System Administration Guide: Security Services

Chapter 21 Introduction to the Kerberos Service

This chapter introduces the Kerberos Service. The following is a list of the overview information in this chapter.

What Is the Kerberos Service?

The Kerberos service is a client-server architecture that provides secure transactions over networks. The service offers strong user authentication, as well as integrity and privacy. Authentication guarantees that the identities of both the sender and the recipient of a network transaction are true. The service can also verify the validity of data being passed back and forth (integrity) and encrypt the data during transmission (privacy). Using the Kerberos service, you can log in to other machines, execute commands, exchange data, and transfer files securely. Additionally, the service provides authorization services, which allows administrators to restrict access to services and machines. Moreover, as a Kerberos user, you can regulate other people's access to your account.

The Kerberos service is a single-sign-on system, which means that you only need to authenticate yourself to the service once per session, and all subsequent transactions during the session are automatically secured. After the service has authenticated you, you do not need to authenticate yourself every time you use a Kerberos-based command such as ftp or rsh, or to access data on an NFS file system. Thus, you do not have to send your password over the network, where it can be intercepted, each time you use these services.

The Solaris Kerberos service is based on the Kerberos V5 network authentication protocol that was developed at the Massachusetts Institute of Technology (MIT). People who have used Kerberos V5 product should therefore find the Solaris version very familiar. Because the Kerberos V5 protocol is a de facto industry standard for network security, the Solaris version promotes interoperability with other systems. In other words, because the Solaris Kerberos service works with systems that use the Kerberos V5 protocol, the service allows for secure transactions even over heterogeneous networks. Moreover, the service provides authentication and security both between domains and within a single domain.

The Kerberos service allows for flexibility in running Solaris applications. You can configure the service to allow both Kerberos-based and non-Kerberos-based requests for network services such as the NFS service, telnet, and ftp. As a result, current Solaris applications still work even if they are running on systems on which the Kerberos service is not enabled. Of course, you can also configure the Kerberos service to allow only Kerberos-based network requests.

The Kerberos service provides a security mechanism which allows the use of Kerberos for authentication, integrity, and privacy when using applications that use the Generic Security Service Application Programming Interface (GSS-API). However, applications do not have to remain committed to the Kerberos service if other security mechanisms are developed. Because the service is designed to integrate modularly into the GSS-API, applications that use the GSS-API can utilize whichever security mechanism best suits their needs.

How the Kerberos Service Works

The following is an overview of the Kerberos authentication system. For a more detailed description, see How the Kerberos Authentication System Works.

From the user's standpoint, the Kerberos service is mostly invisible after the Kerberos session has been started. Commands such as rsh or ftp work about the same. Initializing a Kerberos session often involves no more than logging in and providing a Kerberos password.

The Kerberos system revolves around the concept of a ticket. A ticket is a set of electronic information that identifies a user or a service such as the NFS service. Just as your driver's license identifies you and indicates what driving privileges you have, so a ticket identifies you and your network access privileges. When you perform a Kerberos-based transaction (for example, if you remote log in to another machine), you transparently send a request for a ticket to a Key Distribution Center, or KDC. The KDC accesses a database to authenticate your identity and returns a ticket that grants you permission to access the other machine. “Transparently” means that you do not need to explicitly request a ticket. The request happens as part of the rlogin command. Because only an authenticated client can get a ticket for a specific service, another client cannot use rlogin under an assumed identity.

Tickets have certain attributes associated with them. For example, a ticket can be forwardable, which means that it can be used on another machine without a new authentication process. A ticket can also be postdated, which means that it is not valid until a specified time. How tickets can be used, for example, to specify which users are allowed to obtain which types of ticket, is set by policies. Policies are determined when the Kerberos service is installed or administered.

Note –

You will frequently see the terms credential and ticket. In the greater Kerberos world, they are often used interchangeably. Technically, however, a credential is a ticket plus the session key for that session. This difference is explained in more detail in Gaining Access to a Service Using Kerberos.

The following sections further explain the Kerberos authentication process.

Initial Authentication: the Ticket-Granting Ticket

Kerberos authentication has two phases: an initial authentication that allows for all subsequent authentications, and the subsequent authentications themselves.

The following figure shows how the initial authentication takes place.

Figure 21–1 Initial Authentication for a Kerberos Session

Flow diagram shows a client requesting a TGT from the
KDC, and then decrypting the TGT that the KDC returns to the client.

  1. A client (a user, or a service such as NFS) begins a Kerberos session by requesting a ticket-granting ticket (TGT) from the Key Distribution Center (KDC). This request is often done automatically at login.

    A ticket-granting ticket is needed to obtain other tickets for specific services. Think of the ticket-granting ticket as similar to a passport. Like a passport, the ticket-granting ticket identifies you and allows you to obtain numerous “visas,” where the “visas” (tickets) are not for foreign countries but for remote machines or network services. Like passports and visas, the ticket-granting ticket and the other various tickets have limited lifetimes. The difference is that “Kerberized” commands notice that you have a passport and obtain the visas for you. You don't have to perform the transactions yourself.

    Another analogy for the ticket-granting ticket is that of a three-day ski pass that is good at four different ski resorts. You show the pass at whichever resort you decide to go to and you receive a lift ticket for that resort, as long as the pass has not expired. Once you have the lift ticket, you can ski all you want at that resort. If you go to another resort the next day, you once again show your pass, and you get an additional lift ticket for the new resort. The difference is that the Kerberos-based commands notice that you have the weekend ski pass, and they get the lift ticket for you. So you don't have to perform the transactions yourself.

  2. The KDC creates a ticket–granting ticket and sends it back, in encrypted form, to the client. The client decrypts the ticket-granting ticket by using the client's password.

  3. Now in possession of a valid ticket-granting ticket, the client can request tickets for all sorts of network operations, such as rlogin or telnet, for as long as the ticket-granting ticket lasts. This ticket usually lasts for a few hours. Each time the client performs a unique network operation, it requests a ticket for that operation from the KDC.

Subsequent Kerberos Authentications

After the client has received the initial authentication, each subsequent authentication follows the pattern that is shown in the following figure.

Figure 21–2 Obtaining Access to a Service Using Kerberos Authentication

Flow diagram shows the client using a TGT to request
a ticket from the KDC, and then using the returned ticket for access to the

  1. The client requests a ticket for a particular service, for example, to remote log in to another machine, from the KDC by sending the KDC its ticket-granting ticket as proof of identity.

  2. The KDC sends the ticket for the specific service to the client.

    For example, suppose user joe wants to access an NFS file system that has been shared with krb5 authentication required. Because he is already authenticated (that is, he already has a ticket-granting ticket), as he attempts to access the files, the NFS client system automatically and transparently obtains a ticket from the KDC for the NFS service.

    For example, suppose the user joe uses rlogin on the server boston. Because he is already authenticated, that is, he already has a ticket-granting ticket, he automatically and transparently obtains a ticket as part of the rlogin command. This ticket allows him to remote log in to boston as often as he wants until the ticket expires. If joe wants to remote log in to the machine denver, he obtains another ticket, as in Step 1.

  3. The client sends the ticket to the server.

    When using the NFS service, the NFS client automatically and transparently sends the ticket for the NFS service to the NFS server.

  4. The server allows the client access.

These steps make it appear that the server doesn't ever communicate with the KDC. The server does, though; it registers itself with the KDC, just as the first client does. For simplicity's sake, that part has been left out.

The Kerberos Remote Applications

The Kerberos-based (or “Kerberized”) commands that a user such as joe can use are the following:

These applications are the same as the Solaris applications of the same name. However, they have been extended to use Kerberos principals to authenticate transactions, thereby providing Kerberos-based security. See Kerberos Principals for information on principals.

These commands are discussed further in Kerberos User Commands.

Kerberos Principals

A client in the Kerberos service is identified by its principal. A principal is a unique identity to which the KDC can assign tickets. A principal can be a user, such as joe, or a service, such as nfs or telnet.

By convention, a principal name is divided into three components: the primary, the instance, and the realm. A typical Kerberos principal would be, for example, joe/admin@ENG.EXAMPLE.COM. In this example:

The following are all valid principal names:

Kerberos Realms

A realm is a logical network, similar to a domain, that defines a group of systems under the same master KDC. Figure 21–3 shows how realms can relate to one another. Some realms are hierarchical, where one realm is a superset of the other realm. Otherwise, the realms are nonhierarchical (or “direct”) and the mapping between the two realms must be defined. A feature of the Kerberos service is that it permits authentication across realms. Each realm only needs to have a principal entry for the other realm in its KDC. This Kerberos feature is called cross-realm authentication.

Figure 21–3 Kerberos Realms

Diagram shows the ENG.EXAMPLE.COM realm in a non-hierarchical
relationship with SEAMCO.COM, and in a hierarchical relationship with EXAMPLE.COM.

Kerberos Servers

Each realm must include a server that maintains the master copy of the principal database. This server is called the master KDC server. Additionally, each realm should contain at least one slave KDC server, which contains duplicate copies of the principal database. Both the master KDC server and the slave KDC server create tickets that are used to establish authentication.

The realm can also include a Kerberos application server. This server provides access to Kerberized services (such as ftp, telnet, rsh and NFS). If you have installed SEAM 1.0 or 1.0.1, the realm might include a Kerberos network application server, but this software was not included with these releases.

The following figure shows what a hypothetical realm might contain.

Figure 21–4 A Typical Kerberos Realm

Diagram shows a typical Kerberos realm, EXAMPLE.COM,
which contains a master KDC, three clients, two slave KDCs, and two application

Kerberos Security Services

In addition to providing secure authentication of users, the Kerberos service provides two security services:

Developers can design their RPC-based applications to choose a security service by using the RPCSEC_GSS programming interface.

The Components of Various Kerberos Releases

Components of the Kerberos service have been included in many releases. Originally, the Kerberos service and changes to the base operating system to support the Kerberos service were released using the product name “Sun Enterprise Authentication Mechanism” which was shortened to SEAM. As more parts of the SEAM product were included in the Solaris software, the contents of the SEAM release decreased. For the Solaris 10 release, all parts of the SEAM product are included, so there is no longer a need for the SEAM product. The SEAM product name exists in the documentation for historical reasons.

The following table describes which components are included in each release. Each product release is listed in chronological order. All components are described in the following sections.

Table 21–1 Kerberos Release Contents

Release Name 


SEAM 1.0 in Solaris Easy Access Server 3.0 

Full release of the Kerberos service for the Solaris 2.6 and 7 releases 

The Kerberos service in the Solaris 8 release 

Kerberos client software only 

SEAM 1.0.1 in the Solaris 8 Admin Pack 

Kerberos KDC and remote applications for the Solaris 8 release 

The Kerberos service in the Solaris 9 release 

Kerberos KDC and client software only 

SEAM 1.0.2 

Kerberos remote applications for the Solaris 9 release 

The Kerberos service in the Solaris 10 release 

Full release of the Kerberos service with enhancements 

Kerberos Components

Similar to the MIT distribution of the Kerberos V5 product, the Solaris Kerberos service includes the following:

In addition, the Solaris Kerberos service includes the following:

Kerberos Additions for the Solaris 10 5/08 Release

These enhancements are available starting in the Solaris 10 5/08 release:

Kerberos Additions for the Solaris 10 8/07 Release

The MIT Kerberos V5 application programming interface (krb5-api) is supported in the Solaris 10 8/07 release. See the libkrb5(3LIB) and krb5-config(1) man pages for more information. Also, see the MIT Kerberos V5 project web pages at for more detailed documentation as it becomes available.

Although the krb5-api is now available, Sun strongly encourages the use of the GSS-API for network authentication and integrity and privacy as the GSS-API is security-mechanism independent and an IETF standard. See the libgss(3LIB) man page for more information.

Kerberos Additions for the Solaris 10 6/06 Release

In the Solaris 10 6/06 release, the ktkt_warnd daemon can automatically renew credentials, rather than just warn the user when the credential is about to expire. The user must be logged in for the credential to be renewed automatically.

Kerberos Enhancements in the Solaris 10 3/05 Release

These Kerberos enhancements are included in the Solaris 10 release. Several of the enhancements were introduced in prior Software Express releases and updated in the Solaris 10 Beta releases.

Kerberos Components in the Solaris 9 Release

The Solaris 9 release includes all components included in Kerberos Components, except for the remote applications.

SEAM 1.0.2 Components

The SEAM 1.0.2 release includes the remote applications. These applications are the only part of SEAM 1.0 that have not been incorporated into the Solaris 9 release. The components for the remote applications are as follows:

Kerberos Components in the Solaris 8 Release

The Solaris 8 release includes only the client-side portions of the Kerberos service, so many components are not included. This product enables systems that run the Solaris 8 release to become Kerberos clients without requiring you to install SEAM 1.0.1 separately. To use these capabilities, you must install a KDC that uses either Solaris Easy Access Server 3.0 or the Solaris 8 Admin Pack, the MIT distribution, or Windows 2000. The client-side components are not useful without a configured KDC to distribute tickets. The following components are included in this release:

SEAM 1.0.1 Components

The SEAM 1.0.1 release includes all components of the SEAM 1.0 release that are not already included in the Solaris 8 release. The components are as follows:

SEAM 1.0 Components

The SEAM 1.0 release includes all of the items included in Kerberos Components as well as the following: