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Sun Enterprise Authentication Mechanism 1.0.1 Guide

How SEAM Works

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

From the user's standpoint, SEAM is mostly invisible after the SEAM session has been started. Commands such as rsh or ftp work pretty much in their usual fashion. Initializing a SEAM session is often no more than logging in and providing a Kerberos password.

The SEAM system revolves around the concept of a ticket. A ticket is a set of electronic information that serves as identification for a user or a service such as the NFS service. Just as your driver's license identifies you and indicates what driving permissions you have, so a ticket identifies you and your network access privileges. When you perform a SEAM-based transaction -- for example, if you rlogin in to another machine -- you transparently send a request for a ticket to a Key Distribution Center, or KDC, which accesses a database to authenticate your identity. The KDC returns a ticket granting you permission to access the other machine. "Transparently" means that you do not need to explicitly request a ticket; it happens as part of the rlogin command. Because only the 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 (meaning that it can be used on another machine without a new authentication process), or postdated (not valid until a specified time). How tickets are used -- for example, which users are allowed to obtain which types of ticket -- is set by policies determined when SEAM 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 SEAM".

The following sections briefly explain the SEAM 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.

Figure 1-1 shows how the initial authentication takes place:

Figure 1-1 Initial Authentication for SEAM Session

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

    A ticket-granting ticket is needed to obtain other tickets for specific services. One analogy is to 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.

  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 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 is usually a few hours. Each time the client performs a unique network operation, it requests a ticket for that operation from the KDC.

Subsequent Authentications

After the client has received the initial authentication, each individual authentication follows the pattern shown in Figure 1-2:

Figure 1-2 Obtaining Access to a Service

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  1. The client requests a ticket for a particular service (say, to rlogin into another machine) from the KDC, 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 the user joe uses rlogin on the server boston. Since 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 rlogin into boston as often as he wants until it expires. If joe wants to rlogin into the machine denver, he obtains another ticket, as in Step 1.

  3. The client sends the ticket to the server.

  4. The server allows the client access.

Looking at these steps, you might have noticed that the server doesn't appear to ever communicate with the KDC. It does, though; it registers itself with the KDC, just as the first client does. For simplicity's sake we have left that part out.

The SEAM-Based Commands

What are the SEAM-based (or "Kerberized") commands that a user such as joe can use? They are:

These applications are the same as the Solaris applications of the same name, except that they use Kerberos principals to authenticate transactions, thereby providing Kerberos-based security. (See "Principals" for information on principals.)

These commands are discussed further in "SEAM Commands".

Principals

A client in SEAM 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 parts: the primary, the instance, and the realm. A typical SEAM principal would be, for example, joe/admin@ENG.ACME.COM, where:

The following are all valid principal names:

Realms

A realm is a logical network, like a domain, which defines a group of systems under the same master KDC (see below). Figure 1-3 shows how realms can relate to one another. Some realms are hierarchical (one being a superset of the other). Otherwise the realms are non-hiearchical and the mapping between the two realms must be defined. A feature of SEAM is that it permits authentication across realms; each realm only needs to have a principal entry for the other realm in its KDC.

Figure 1-3 Realms

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Realms and Servers

Each realm must include a server that maintains the master copy of the principal database. This 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 and the slave KDC servers create tickets used to establish authentication.

The realm can also include two additional types of SEAM servers. A SEAM network application server is a server that provides access to Kerberized applications (such as ftp, telnet and rsh). Realms can also include NFS servers, which provide NFS services, using Kerberos authentication.

Figure 1-4 shows what a hypothetical realm might contain.

Figure 1-4 A Typical Realm

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