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   /*
    * Copyright 2018 The Netty Project
    *
    * The Netty Project licenses this file to you under the Apache License,
    * version 2.0 (the "License"); you may not use this file except in compliance
    * with the License. You may obtain a copy of the License at:
    *
    *   https://www.apache.org/licenses/LICENSE-2.0
    *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
   * License for the specific language governing permissions and limitations
   * under the License.
   */
  package io.netty.resolver.dns;
  
  import io.netty.util.internal.SocketUtils;
  import io.netty.util.internal.logging.InternalLogger;
  import io.netty.util.internal.logging.InternalLoggerFactory;
  
  import javax.naming.Context;
  import javax.naming.NamingException;
  import javax.naming.directory.DirContext;
  import javax.naming.directory.InitialDirContext;
  import java.net.InetSocketAddress;
  import java.net.URI;
  import java.net.URISyntaxException;
  import java.util.Hashtable;
  import java.util.List;
  
  final class DirContextUtils {
      private static final InternalLogger logger =
              InternalLoggerFactory.getInstance(DirContextUtils.class);
  
      private DirContextUtils() { }
  
      static void addNameServers(List<InetSocketAddress> defaultNameServers, int defaultPort) {
          // Using jndi-dns to obtain the default name servers.
          //
          // See:
          // - https://docs.oracle.com/javase/8/docs/technotes/guides/jndi/jndi-dns.html
          // - https://mail.openjdk.java.net/pipermail/net-dev/2017-March/010695.html
          Hashtable<String, String> env = new Hashtable<String, String>();
          env.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.dns.DnsContextFactory");
          env.put("java.naming.provider.url", "dns://");
  
          try {
              DirContext ctx = new InitialDirContext(env);
              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 = uri.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 ?
                                  defaultPort : port));
                      } catch (URISyntaxException e) {
                          logger.debug("Skipping a malformed nameserver URI: {}", server, e);
                      }
                  }
              }
          } catch (NamingException ignore) {
              // Will try reflection if this fails.
          }
      }
  }