Cloudera Docs

Configuring a dedicated MIT KDC for cross-realm trust

How to configure a cluster-dedicated KDC and default realm in Cloudera Manager.

Using Cloudera Manager to configure Kerberos authentication for the cluster creates several principals and keytabs automatically. Cloudera Manager also deploys the keytab files to every host in the cluster. See Hadoop Users (user:group) and Kerberos Principals for complete listing.

Local and Remote Kerberos Admin Tools

Kerberos administrator commands can be run directly on the KDC server host or remotely, as shown in the table:

kadmin.local Requires root access or Kerberos admin account. Use to log on directly to the KDC host.
kadmin Use the logon to the KDC host system from another remote host over the network.
  • To run Kerberos administration commands locally on the KRB host system: 
    sudo kadmin.local
    Enter your Linux system password (for the sudo).
  • To run Kerberos administration commands from any host: 
    kadmin
    Enter your Kerberos administrator password.

Setting up a Cluster-Dedicated KDC and Default Realm for the Hadoop Cluster

Cloudera has tested the following configuration approaches to Kerberos security for clusters managed by Cloudera Manager. For administration teams that are just getting started with Kerberos security, we recommend starting with these approaches to the configuration of KDC services for a number of reasons.

The number of Service Principal Names (SPNs) that are created and managed by the Cloudera Manager server for a CDP cluster can be significant, so it is important to realize the potential impact on cluster uptime and overall operations if you choose to manage keytabs manually instead. The Cloudera Manager server manages the creation of service keytabs on the proper hosts based on the current configuration of the database. Manual keytab management can be error prone and introduce delays when deploying or moving services within the cluster, especially under time-sensitive conditions.

Cloudera Manager creates SPNs within a KDC that it can access with the kadmin command based on configuration of the /etc/krb5.conf file on the Cloudera Manager host. SPNs are created with the format service-name/host.fqdn.name@EXAMPLE.COM where service-name is the relevant CDP service name such as hue or hbase or hdfs.

If your site already has a working KDC, and any existing principals share the same name as any of the principals that Cloudera Manager creates, the Cloudera Manager Server generates a new randomized key for those principals, and consequently causes existing keytabs to become invalid.

This is why Cloudera recommends using a dedicated local MIT Kerberos KDC and realm for the Hadoop cluster. You can set up a one-way cross-realm trust from the cluster-dedicated KDC and realm to your existing central MIT Kerberos KDC, or to an existing Active Directory realm. Using this method, there is no need to create Hadoop service principals in the central MIT Kerberos KDC or in Active Directory, but principals (users) in the central MIT KDC or in Active Directory can be authenticated to Hadoop. The steps to implement this approach are as follows:

  1. Install and configure a cluster-dedicated MIT Kerberos KDC that will be managed by Cloudera Manager for creating and storing principals for the services supported by the cluster.
  2. See the example kdc.conf and krb5.conf files in in Sample Kerberos Configuration Files for configuration considerations for the KDC and Kerberos clients.
  3. Configure a default Kerberos realm for the cluster you want Cloudera Manager to manage and set up one-way cross-realm trust between the cluster-dedicated KDC and either your central KDC or Active Directory, using the appropriate steps:
    • Using a Cluster-Dedicated KDC with a Central MIT KDC
    • Using a Cluster-Dedicated MIT KDC with Active Directory
    Cloudera strongly recommends the method above because:
    • It requires minimal configuration in Active Directory.
    • It is comparatively easy to script the creation of many principals and keytabs. A principal and keytab must be created for every daemon in the cluster, and in a large cluster this can be extremely onerous to do directly in Active Directory.
    • There is no need to involve central Active Directory administrators to get service principals created.
    • It allows for incremental configuration. The Hadoop administrator can completely configure and verify the functionality the cluster independently of integrating with Active Directory.

Using a Cluster-Dedicated KDC with a Central MIT KDC

  1. In the /var/kerberos/krb5kdc/kdc.conf file on the local dedicated KDC server host, configure the default realm for the Hadoop cluster by substituting your Kerberos realm in the following realms property: 
    [realms]
 HADOOP.EXAMPLE.COM = {
  2. In the /etc/krb5.conf file on all cluster hosts and all Hadoop client user hosts, configure the default realm for the Hadoop cluster by substituting your Kerberos realm in the following realms property. Also specify the local dedicated KDC server hostname in the /etc/krb5.conf file (for example, kdc01.example.com). 
    [libdefaults]
  default_realm = HADOOP.EXAMPLE.COM
[realms]
  HADOOP.EXAMPLE.COM = {
    kdc = kdc01.hadoop.example.com:88
    admin_server = kdc01.hadoop.example.com:749
    default_domain = hadoop.example.com
  }
  EXAMPLE.COM = {
    kdc = kdc01.example.com:88
    admin_server = kdc01.example.com:749
    default_domain = example.com
  }
[domain_realm]
  .hadoop.example.com = HADOOP.EXAMPLE.COM
  hadoop.example.com = HADOOP.EXAMPLE.COM
  .example.com = EXAMPLE.COM
  example.com = EXAMPLE.COM
  3. To set up the cross-realm trust in the cluster-dedicated KDC, type the following command in the kadmin.local or kadmin shell on the cluster-dedicated KDC host to create a krbtgt principal. Substitute your cluster-dedicated KDC realm for HADOOP.EXAMPLE.COM, and substitute your central KDC realm for EXAMPLE.COM. Enter a trust password when prompted. Note the password because you will need to enter the exact same password in the central KDC in the next step. 
    kadmin:  addprinc krbtgt/HADOOP.EXAMPLE.COM@EXAMPLE.COM
  4. To set up the cross-realm trust in the central KDC, type the same command in the kadmin.local or kadmin shell on the central KDC host to create the exact same krbtgt principal and password. 
    kadmin:  addprinc krbtgt/HADOOP.EXAMPLE.COM@EXAMPLE.COM
  5. To properly translate principal names from the central KDC realm into the cluster-dedicated KDC realm for the Hadoop cluster, configure the Trusted Kerberos Realms property of the HDFS service.
    1. Open the Cloudera Manager Admin Console.
    2. Go to the HDFS service.
    3. Click the Configuration tab.
    4. Select Scope > HDFS (Service Wide).
    5. Select Category > Security.
    6. Type Kerberos in the Search box.
    7. Edit the Trusted Kerberos Realms property to add the name of your central KDC realm. If you need to use more advanced mappings which do more than just allow principals from another domain, you may enter them in the Additional Rules to Map Kerberos Principals to Short Names property.
  6. Each of your Hadoop client users must also place this information in their local core-site.xml file. The easiest way to do so is by using the Cloudera Manager Admin Console to generate a client configuration file.
  7. Later in this procedure, you will restart the services to have the configuration changes in core-site.xml take effect.

Using a Cluster-Dedicated MIT KDC with Active Directory

For Cloudera Manager clusters, the openldap-clients package must be installed on the Cloudera Manager Server host before configuring the cluster to use Kerberos for authentication.

On the Active Directory Server

  1. On the Active Directory server host, type the following command to add the local realm trust to Active Directory: 
    netdom trust HADOOP.EXAMPLE.COM /Domain:EXAMPLE.COM /add /realm /passwordt:TrustPassword
  2. On the Active Directory server host, type the following command to set the proper encryption type:

    Windows 2003 RC2

    Windows 2003 server installations do not support AES encryption for Kerberos. 
    ktpass /MITRealmName HADOOP.EXAMPLE.COM /TrustEncryp RC4
    Windows 2008
    ksetup /SetEncTypeAttr HADOOP.EXAMPLE.COM <enc_type>
    Where the <enc_type> parameter can be replaced with parameter strings for AES, DES, or RC4 encryption modes. For example, for AES encryption, replace <enc_type> with AES256-CTS-HMAC-SHA1-96 or AES128-CTS-HMAC-SHA1-96 and for RC4 encryption, replace with RC4-HMAC-MD5.

On the MIT KDC Server

  1. In the /var/kerberos/krb5kdc/kdc.conf file on the local dedicated KDC server host, configure the default realm for the Hadoop cluster by substituting your Kerberos realm in the following realms property: 
    [realms]
 HADOOP.EXAMPLE.COM = {
  2. Each of your Hadoop client users must also place this information in their local core-site.xml file. The easiest way to do so is by using the Cloudera Manager Admin Console to generate a client configuration file.
  3. On the local MIT KDC server host, type the following command in the kadmin.local or kadmin shell to add the cross-realm krbtgt principal: 
    kadmin:  addprinc -e "<keysalt_list>" krbtgt/HADOOP.EXAMPLE.COM@EXAMPLE.COM

    where the <keysalt_list> parameter specifies the types of keys and their salt to be used for encryption of the password for this cross-realm krbtgt principal. It can be set to AES, or RC4 keytypes with a salt value of :normal. Note that DES is deprecated and should no longer be used. You can specify multiple keysalt types using the parameter in the command above. Make sure that at least one of the encryption types corresponds to the encryption types found in the tickets granted by the KDC in the remote realm. For an example of the values to use, see the examples based on the Active Directory functional domain level, below.

    Examples by Active Directory Domain or Forest "Functional level"

    Active Directory will, based on the Domain or Forest functional level, use encryption types supported by that release of the Windows Server operating system. It is not possible to use AES encryption types with an AD 2003 functional level. If you notice that DES encryption types are being used when authenticating or requesting service tickets to Active Directory then it might be necessary to enable weak encryption types in the /etc/krb5.conf.
    • Windows 2003
      kadmin: addprinc -e "rc4-hmac:normal" krbtgt/HADOOP.EXAMPLE.COM@EXAMPLE.COM
    • Windows 2008
      kadmin: addprinc -e "aes256-cts:normal aes128-cts:normal rc4-hmac:normal" krbtgt/HADOOP.EXAMPLE.COM@EXAMPLE.COM

On All Cluster Hosts

  1. In the /etc/krb5.conf file on all cluster hosts and all Hadoop client user hosts, configure both Kerberos realms. Note that default_realm should be configured as the local MIT Kerberos realm for the cluster. Your krb5.conf may contain more configuration properties than those demonstrated below. This example is provided to clarify configuration parameters. 
    [libdefaults]
  default_realm = HADOOP.EXAMPLE.COM
[realms]
  EXAMPLE.COM = {
    kdc = dc01.example.com:88
    admin_server = dc01.example.com:749
  }
  HADOOP.EXAMPLE.COM = {
    kdc = kdc01.hadoop.example.com:88
    admin_server = kdc01.hadoop.example.com:749
  }
[domain_realm]
  .hadoop.example.com = HADOOP.EXAMPLE.COM
  hadoop.example.com = HADOOP.EXAMPLE.COM
  .example.com = EXAMPLE.COM
  example.com = EXAMPLE.COM
  2. Use one of the following methods to properly translate principal names from the Active Directory realm into the cluster-dedicated KDC realm for the Hadoop cluster.
    • Using Cloudera Manager: Configure the Trusted Kerberos realms property of the HDFS service:
      1. Open the Cloudera Manager Admin Console.
      2. Go to the HDFS service.
      3. Click the Configuration tab.
      4. Select Scope > HDFS (Service Wide)
      5. Select Category > Security.
      6. Type Kerberos in the Search box.
      7. Edit the Trusted Kerberos Realms property to add the name of your central KDC realm. If you need to use more advanced mappings which do more than just allow principals from another domain, you may enter them in the Additional Rules to Map Kerberos Principals to Short Names property.
    • Using the Command Line: Configure the hadoop.security.auth_to_local setting in the core-site.xml file on all of the cluster hosts. The following example translates all principal names with the realm EXAMPLE.COM into the first component of the principal name only. It also preserves the standard translation for the default realm (the cluster realm).
      <property>
  <name>hadoop.security.auth_to_local</name>
  <value>
    RULE:[1:$1@$0](^.*@EXAMPLE\.COM$)s/^(.*)@EXAMPLE\.COM$/$1/g
    RULE:[2:$1@$0](^.*@EXAMPLE\.COM$)s/^(.*)@EXAMPLE\.COM$/$1/g
    DEFAULT
  </value>
</property>