Hadoop的典型运行场景在集群（cluster）上，其中每台机器都称为节点（node），节点之间连成一个局域网。

集群内的节点之间可以通过IP地址通讯，也可以通过节点的域名即URL通信，这就需要有DNS的帮助。这意味着，在网络可以通达的某处存在着DNS服务，因而可以根据对方的URL，查找到其IP地址。着也意味着集群内的这些节点都应有个域名，并且登记在DNS中。

一般涉及到DNS的操作都在操作系统或底层的库程序中，对于应用层是透明的。比方说我们通过HTTP访问网站就只需要提供其域名，而HTTP驱动层自然会与DNS服务器交互以获得目标网站的IP地址。但是Hadoop并不甘于DNS对其保持透明，因为它的有些操作需要知道具体节点的IP地址，因此Hadoop定义了一个名为DNS的类。

源码如下：

package org.apache.hadoop.net;

//1.如果标注的是Public，说明被注解的类型对多有工程和应用可用。 
//2.如果标注的是LimitedPrivate,说明被注解的类型只能用于某些特定的工程或应用，如Common,HDFS,MapReduce,ZooKeeper,HBase等。 
//3.如果标注的是Private，说明被注解的类型只能用于Hadoop。
@InterfaceAudience.LimitedPrivate({"HDFS", "MapReduce"})

//1.如果标注的是Stable，说明主版本是稳定的，不同主版本之间可能不兼容。 
//2.如果标注的是Evolving，说明是不停在变化的，不同小版本之间也可能不兼容。 
//3.如果标注的是Unstable，说明稳定性没有任何保证。
@InterfaceStability.Unstable
public class DNS {

  private static final Log LOG = LogFactory.getLog(DNS.class);

  /**
   * The cached hostname -initially null.
   */

  private static final String cachedHostname = resolveLocalHostname();
  private static final String cachedHostAddress = resolveLocalHostIPAddress();
  private static final String LOCALHOST = "localhost";

  /**
   * Returns the hostname associated with the specified IP address by the
   * provided nameserver.
   *
   * Loopback addresses 
   * @param hostIp The address to reverse lookup
   * @param ns The host name of a reachable DNS server
   * @return The host name associated with the provided IP
   * @throws NamingException If a NamingException is encountered
   */
//这是逆向查询，从IP地址查找其域名，参数ns是DNS服务器的地址
  public static String reverseDns(InetAddress hostIp, String ns)
    throws NamingException {
    //
    // Builds the reverse IP lookup form
    // This is formed by reversing the IP numbers and appending in-addr.arpa
    //
//倒排IP地址的4个字段，例如“192.188.0.1”就变成“1.0.188.192.in-addr.arpa”
    String[] parts = hostIp.getHostAddress().split("\\.");
    String reverseIP = parts[3] + "." + parts[2] + "." + parts[1] + "."
      + parts[0] + ".in-addr.arpa";

//形成一个DNS查询语句
    DirContext ictx = new InitialDirContext();
    Attributes attribute;
    try {
      attribute = ictx.getAttributes("dns://"               // Use "dns:///" if the default
                         + ((ns == null) ? "" : ns) +
                         // nameserver is to be used
                         "/" + reverseIP, new String[] { "PTR" });
    } finally {
      ictx.close();
    }

    String hostname = attribute.get("PTR").get().toString();
    int hostnameLength = hostname.length();
    if (hostname.charAt(hostnameLength - 1) == '.') {
      hostname = hostname.substring(0, hostnameLength - 1);
    }
    return hostname;
  }

  /**
   * @return NetworkInterface for the given subinterface name (eg eth0:0)
   *    or null if no interface with the given name can be found  
   */
  private static NetworkInterface getSubinterface(String strInterface)
      throws SocketException {
    Enumeration<NetworkInterface> nifs = 
      NetworkInterface.getNetworkInterfaces();
      
    while (nifs.hasMoreElements()) {
      Enumeration<NetworkInterface> subNifs = 
        nifs.nextElement().getSubInterfaces();

      while (subNifs.hasMoreElements()) {
        NetworkInterface nif = subNifs.nextElement();
        if (nif.getName().equals(strInterface)) {
          return nif;
        }
      }
    }
    return null;
  }

  /**
   * @param nif network interface to get addresses for
   * @return set containing addresses for each subinterface of nif,
   *    see below for the rationale for using an ordered set
   */
  private static LinkedHashSet<InetAddress> getSubinterfaceInetAddrs(
      NetworkInterface nif) {
    LinkedHashSet<InetAddress> addrs = new LinkedHashSet<InetAddress>();
    Enumeration<NetworkInterface> subNifs = nif.getSubInterfaces();
    while (subNifs.hasMoreElements()) {
      NetworkInterface subNif = subNifs.nextElement();
      addrs.addAll(Collections.list(subNif.getInetAddresses()));
    }
    return addrs;
  }

  /**
   * Like {@link DNS#getIPs(String, boolean), but returns all
   * IPs associated with the given interface and its subinterfaces.
   */
  public static String[] getIPs(String strInterface)
      throws UnknownHostException {
    return getIPs(strInterface, true);
  }

  /**
   * Returns all the IPs associated with the provided interface, if any, in
   * textual form.
   * 
   * @param strInterface
   *            The name of the network interface or sub-interface to query
   *            (eg eth0 or eth0:0) or the string "default"
   * @param returnSubinterfaces
   *            Whether to return IPs associated with subinterfaces of
   *            the given interface
   * @return A string vector of all the IPs associated with the provided
   *         interface. The local host IP is returned if the interface
   *         name "default" is specified or there is an I/O error looking
   *         for the given interface.
   * @throws UnknownHostException
   *             If the given interface is invalid
   * 
   */
//获取绑定在某个网口（例如eth0）上的所有IP地址
  public static String[] getIPs(String strInterface,
      boolean returnSubinterfaces) throws UnknownHostException {
    if ("default".equals(strInterface)) {
      return new String[] { cachedHostAddress };
    }
    NetworkInterface netIf;
    try {
      netIf = NetworkInterface.getByName(strInterface);
      if (netIf == null) {
        netIf = getSubinterface(strInterface);
      }
    } catch (SocketException e) {
      LOG.warn("I/O error finding interface " + strInterface +
          ": " + e.getMessage());
      return new String[] { cachedHostAddress };
    }
    if (netIf == null) {
      throw new UnknownHostException("No such interface " + strInterface);
    }

    // NB: Using a LinkedHashSet to preserve the order for callers
    // that depend on a particular element being 1st in the array.
    // For example, getDefaultIP always returns the first element.
    LinkedHashSet<InetAddress> allAddrs = new LinkedHashSet<InetAddress>();
    allAddrs.addAll(Collections.list(netIf.getInetAddresses()));
    if (!returnSubinterfaces) {
      allAddrs.removeAll(getSubinterfaceInetAddrs(netIf));
    }

    String ips[] = new String[allAddrs.size()];
    int i = 0;
    for (InetAddress addr : allAddrs) {
      ips[i++] = addr.getHostAddress();
    }
    return ips;
  }


  /**
   * Returns the first available IP address associated with the provided
   * network interface or the local host IP if "default" is given.
   *
   * @param strInterface
   *            The name of the network interface or subinterface to query
   *             (e.g. eth0 or eth0:0) or the string "default"
   * @return The IP address in text form, the local host IP is returned
   *         if the interface name "default" is specified
   * @throws UnknownHostException
   *             If the given interface is invalid
   */
//获取绑定在某个网口上的默认IP地址
  public static String getDefaultIP(String strInterface)
    throws UnknownHostException {
    String[] ips = getIPs(strInterface);
    return ips[0];
  }

  /**
   * Returns all the host names associated by the provided nameserver with the
   * address bound to the specified network interface
   *
   * @param strInterface
   *            The name of the network interface or subinterface to query
   *            (e.g. eth0 or eth0:0)
   * @param nameserver
   *            The DNS host name
   * @return A string vector of all host names associated with the IPs tied to
   *         the specified interface
   * @throws UnknownHostException if the given interface is invalid
   */
//获取绑定于网口strlnterface的所有域名，参数nameserver是DNS的服务器地址或IP地址，可以为null
  public static String[] getHosts(String strInterface, String nameserver)
    throws UnknownHostException {
    String[] ips = getIPs(strInterface);//获取该网口的所有IP地址
    Vector<String> hosts = new Vector<String>();
    for (int ctr = 0; ctr < ips.length; ctr++) {//逐一查询绑定于这些地址的域名
      try {
        hosts.add(reverseDns(InetAddress.getByName(ips[ctr]),
                             nameserver));
      } catch (UnknownHostException ignored) {
      } catch (NamingException ignored) {
      }
    }
    if (hosts.isEmpty()) {
      LOG.warn("Unable to determine hostname for interface " + strInterface);
      return new String[] { cachedHostname };
    } else {
      return hosts.toArray(new String[hosts.size()]);//返回这些域名
    }
  }


  /**
   * Determine the local hostname; retrieving it from cache if it is known
   * If we cannot determine our host name, return "localhost"
   * @return the local hostname or "localhost"
   */
  private static String resolveLocalHostname() {//获取本机的主机名
    String localhost;
    try {
      localhost = InetAddress.getLocalHost().getCanonicalHostName();
    } catch (UnknownHostException e) {
      LOG.warn("Unable to determine local hostname "
          + "-falling back to \"" + LOCALHOST + "\"", e);
      localhost = LOCALHOST;
    }
    return localhost;
  }


  /**
   * Get the IPAddress of the local host as a string.
   * This will be a loop back value if the local host address cannot be
   * determined.
   * If the loopback address of "localhost" does not resolve, then the system's
   * network is in such a state that nothing is going to work. A message is
   * logged at the error level and a null pointer returned, a pointer
   * which will trigger failures later on the application
   * @return the IPAddress of the local host or null for a serious problem.
   */
  private static String resolveLocalHostIPAddress() {//获取本机的IP地址
    String address;
      try {
        address = InetAddress.getLocalHost().getHostAddress();
      } catch (UnknownHostException e) {
        LOG.warn("Unable to determine address of the host"
                + "-falling back to \"" + LOCALHOST + "\" address", e);
        try {
          address = InetAddress.getByName(LOCALHOST).getHostAddress();
        } catch (UnknownHostException noLocalHostAddressException) {
          //at this point, deep trouble
          LOG.error("Unable to determine local loopback address "
                  + "of \"" + LOCALHOST + "\" " +
                  "-this system's network configuration is unsupported", e);
          address = null;
        }
      }
    return address;
  }

  /**
   * Returns all the host names associated by the default nameserver with the
   * address bound to the specified network interface
   * 
   * @param strInterface
   *            The name of the network interface to query (e.g. eth0)
   * @return The list of host names associated with IPs bound to the network
   *         interface
   * @throws UnknownHostException
   *             If one is encountered while querying the default interface
   * 
   */
  public static String[] getHosts(String strInterface)
    throws UnknownHostException {
    return getHosts(strInterface, null);
  }

  /**
   * Returns the default (first) host name associated by the provided
   * nameserver with the address bound to the specified network interface
   * 
   * @param strInterface
   *            The name of the network interface to query (e.g. eth0)
   * @param nameserver
   *            The DNS host name
   * @return The default host names associated with IPs bound to the network
   *         interface
   * @throws UnknownHostException
   *             If one is encountered while querying the default interface
   */
//参数nameserver为DNS服务器地址
  public static String getDefaultHost(String strInterface, String nameserver)
    throws UnknownHostException {
    if ("default".equals(strInterface)) {
      return cachedHostname;
    }

    if ("default".equals(nameserver)) {
      return getDefaultHost(strInterface);
    }

    String[] hosts = getHosts(strInterface, nameserver);
    return hosts[0];
  }

  /**
   * Returns the default (first) host name associated by the default
   * nameserver with the address bound to the specified network interface
   * 
   * @param strInterface
   *            The name of the network interface to query (e.g. eth0).
   *            Must not be null.
   * @return The default host name associated with IPs bound to the network
   *         interface
   * @throws UnknownHostException
   *             If one is encountered while querying the default interface
   */
  public static String getDefaultHost(String strInterface)
    throws UnknownHostException {
    return getDefaultHost(strInterface, null);
  }

}

模块外程序在使用时直接使用DNS.getDefaultHost这样的方式调用。举个使用的例子，DataNode中有一个getHostName()的方法，用来获取本节点的主机名，代码如下：

  package org.apache.hadoop.hdfs.server.datanode;

  private static String getHostName(Configuration config)
      throws UnknownHostException {
    String name = config.get(DFS_DATANODE_HOST_NAME_KEY);//本节点主机名
//先查看配置文件是否已经设定
//如果没有设定，就求助于DNS服务
    if (name == null) {
      name = DNS.getDefaultHost(
          config.get(DFS_DATANODE_DNS_INTERFACE_KEY,//网卡名称
                     DFS_DATANODE_DNS_INTERFACE_DEFAULT),
          config.get(DFS_DATANODE_DNS_NAMESERVER_KEY,//DNS服务器
                     DFS_DATANODE_DNS_NAMESERVER_DEFAULT));
    }
    return name;
  }