reverse resolve the namenode hostname for kerberos spn

[realityone]

I find that hadoop will use the reverse-resolved hostname to make the kerberos SPN. This PR just make the same behavior.

• https://github.com/apache/hadoop/blob/7a7db7f0dc4107f44b281eb834fdffc9fd9b08b3/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/DFSUtil.java#L445

  /**
   * @return a collection of all configured NN Kerberos principals.
   */
  public static Set<String> getAllNnPrincipals(Configuration conf) throws IOException {
    Set<String> principals = new HashSet<String>();
    for (String nsId : DFSUtilClient.getNameServiceIds(conf)) {
      if (HAUtil.isHAEnabled(conf, nsId)) {
        for (String nnId : DFSUtilClient.getNameNodeIds(conf, nsId)) {
          Configuration confForNn = new Configuration(conf);
          NameNode.initializeGenericKeys(confForNn, nsId, nnId);
          String principal = SecurityUtil.getServerPrincipal(confForNn
              .get(DFSConfigKeys.DFS_NAMENODE_KERBEROS_PRINCIPAL_KEY),
              DFSUtilClient.getNNAddress(confForNn).getHostName());
          principals.add(principal);
        }
      } else {
        Configuration confForNn = new Configuration(conf);
        NameNode.initializeGenericKeys(confForNn, nsId, null);
        String principal = SecurityUtil.getServerPrincipal(confForNn
            .get(DFSConfigKeys.DFS_NAMENODE_KERBEROS_PRINCIPAL_KEY),
            DFSUtilClient.getNNAddress(confForNn).getHostName());
        principals.add(principal);
      }
    }

    return principals;
  }

And this is a potential reason for this error:

# go run ./cmd/hdfs ls /
Couldn't connect to namenode: no available namenodes: SASL handshake: [Root cause: KDC_Error] KDC_Error: TGS Exchange Error: kerberos error response from KDC when requesting for nn/<hostname>: KRB Error: (7) KDC_ERR_S_PRINCIPAL_UNKNOWN Server not found in Kerberos database - LOOKING_UP_SERVER
exit status 1

[lishunan246]

Works for me.