A user in Kerberos is called a principal, which is made up of three distinct components: the primary, instance, and realm. A Kerberos principal is used in a Kerberos-secured system to represent a unique identity. The first component of the principal is called the primary, or sometimes the user component. The primary component is an arbitrary string and may be the operating system username of the user or the name of a service. The primary component is followed by an optional section called the instance, which is used to create principals that are used by users in special roles or to define the host on which a service runs, for example. An instance, if it exists, is separated from the primary by a slash and then the content is used to disambiguate multiple principals for a single user or service. The final component of the principal is the realm. The realm is similar to a domain in DNS in that it logically defines a related group of objects, although rather than hostnames as in DNS, the Kerberos realm defines a group of principals . Each realm can have its own settings including the location of the KDC on the network and supported encryption algorithms. Large organizations commonly create distinct realms to delegate administration of a realm to a group within the enterprise. Realms, by convention, are written in uppercase characters.

Kerberos assigns tickets to Kerberos principals to enable them to access Kerberos-secured Hadoop services. For the Hadoop daemon principals, the principal names should be of the format username/fully.qualified.domain.name@YOUR-REALM.COM. In this guide, username in the username/fully.qualified.domain.name@YOUR-REALM.COM principal refers to the username of an existing Unix account that is used by Hadoop daemons, such as hdfs or mapred. Human users who want to access the Hadoop cluster also need to have Kerberos principals; in this case, username refers to the username of the user's Unix account, such as joe or jane. Single-component principal names (such as joe@YOUR-REALM.COM) are acceptable for client user accounts. Hadoop does not support more than two-component principal names.

A keytab is a file containing pairs of Kerberos principals and an encrypted copy of that principal's key. A keytab file for a Hadoop daemon is unique to each host since the principal names include the hostname. This file is used to authenticate a principal on a host to Kerberos without human interaction or storing a password in a plain text file. Because having access to the keytab file for a principal allows one to act as that principal, access to the keytab files should be tightly secured. They should be readable by a minimal set of users, should be stored on local disk, and should not be included in host backups, unless access to those backups is as secure as access to the local host.

Users in a Hadoop cluster authenticate themselves to the NameNode using their Kerberos credentials. However, once the user is authenticated, each job subsequently submitted must also be checked to ensure it comes from an authenticated user. Since there could be a time gap between a job being submitted and the job being executed, during which the user could have logged off, user credentials are passed to the NameNode using delegation tokens that can be used for authentication in the future.

Delegation tokens are a secret key shared with the NameNode, that can be used to impersonate a user to get a job executed. While these tokens can be renewed, new tokens can only be obtained by clients authenticating to the NameNode using Kerberos credentials. By default, delegation tokens are only valid for a day. However, since jobs can last longer than a day, each token specifies a JobTracker as a renewer which is allowed to renew the delegation token once a day, until the job completes, or for a maximum period of 7 days. When the job is complete, the JobTracker requests the NameNode to cancel the delegation token.

TokenID = {ownerID, renewerID, issueDate, maxDate, sequenceNumber}
TokenAuthenticator = HMAC-SHA1(masterKey, TokenID) 
Delegation Token = {TokenID, TokenAuthenticator}

This approach uses an MIT KDC that is local to the cluster. Users and services will have to authenticate with this local KDC before they can interact with the CDH components on the cluster.

This approach uses an MIT KDC and Kerberos realm that is local to the cluster. However, Active Directory stores the user principals that will access the cluster in a central realm. Users will have to authenticate with this central AD realm to obtain TGTs before they can interact with CDH services on the cluster. Note that CDH service principals reside only in the local KDC realm.

This approach uses the central Active Directory as the KDC. No local KDC is required. Before you decide upon an AD KDC deployment, make sure you are aware of the following possible ramifications of that decision.

Architecture Summary

• All service and user principals are created in the Active Directory KDC.
• All cluster hosts are configured with the central AD Kerberos realm using krb5.conf.
• Cloudera Manager connects to the Active Directory KDC to create and manage the principals for the CDH services running on the cluster. To do this, Cloudera Manager uses a principal that has the privileges to create other accounts within the given Organisational Unit (OU). This step has been automated by the Kerberos wizard. Instructions for manually creating the Cloudera Manager admin principal are provided in the Cloudera Manager security documentation.
• All service and user principals are authenticated by the Active Directory KDC.

  Note: If it is not possible to create the Cloudera Manager admin principal with the required privileges in the Active Directory KDC, then the CDH services principals will need to be created manually. The corresponding keytab files should then be stored securely on the Cloudera Manager Server host. Cloudera Manager's Custom Kerberos Keytab Retrieval script can be used to retrieve the keytab files from the local filesystem.