEntries entries;
if (options.getHostsPath() != null) {
File file = vertx.resolveFile(options.getHostsPath()).getAbsoluteFile();
try {
if (!file.exists() || !file.isFile()) {
throw new IOException();
}
entries = HostsFileParser.parse(file);
} catch (IOException e) {
throw new VertxException("Cannot read hosts file " + file.getAbsolutePath());
}
} else if (options.getHostsValue() != null) {
try {
entries = HostsFileParser.parse(new StringReader(options.getHostsValue().toString()));
} catch (IOException e) {
throw new VertxException("Cannot read hosts config ", e);
}
} else {
entries = HostsFileParser.parseSilently();
}

int minTtl = intValue(options.getCacheMinTimeToLive(), 0);
int maxTtl = intValue(options.getCacheMaxTimeToLive(), Integer.MAX_VALUE);
int negativeTtl = intValue(options.getCacheNegativeTimeToLive(), 0);
DnsCache resolveCache = new DefaultDnsCache(minTtl, maxTtl, negativeTtl);
DnsCache authoritativeDnsServerCache = new DefaultDnsCache(minTtl, maxTtl, negativeTtl);

this.vertx = vertx;
// 初始化AddressResolverGroup
this.resolverGroup = new AddressResolverGroup() {
  @Override
  protected io.netty.resolver.AddressResolver newResolver(EventExecutor executor) throws Exception {
    ChannelFactory channelFactory = () -> vertx.transport().datagramChannel();
    DnsAddressResolverGroup group = new DnsAddressResolverGroup(channelFactory, nameServerAddressProvider) {
      @Override
      protected NameResolver newNameResolver(EventLoop eventLoop, ChannelFactory channelFactory, DnsServerAddressStreamProvider nameServerProvider) throws Exception {
        DnsNameResolverBuilder builder = new DnsNameResolverBuilder((EventLoop) executor);
        builder.hostsFileEntriesResolver(new HostsFileEntriesResolver() {
          @Override
          public InetAddress address(String inetHost, ResolvedAddressTypes resolvedAddressTypes) {
            InetAddress address = lookup(inetHost, resolvedAddressTypes);
            if (address == null) {
              address = lookup(inetHost.toLowerCase(Locale.ENGLISH), resolvedAddressTypes);
            }
            return address;
          }
          InetAddress lookup(String inetHost, ResolvedAddressTypes resolvedAddressTypes) {
            switch (resolvedAddressTypes) {
              case IPV4_ONLY:
                return entries.inet4Entries().get(inetHost);
              case IPV6_ONLY:
                return entries.inet6Entries().get(inetHost);
              case IPV4_PREFERRED:
                Inet4Address inet4Address = entries.inet4Entries().get(inetHost);
                return inet4Address != null? inet4Address : entries.inet6Entries().get(inetHost);
              case IPV6_PREFERRED:
                Inet6Address inet6Address = entries.inet6Entries().get(inetHost);
                return inet6Address != null? inet6Address : entries.inet4Entries().get(inetHost);
              default:
                throw new IllegalArgumentException("Unknown ResolvedAddressTypes " + resolvedAddressTypes);
            }
          }
        });
        builder.channelFactory(channelFactory);
        builder.nameServerProvider(nameServerAddressProvider);
        builder.optResourceEnabled(options.isOptResourceEnabled());
        builder.resolveCache(resolveCache);
        builder.authoritativeDnsServerCache(authoritativeDnsServerCache);
        builder.queryTimeoutMillis(options.getQueryTimeout());
        builder.maxQueriesPerResolve(options.getMaxQueries());
        builder.recursionDesired(options.getRdFlag());
        // 必须配置了searchDomains，ndots才起作用
        if (options.getSearchDomains() != null) {
          builder.searchDomains(options.getSearchDomains());
          int ndots = options.getNdots();
          if (ndots == -1) {
            ndots = AddressResolver.DEFAULT_NDOTS_RESOLV_OPTION;
          }
          builder.ndots(ndots);
        }
        return builder.build();
      }
    };

    io.netty.resolver.AddressResolver resolver = group.getResolver(executor);
    resolvers.add(new ResolverRegistration(resolver, (EventLoop) executor));

    return resolver;
  }
};

}

2.3 DefaultDnsServerAddressStreamProvider 读取系统默认域名解析地址，使用static块，利用jndi-DNS获取域名地址列表

static {
final List defaultNameServers = new ArrayList(2);

    // Using jndi-dns to obtain the default name servers.
    //
    // See:
    // - http://docs.oracle.com/javase/8/docs/technotes/guides/jndi/jndi-dns.html
    // - http://mail.openjdk.java.net/pipermail/net-dev/2017-March/010695.html
    Hashtable env = new Hashtable();
    env.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.dns.DnsContextFactory");
    env.put("java.naming.provider.url", "dns://");
    try {
        DirContext ctx = new InitialDirContext(env);
        // 获取dnsUrl地址
        String dnsUrls = (String) ctx.getEnvironment().get("java.naming.provider.url");
        // Only try if not empty as otherwise we will produce an exception
        if (dnsUrls != null && !dnsUrls.isEmpty()) {
            String[] servers = dnsUrls.split(" ");
            for (String server : servers) {
                try {
                    URI uri = new URI(server);
                    String host = new URI(server).getHost();

                    if (host == null || host.isEmpty()) {
                        logger.debug(
                                "Skipping a nameserver URI as host portion could not be extracted: {}", server);
                        // If the host portion can not be parsed we should just skip this entry.
                        continue;
                    }
                    int port  = uri.getPort();
                    defaultNameServers.add(SocketUtils.socketAddress(uri.getHost(), port == -1 ? DNS_PORT : port));
                } catch (URISyntaxException e) {
                    logger.debug("Skipping a malformed nameserver URI: {}", server, e);
                }
            }
        }
    } catch (NamingException ignore) {
        // Will try reflection if this fails.
    }

    if (defaultNameServers.isEmpty()) {
        try {
            Class configClass = Class.forName("sun.net.dns.ResolverConfiguration");
            Method open = configClass.getMethod("open");
            Method nameservers = configClass.getMethod("nameservers");
            Object instance = open.invoke(null);

            @SuppressWarnings("unchecked")
            final List list = (List) nameservers.invoke(instance);
            for (String a: list) {
                if (a != null) {
                    defaultNameServers.add(new InetSocketAddress(SocketUtils.addressByName(a), DNS_PORT));
                }
            }
        } catch (Exception ignore) {
            // Failed to get the system name server list via reflection.
            // Will add the default name servers afterwards.
        }
    }

    if (!defaultNameServers.isEmpty()) {
        if (logger.isDebugEnabled()) {
            logger.debug(
                    "Default DNS servers: {} (sun.net.dns.ResolverConfiguration)", defaultNameServers);
        }
    } else {
        // Depending if IPv6 or IPv4 is used choose the correct DNS servers provided by google:
        // https://developers.google.com/speed/public-dns/docs/using
        // https://docs.oracle.com/javase/7/docs/api/java/net/doc-files/net-properties.html
        if (NetUtil.isIpV6AddressesPreferred() ||
                (NetUtil.LOCALHOST instanceof Inet6Address && !NetUtil.isIpV4StackPreferred())) {
            Collections.addAll(
                    defaultNameServers,
                    SocketUtils.socketAddress("2001:4860:4860::8888", DNS_PORT),
                    SocketUtils.socketAddress("2001:4860:4860::8844", DNS_PORT));
        } else {
            Collections.addAll(
                    defaultNameServers,
                    SocketUtils.socketAddress("8.8.8.8", DNS_PORT),
                    SocketUtils.socketAddress("8.8.4.4", DNS_PORT));
        }

        if (logger.isWarnEnabled()) {
            logger.warn(
                    "Default DNS servers: {} (Google Public DNS as a fallback)", defaultNameServers);
        }
    }

    DEFAULT_NAME_SERVER_LIST = Collections.unmodifiableList(defaultNameServers);
    DEFAULT_NAME_SERVER_ARRAY = defaultNameServers.toArray(new InetSocketAddress[defaultNameServers.size()]);
    DEFAULT_NAME_SERVERS = sequential(DEFAULT_NAME_SERVER_ARRAY);
}

3、潜在的问题
3.1 jndi-DNS获取的域名解析机制，底层使用native code方法，具体实现不详。猜测Linux是直接获取/etc/resolv.conf里面配置的地址，windows获取网卡配置的域名地址。在windows下面，如果有多个网卡，并且有网卡是不通外网的。

3.2 DNS默认的searchDomain没有配置，也没有获取系统的配置，单独设置ndots是不生效的。