# Kerberos V5 UNIX User's Guide

## Copyright

Copyright © 1985-2002 by the Massachusetts Institute of Technology.

> Export of software employing encryption from the United States of America may require a specific license from the United States Government. It is the responsibility of any person or organization contemplating export to obtain such a license before exporting.

WITHIN THAT CONSTRAINT, permission to use, copy, modify, and distribute this software and its documentation for any purpose and without fee is hereby granted, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation, and that the name of M.I.T. not be used in advertising or publicity pertaining to distribution of the software without specific, written prior permission. Furthermore if you modify this software you must label your software as modified software and not distribute it in such a fashion that it might be confused with the original MIT software. M.I.T. makes no representations about the suitability of this software for any purpose. It is provided "as is" without express or implied warranty.

The following copyright and permission notice applies to the OpenVision Kerberos Administration system located in kadmin/create, kadmin/dbutil, kadmin/passwd, kadmin/server, lib/kadm5, and portions of lib/rpc:

> Copyright, OpenVision Technologies, Inc., 1996, All Rights Reserved

> WARNING: Retrieving the OpenVision Kerberos Administration system source code, as described below, indicates your acceptance of the following terms. If you do not agree to the following terms, do not retrieve the OpenVision Kerberos administration system.

> You may freely use and distribute the Source Code and Object Code compiled from it, with or without modification, but this Source Code is provided to you "AS IS" EXCLUSIVE OF ANY WARRANTY, INCLUDING, WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, OR ANY OTHER WARRANTY, WHETHER EXPRESS OR IMPLIED. IN NO EVENT WILL OPENVISION HAVE ANY LIABILITY FOR ANY LOST PROFITS, LOSS OF DATA OR COSTS OF PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES, OR FOR ANY SPECIAL, INDIRECT, OR CONSEQUENTIAL DAMAGES ARISING OUT OF THIS AGREEMENT, INCLUDING, WITHOUT LIMITATION, THOSE RESULTING FROM THE USE OF THE SOURCE CODE, OR THE FAILURE OF THE SOURCE CODE TO PERFORM, OR FOR ANY OTHER REASON.

> OpenVision retains all copyrights in the donated Source Code. OpenVision also retains copyright to derivative works of the Source Code, whether created by OpenVision or by a third party. The OpenVision copyright notice must be preserved if derivative works are made based on the donated Source Code.

> OpenVision Technologies, Inc. has donated this Kerberos Administration system to MIT for inclusion in the standard Kerberos 5 distribution. This donation underscores our commitment to continuing Kerberos technology development and our gratitude for the valuable work which has been performed by MIT and the Kerberos community.

The implementation of the Yarrow pseudo-random number generator in src/lib/crypto/yarrow has the following copyright:

> Copyright 2000 by Zero-Knowledge Systems, Inc.

> Permission to use, copy, modify, distribute, and sell this software and its documentation for any purpose is hereby granted without fee, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation, and that the name of Zero-Knowledge Systems, Inc. not be used in advertising or publicity pertaining to distribution of the software without specific, written prior permission. Zero-Knowledge Systems, Inc. makes no representations about the suitability of this software for any purpose. It is provided "as is" without express or implied warranty.

> ZERO-KNOWLEDGE SYSTEMS, INC. DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO EVENT SHALL ZERO-KNOWLEDGE SYSTEMS, INC. BE LIABLE FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTUOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

The implementation of the AES encryption algorithm in src/lib/crypto/aes has the following copyright:

> Copyright (c) 2001, Dr Brian Gladman <brg@gladman.uk.net>, Worcester, UK. All rights reserved.

> LICENSE TERMS

> The free distribution and use of this software in both source and binary form is allowed (with or without changes) provided that:

> 1. distributions of this source code include the above copyright notice, this list of conditions and the following disclaimer;
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> DISCLAIMER

> This software is provided 'as is' with no explcit or implied warranties in respect of any properties, including, but not limited to, correctness and fitness for purpose.

Kerberos V5 includes documentation and software developed at the University of California at Berkeley, which includes this copyright notice:

Copyright © 1983 Regents of the University of California.
All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
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   > This product includes software developed by the University of California, Berkeley and its contributors.

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## Introduction

Kerberos V5 is an authentication system developed at MIT. Kerberos is named for the three-headed watchdog from Greek mythology, who guarded the entrance to the underworld.

Under Kerberos, a client (generally either a user or a service) sends a request for a ticket to the Key Distribution Center (KDC). The KDC creates a ticket-granting ticket (TGT) for the client, encrypts it using the client's password as the key, and sends the encrypted TGT back to the client. The client then attempts to decrypt the TGT, using its password. If the client successfully decrypts the TGT (i.e., if the client gave the correct password), it keeps the decrypted TGT, which indicates proof of the client's identity.

The TGT, which expires at a specified time, permits the client to obtain additional tickets, which give permission for specific services. The requesting and granting of these additional tickets is user-transparent.

Since Kerberos negotiates authenticated, and optionally encrypted, communications between two points anywhere on the internet, it provides a layer of security that is not dependent on which side of a firewall either client is on. Since studies have shown that half of the computer security breaches in industry happen from inside firewalls, MIT's Kerberos V5 plays a vital role in maintaining your network security.

The Kerberos V5 package is designed to be easy to use. Most of the commands are nearly identical to UNIX network programs you already use. Kerberos V5 is a single-sign-on system, which means that you have to type your password only once per session, and Kerberos does the authenticating and encrypting transparently.

### What is a Ticket?

Your Kerberos credentials, or "tickets", are a set of electronic information that can be used to verify your identity. Your Kerberos tickets may be stored in a file, or they may exist only in memory.

The first ticket you obtain is a ticket-granting ticket, which permits you to obtain additional tickets. These additional tickets give you permission for specific services. The requesting and granting of these additional tickets happens transparently.

A good analogy for the ticket-granting ticket is a three-day ski pass that is good at four different resorts. You show the pass at whichever resort you decide to go to (until it expires), and you receive a lift ticket for that resort. 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 V5 programs notice that you have the weekend ski pass, and get the lift ticket for you, so you don't have to perform the transactions yourself.

### What is a Kerberos Principal?

A Kerberos principal is a unique identity to which Kerberos can assign tickets. Principals can have an arbitrary number of components. Each component is separated by a component separator, generally `/'. The last component is the realm, separated from the rest of the principal by the realm separator, generally `@'. If there is no realm component in the principal, then it will be assumed that the principal is in the default realm for the context in which it is being used.

Traditionally, a principal is divided into three parts: the primary, the instance, and the realm. The format of a typical Kerberos V5 principal is `primary/instance@REALM`.

- The primary is the first part of the principal. In the case of a user, it's the same as your username. For a host, the primary is the word `host`.
- The instance is an optional string that qualifies the primary. The instance is separated from the primary by a slash (`/`). In the case of a user, the instance is usually null, but a user might also have an additional principal, with an instance called `admin`, which he/she uses to administrate a database. The principal `jennifer@ATHENA.MIT.EDU` is completely separate from the principal `jennifer/admin@ATHENA.MIT.EDU`, with a separate password, and separate permissions. In the case of a host, the instance is the fully qualified hostname, e.g., `daffodil.mit.edu`.
- The realm is your Kerberos realm. In most cases, your Kerberos realm is your domain name, in upper-case letters. For example, the machine `daffodil.example.com` would be in the realm `EXAMPLE.COM`.

## Kerberos V5 Tutorial

This tutorial is intended to familiarize you with the Kerberos V5 client programs. We will represent your prompt as "`shell%`". So an instruction to type the "`ls`" command would be represented as follows:

         shell% ls

In these examples, we will use sample usernames, such as `jennifer` and `david`, sample hostnames, such as `daffodil` and `trillium`, and sample domain names, such as `mit.edu` and `example.com`. When you see one of these, substitute your username, hostname, or domain name accordingly.

### Setting Up to Use Kerberos V5

Your system administrator will have installed the Kerberos V5 programs in whichever directory makes the most sense for your system. We will use `/usr/local` throughout this guide to refer to the top-level directory Kerberos V5 directory. We will therefor use `/usr/local/bin` to denote the location of the Kerberos V5 user programs. In your installation, the directory name may be different, but whatever the directory name is, you should make sure it is included in your path. You will probably want to put it ahead of the directories `/bin` and `/usr/bin` so you will get the Kerberos V5 network programs, rather than the standard UNIX versions, when you type their command names.

### Ticket Management

On many systems, Kerberos is built into the login program, and you get tickets automatically when you log in. Other programs, such as `rsh`, `rcp`, `telnet`, and `rlogin`, can forward copies of your tickets to the remote host. Most of these programs also automatically destroy your tickets when they exit. However, MIT recommends that you explicitly destroy your Kerberos tickets when you are through with them, just to be sure. One way to help ensure that this happens is to add the `kdestroy` command to your `.logout` file. Additionally, if you are going to be away from your machine and are concerned about an intruder using your permissions, it is safest to either destroy all copies of your tickets, or use a screensaver that locks the screen.

### Kerberos Ticket Properties

There are various properties that Kerberos tickets can have:

If a ticket is forwardable, then the KDC can issue a new ticket with a different network address based on the forwardable ticket. This allows for authentication forwarding without requiring a password to be typed in again. For example, if a user with a forwardable TGT logs into a remote system, the KDC could issue a new TGT for that user with the netword address of the remote system, allowing authentication on that host to work as though the user were logged in locally.

When the KDC creates a new ticket based on a forwardable ticket, it sets the forwarded flag on that new ticket. Any tickets that are created based on a ticket with the forwarded flag set will also have their forwarded flags set.

A proxiable ticket is similar to a forwardable ticket in that it allows a service to take on the identity of the client. Unlike a forwardable ticket, however, a proxiable ticket is only issued for specific services. In other words, a ticket-granting ticket cannot be issued based on a ticket that is proxiable but not forwardable.

A proxy ticket is one that was issued based on a proxiable ticket.

A postdated ticket is issued with the invalid flag set. After the starting time listed on the ticket, it can be presented to the KDC to obtain valid tickets.

Tickets with the postdateable flag set can be used to issue postdated tickets.

Renewable tickets can be used to obtain new session keys without the user entering their password again. A renewable ticket has two expiration times. The first is the time at which this particular ticket expires. The second is the latest possible expiration time for any ticket issued based on this renewable ticket.

A ticket with the initial flag set was issued based on the authentication protocol, and not on a ticket-granting ticket. Clients that wish to ensure that the user's key has been recently presented for verification could specify that this flag must be set to accept the ticket.

An invalid ticket must be rejected by application servers. Postdated tickets are usually issued with this flag set, and must be validated by the KDC before they can be used.

A preauthenticated ticket is one that was only issued after the client requesting the ticket had authenticated itself to the KDC.

The hardware authentication flag is set on a ticket which required the use of hardware for authentication. The hardware is expected to be possessed only by the client which requested the tickets.

If a ticket has the transit policy checked flag set, then the KDC that issued this ticket implements the transited-realm check policy and checked the transited-realms list on the ticket. The transited-realms list contains a list of all intermediate realms between the realm of the KDC that issued the first ticket and that of the one that issued the current ticket. If this flag is not set, then the application server must check the transited realms itself or else reject the ticket.

The okay as delegate flag indicates that the server specified in the ticket is suitable as a delegate as determined by the policy of that realm. A server that is acting as a delegate has been granted a proxy or a forwarded TGT. This flag is a new addition to the Kerberos V5 protocol and is not yet implemented on MIT servers.

An anonymous ticket is one in which the named principal is a generic principal for that realm; it does not actually specify the individual that will be using the ticket. This ticket is meant only to securely distribute a session key. This is a new addition to the Kerberos V5 protocol and is not yet implemented on MIT servers.

### Obtaining Tickets with kinit

If your site is using the Kerberos V5 login program, you will get Kerberos tickets automatically when you log in. If your site uses a different login program, you may need to explicitly obtain your Kerberos tickets, using the `kinit` program. Similarly, if your Kerberos tickets expire, use the `kinit` program to obtain new ones.

To use the `kinit` program, simply type `kinit` and then type your password at the prompt. For example, Jennifer (whose username is `jennifer`) works for Bleep, Inc. (a fictitious company with the domain name `mit.edu` and the Kerberos realm `ATHENA.MIT.EDU`). She would type:

         shell% kinit
         Password for jennifer@ATHENA.MIT.EDU: <-- [Type jennifer's password here.]
         shell%

If you type your password incorrectly, kinit will give you the following error message:

         shell% kinit
         Password for jennifer@ATHENA.MIT.EDU: <-- [Type the wrong password here.]
         kinit: Password incorrect
         shell%

and you won't get Kerberos tickets.

Notice that `kinit` assumes you want tickets for your own username in your default realm. Suppose Jennifer's friend David is visiting, and he wants to borrow a window to check his mail. David needs to get tickets for himself in his own realm, EXAMPLE.COM.1 He would type:

         shell% kinit david@EXAMPLE.COM
         Password for david@EXAMPLE.COM: <-- [Type david's password here.]
         shell%

David would then have tickets which he could use to log onto his own machine. Note that he typed his password locally on Jennifer's machine, but it never went over the network. Kerberos on the local host performed the authentication to the KDC in the other realm.

If you want to be able to forward your tickets to another host, you need to request forwardable tickets. You do this by specifying the `-f` option:

         shell% kinit -f
         Password for jennifer@ATHENA.MIT.EDU: <-- [Type your password here.]
         shell%

Note that `kinit` does not tell you that it obtained forwardable tickets; you can verify this using the `klist` command (see Viewing Your Tickets with klist).

Normally, your tickets are good for your system's default ticket lifetime, which is ten hours on many systems. You can specify a different ticket lifetime with the `-l` option. Add the letter `s` to the value for seconds, `m` for minutes, `h` for hours, or `d` for days. For example, to obtain forwardable tickets for `david@EXAMPLE.COM` that would be good for three hours, you would type:

         shell% kinit -f -l 3h david@EXAMPLE.COM
         Password for david@EXAMPLE.COM: <-- [Type david's password here.]
         shell%

You cannot mix units; specifying a lifetime of `3h30m` would result in an error. Note also that most systems specify a maximum ticket lifetime. If you request a longer ticket lifetime, it will be automatically truncated to the maximum lifetime.

### Viewing Your Tickets with klist

The `klist` command shows your tickets. When you first obtain tickets, you will have only the ticket-granting ticket. (See What is a Ticket?.) The listing would look like this:

         shell% klist
         Ticket cache: /tmp/krb5cc_ttypa
         Default principal: jennifer@ATHENA.MIT.EDU

         Valid starting     Expires            Service principal
         06/07/04 19:49:21  06/08/04 05:49:19  krbtgt/ATHENA.MIT.EDU@ATHENA.MIT.EDU
         shell%

The ticket cache is the location of your ticket file. In the above example, this file is named `/tmp/krb5cc_ttypa`. The default principal is your kerberos principal. (see What is a Kerberos Principal?)

The "valid starting" and "expires" fields describe the period of time during which the ticket is valid. The service principal describes each ticket. The ticket-granting ticket has the primary `krbtgt`, and the instance is the realm name.

Now, if jennifer connected to the machine `daffodil.mit.edu`, and then typed `klist` again, she would have gotten the following result:

         shell% klist
         Ticket cache: /tmp/krb5cc_ttypa
         Default principal: jennifer@ATHENA.MIT.EDU

         Valid starting     Expires            Service principal
         06/07/04 19:49:21  06/08/04 05:49:19  krbtgt/ATHENA.MIT.EDU@ATHENA.MIT.EDU
         06/07/04 20:22:30  06/08/04 05:49:19  host/daffodil.mit.edu@ATHENA.MIT.EDU
         shell%

Here's what happened: when jennifer used telnet to connect to the host `daffodil.mit.edu`, the telnet program presented her ticket-granting ticket to the KDC and requested a host ticket for the host `daffodil.mit.edu`. The KDC sent the host ticket, which telnet then presented to the host `daffodil.mit.edu`, and she was allowed to log in without typing her password.

Suppose your Kerberos tickets allow you to log into a host in another domain, such as `trillium.example.com`, which is also in another Kerberos realm, `EXAMPLE.COM`. If you telnet to this host, you will receive a ticket-granting ticket for the realm `EXAMPLE.COM`, plus the new `host` ticket for `trillium.example.com`. `klist` will now show:

         shell% klist
         Ticket cache: /tmp/krb5cc_ttypa
         Default principal: jennifer@ATHENA.MIT.EDU

         Valid starting     Expires            Service principal
         06/07/04 19:49:21  06/08/04 05:49:19  krbtgt/ATHENA.MIT.EDU@ATHENA.MIT.EDU
         06/07/04 20:22:30  06/08/04 05:49:19  host/daffodil.mit.edu@ATHENA.MIT.EDU
         06/07/04 20:24:18  06/08/04 05:49:19  krbtgt/EXAMPLE.COM@ATHENA.MIT.EDU
         06/07/04 20:24:18  06/08/04 05:49:19  host/trillium.example.com@ATHENA.MIT.EDU
         shell%

You can use the `-f` option to view the flags that apply to your tickets. The flags are:

F
  Forwardable
f
  forwarded
P
  Proxiable
p
  proxy
D
  postDateable
d
  postdated
R
  Renewable
I
  Initial
i
  invalid
H
  Hardware authenticated
A
  preAuthenticated
T
  Transit policy checked
O
  Okay as delegate
a
  anonymous

Here is a sample listing. In this example, the user jennifer obtained her initial tickets (`I`), which are forwardable (`F`) and postdated (`d`) but not yet validated (`i`). (See kinit Reference, for more information about postdated tickets.)

         shell% klist -f
         Ticket cache: /tmp/krb5cc_320
         Default principal: jennifer@ATHENA.MIT.EDU

         Valid starting      Expires             Service principal
         31/07/05 19:06:25  31/07/05 19:16:25  krbtgt/ATHENA.MIT.EDU@ATHENA.MIT.EDU
                 Flags: FdiI
         shell%

In the following example, the user david's tickets were forwarded (`f`) to this host from another host. The tickets are reforwardable (`F`).

         shell% klist -f
         Ticket cache: /tmp/krb5cc_p11795
         Default principal: david@EXAMPLE.COM

         Valid starting     Expires            Service principal
         07/31/05 11:52:29  07/31/05 21:11:23  krbtgt/EXAMPLE.COM@EXAMPLE.COM
                 Flags: Ff
         07/31/05 12:03:48  07/31/05 21:11:23  host/trillium.example.com@EXAMPLE.COM
                 Flags: Ff
         shell%

### Destroying Your Tickets with kdestroy

Your Kerberos tickets are proof that you are indeed yourself, and tickets can be stolen. If this happens, the person who has them can masquerade as you until they expire. For this reason, you should destroy your Kerberos tickets when you are away from your computer.

Destroying your tickets is easy. Simply type `kdestroy`.

         shell% kdestroy
         shell%

If `kdestroy` fails to destroy your tickets, it will beep and give an error message. For example, if `kdestroy` can't find any tickets to destroy, it will give the following message:

         shell% kdestroy
         kdestroy: No credentials cache file found while destroying cache
         shell%

### Password Management

Your password is the only way Kerberos has of verifying your identity. If someone finds out your password, that person can masquerade as you--send email that comes from you, read, edit, or delete your files, or log into other hosts as you--and no one will be able to tell the difference. For this reason, it is important that you choose a good password (see Password Advice), and keep it secret. If you need to give access to your account to someone else, you can do so through Kerberos. (See Granting Access to Your Account.) You should never tell your password to anyone, including your system administrator, for any reason. You should change your password frequently, particularly any time you think someone may have found out what it is.

### Changing Your Password

To change your Kerberos password, use the `kpasswd` command. It will ask you for your old password (to prevent someone else from walking up to your computer when you're not there and changing your password), and then prompt you for the new one twice. (The reason you have to type it twice is to make sure you have typed it correctly.) For example, user `david` would do the following:

         shell% kpasswd
         Password for david:    <- Type your old password.
         Enter new password:    <- Type your new password.
         Enter it again:  <- Type the new password again.
         Password changed.
         shell%

If david typed the incorrect old password, he would get the following message:

         shell% kpasswd
         Password for david:  <- Type the incorrect old password.
         kpasswd: Password incorrect while getting initial ticket
         shell%

If you make a mistake and don't type the new password the same way twice, `kpasswd` will ask you to try again:

         shell% kpasswd
         Password for david:  <- Type the old password.
         Enter new password:  <- Type the new password.
         Enter it again: <- Type a different new password.
         kpasswd: Password mismatch while reading password
         shell%

Once you change your password, it takes some time for the change to propagate through the system. Depending on how your system is set up, this might be anywhere from a few minutes to an hour or more. If you need to get new Kerberos tickets shortly after changing your password, try the new password. If the new password doesn't work, try again using the old one.

### Password Advice

Your password can include almost any character you can type (except control keys and the "enter" key). A good password is one you can remember, but that no one else can easily guess. Examples of bad passwords are words that can be found in a dictionary, any common or popular name, especially a famous person (or cartoon character), your name or username in any form (e.g., forward, backward, repeated twice, etc.), your spouse's, child's, or pet's name, your birth date, your social security number, and any sample password that appears in this (or any other) manual. MIT recommends that your password be at least 6 characters long, and contain UPPER- and lower-case letters, numbers, and/or punctuation marks. Some passwords that would be good if they weren't listed in this manual include:

- some initials, like "GykoR-66." for "Get your kicks on Route 66."
- an easy-to-pronounce nonsense word, like "slaRooBey" or "krang-its"
- a misspelled phrase, like "2HotPeetzas!" or "ItzAGurl!!!"

Note: don't actually use any of the above passwords. They're only meant to show you how to make up a good password. Passwords that appear in a manual are the first ones intruders will try. Kerberos V5 allows your system administrators to automatically reject bad passwords, based on certain criteria, such as a password dictionary or a minimum length. For example, if the user `jennifer`, who had a policy "strict" that required a minimum of 8 characaters, chose a password that was less than 8 characters, Kerberos would give an error message like the following:

         shell% kpasswd
         Password for jennifer:  <- Type your old password here.

         jennifer's password is controlled by the policy strict, which
         requires a minimum of 8 characters from at least 3 classes (the five classes
         are lowercase, uppercase, numbers, punctuation, and all other characters).

         Enter new password:  <- Type an insecure new password.
         Enter it again:  <- Type it again.

         kpasswd: Password is too short while attempting to change password.
         Please choose another password.

         Enter new password:  <- Type a good password here.
         Enter it again:  <- Type it again.
         Password changed.
         shell%

Your system administrators can choose the message that is displayed if you choose a bad password, so the message you see may be different from the above example.

### Granting Access to Your Account

If you need to give someone access to log into your account, you can do so through Kerberos, without telling the person your password. Simply create a file called `.k5login` in your home directory. This file should contain the Kerberos principal (See What is a Kerberos Principal?.) of each person to whom you wish to give access. Each principal must be on a separate line. Here is a sample `.k5login` file:

         jennifer@ATHENA.MIT.EDU
         david@EXAMPLE.COM

This file would allow the users `jennifer` and `david` to use your user ID, provided that they had Kerberos tickets in their respective realms. If you will be logging into other hosts across a network, you will want to include your own Kerberos principal in your `.k5login` file on each of these hosts.

Using a `.k5login` file is much safer than giving out your password, because:

- You can take access away any time simply by removing the principal from your `.k5login` file.
- Although the user has full access to your account on one particular host (or set of hosts if your `.k5login` file is shared, e.g., over NFS), that user does not inherit your network privileges.
- Kerberos keeps a log of who obtains tickets, so a system administrator could find out, if necessary, who was capable of using your user ID at a particular time.

One common application is to have a `.k5login` file in `root`'s home directory, giving root access to that machine to the Kerberos principals listed. This allows system administrators to allow users to become root locally, or to log in remotely as `root`, without their having to give out the root password, and without anyone having to type the root password over the network.

### Kerberos V5 Applications

Kerberos V5 is a single-sign-on system. This means that you only have to type your password once, and the Kerberos V5 programs do the authenticating (and optionally encrypting) for you. The way this works is that Kerberos has been built into each of a suite of network programs. For example, when you use a Kerberos V5 program to connect to a remote host, the program, the KDC, and the remote host perform a set of rapid negotiations. When these negotiations are completed, your program has proven your identity on your behalf to the remote host, and the remote host has granted you access, all in the space of a few seconds.

The Kerberos V5 applications are versions of existing UNIX network programs with the Kerberos features added.

### Overview of Additional Features

The Kerberos V5 network programs are those programs that connect to another host somewhere on the internet. These programs include `rlogin`, `telnet`, `ftp`, `rsh`, `rcp`, and `ksu`. These programs have all of the original features of the corresponding non-Kerberos `rlogin`, `telnet`, `ftp`, `rsh`, `rcp`, and `su` programs, plus additional features that transparently use your Kerberos tickets for negotiating authentication and optional encryption with the remote host. In most cases, all you'll notice is that you no longer have to type your password, because Kerberos has already proven your identity.

The Kerberos V5 network programs allow you the options of forwarding your tickets to the remote host (if you obtained forwardable tickets with the `kinit` program; see Obtaining Tickets with kinit), and encrypting data transmitted between you and the remote host.

This section of the tutorial assumes you are familiar with the non-Kerberos versions of these programs, and highlights the Kerberos functions added in the Kerberos V5 package.

### telnet

The Kerberos V5 `telnet` command works exactly like the standard UNIX telnet program, with the following Kerberos options added:

`-f`
  forwards a copy of your tickets to the remote host.
`-F`
  forwards a copy of your tickets to the remote host, and marks them re-forwardable from the remote host.
`-k` realm``
  requests tickets for the remote host in the specified realm, instead of determining the realm itself.
`-K`
  uses your tickets to authenticate to the remote host, but does not log you in.
`-a`
  attempt automatic login using your tickets. `telnet` will assume the same username unless you explicitly specify another.
`-x`
  turns on encryption.

For example, if `david` wanted to use the standard UNIX telnet to connect to the machine `daffodil.mit.edu`, he would type:

         shell% telnet daffodil.example.com
         Trying 128.0.0.5 ...
         Connected to daffodil.example.com.
         Escape character is '^]'.

         NetBSD/i386 (daffodil) (ttyp3)

         login: david
         Password:    <- david types his password here
         Last login: Fri Jun 21 17:13:11 from trillium.mit.edu
         Copyright (c) 1980, 1983, 1986, 1988, 1990, 1991, 1993, 1994
                 The Regents of the University of California.   All rights reserved.

         NetBSD 1.1: Tue May 21 00:31:42 EDT 1996

         Welcome to NetBSD!
         shell%

Note that the machine `daffodil.example.com` asked for `david`'s password. When he typed it, his password was sent over the network unencrypted. If an intruder were watching network traffic at the time, that intruder would know `david`'s password.

If, on the other hand, `jennifer` wanted to use the Kerberos V5 telnet to connect to the machine `trillium.mit.edu`, she could forward a copy of her tickets, request an encrypted session, and log on as herself as follows:

         shell% telnet -a -f -x trillium.mit.edu
         Trying 128.0.0.5...
         Connected to trillium.mit.edu.
         Escape character is '^]'.
         [ Kerberos V5 accepts you as ``jennifer@mit.edu'' ]
         [ Kerberos V5 accepted forwarded credentials ]
         What you type is protected by encryption.
         Last login: Tue Jul 30 18:47:44 from {No value for `RANDOMHOST'}.example.com
         Athena Server (sun4) Version 9.1.11 Tue Jul 30 14:40:08 EDT 2002

         shell%

Note that `jennifer`'s machine used Kerberos to authenticate her to `trillium.mit.edu`, and logged her in automatically as herself. She had an encrypted session, a copy of her tickets already waiting for her, and she never typed her password.

If you forwarded your Kerberos tickets, `telnet` automatically destroys them when it exits. The full set of options to Kerberos V5 `telnet` are discussed in the Reference section of this manual. (see telnet Reference)

### rlogin

The Kerberos V5 `rlogin` command works exactly like the standard UNIX rlogin program, with the following Kerberos options added:

`-f`
  forwards a copy of your tickets to the remote host.
`-F`
  forwards a copy of your tickets to the remote host, and marks them re-forwardable from the remote host.
`-k` realm``
  requests tickets for the remote host in the specified realm, instead of determining the realm itself.
`-x`
  encrypts the input and output data streams (the username is sent unencrypted)

For example, if `david` wanted to use the standard UNIX rlogin to connect to the machine `daffodil.example.com`, he would type:

         shell% rlogin daffodil.example.com -l david
         Password:  <- david types his password here
         Last login: Fri Jun 21 10:36:32 from :0.0
         Copyright (c) 1980, 1983, 1986, 1988, 1990, 1991, 1993, 1994
                 The Regents of the University of California.   All rights reserved.

         NetBSD 1.1: Tue May 21 00:31:42 EDT 1996

         Welcome to NetBSD!
         shell%

Note that the machine `daffodil.example.com` asked for `david`'s password. When he typed it, his password was sent over the network unencrypted. If an intruder were watching network traffic at the time, that intruder would know `david`'s password.

If, on the other hand, `jennifer` wanted to use Kerberos V5 rlogin to connect to the machine `trillium.mit.edu`, she could forward a copy of her tickets, mark them as not forwardable from the remote host, and request an encrypted session as follows:

         shell% rlogin trillium.mit.edu -f -x
         This rlogin session is using DES encryption for all data transmissions.
         Last login: Thu Jun 20 16:20:50 from daffodil
         Athena Server (sun4) Version 9.1.11 Tue Jul 30 14:40:08 EDT 2002
         shell%

Note that `jennifer`'s machine used Kerberos to authenticate her to `trillium.mit.edu`, and logged her in automatically as herself. She had an encrypted session, a copy of her tickets were waiting for her, and she never typed her password.

If you forwarded your Kerberos tickets, `rlogin` automatically destroys them when it exits. The full set of options to Kerberos V5 `rlogin` are discussed in the Reference section of this manual. (see rlogin Reference)

### FTP

The Kerberos V5 `FTP` program works exactly like the standard UNIX FTP program, with the following Kerberos features added:

`-k` realm``
  requests tickets for the remote host in the specified realm, instead of determining the realm itself.
`-f`
  requests that your tickets be forwarded to the remote host. The `-f` argument must be the last argument on the command line.
`protect` level``
  (issued at the `ftp>` prompt) sets the protection level. "Clear" is no protection; "safe" ensures data integrity by verifying the checksum, and "private" encrypts the data. Encryption also ensures data integrity.

For example, suppose `jennifer` wants to get her `RMAIL` file from the directory `~jennifer/Mail`, on the host `daffodil.mit.edu`. She wants to encrypt the file transfer. The exchange would look like the following:

         shell% ftp daffodil.mit.edu
         Connected to daffodil.mit.edu.
         220 daffodil.mit.edu FTP server (Version 5.60) ready.
         334 Using authentication type GSSAPI; ADAT must follow
         GSSAPI accepted as authentication type
         GSSAPI authentication succeeded
         200 Data channel protection level set to private.
         Name (daffodil.mit.edu:jennifer):
         232 GSSAPI user jennifer@ATHENA.MIT.EDU is authorized as jennifer
         230 User jennifer logged in.
         Remote system type is UNIX.
         Using binary mode to transfer files.
         ftp> protect private
         200 Protection level set to Private.
         ftp> cd ~jennifer/MAIL
         250 CWD command successful.
         ftp> get RMAIL
         227 Entering Passive Mode (128,0,0,5,16,49)
         150 Opening BINARY mode data connection for RMAIL (361662 bytes).
         226 Transfer complete.
         361662 bytes received in 2.5 seconds (1.4e+02 Kbytes/s)
         ftp> quit
         shell%

The full set of options to Kerberos V5 `FTP` are discussed in the Reference section of this manual. (see FTP Reference)

### rsh

The Kerberos V5 `rsh` program works exactly like the standard UNIX rlogin program, with the following Kerberos features added:

`-f`
  forwards a copy of your tickets to the remote host.
`-F`
  forwards a copy of your tickets to the remote host, and marks them re-forwardable from the remote host.
`-k` realm``
  requests tickets for the remote host in the specified realm, instead of determining the realm itself.
`-x`
  encrypts the input and output data streams (the command line is not encrypted)

For example, if your Kerberos tickets allowed you to run programs on the host
`trillium@example.com` as root, you could run the `date` program as follows:

         shell% rsh trillium.example.com -l root -x date
         This rsh session is using DES encryption for all data transmissions.
         Tue Jul 30 19:34:21 EDT 2002
         shell%

If you forwarded your Kerberos tickets, `rsh` automatically destroys them when it exits. The full set of options to Kerberos V5 `rsh` are discussed in the Reference section of this manual. (see rsh Reference)

### rcp

The Kerberos V5 `rcp` program works exactly like the standard UNIX rcp program, with the following Kerberos features added:

`-k` realm``
  requests tickets for the remote host in the specified realm, instead of determining the realm itself.
`-x`
  turns on encryption.

For example, if you wanted to copy the file `/etc/motd` from the host `daffodil.mit.edu` into the current directory, via an encrypted connection, you would simply type:

         shell% rcp -x daffodil.mit.edu:/etc/motd .

The `rcp` program negotiates authentication and encryption transparently. The full set of options to Kerberos V5 `rcp` are discussed in the Reference section of this manual. (see rcp Reference)

### ksu

The Kerberos V5 `ksu` program replaces the standard UNIX su program. `ksu` first authenticates you to Kerberos. Depending on the configuration of your system, `ksu` may ask for your Kerberos password if authentication fails. Note that you should never type your password if you are remotely logged in using an unencrypted connection.

Once `ksu` has authenticated you, if your Kerberos principal appears in the target's `.k5login` file (see Granting Access to Your Account) or in the target's `.k5users` file (see below), it switches your user ID to the target user ID.

For example, `david` has put `jennifer`'s Kerberos principal in his `.k5login` file. If `jennifer` uses `ksu` to become `david`, the exchange would look like this. (To differentiate between the two shells, `jennifer`'s prompt is represented as `jennifer%` and `david`'s prompt is represented as `david%`.)

         jennifer% ksu david
         Account david: authorization for jennifer@ATHENA.MIT.EDU successful
         Changing uid to david (3382)
         david%

Note that the new shell has a copy of `jennifer`'s tickets. The ticket filename contains `david`'s UID with `.1` appended to it:

         david% klist
         Ticket cache: /tmp/krb5cc_3382.1
         Default principal: jennifer@ATHENA.MIT.EDU

         Valid starting      Expires             Service principal
         07/31/04 21:53:01  08/01/04 07:52:53  krbtgt/ATHENA.MIT.EDU@ATHENA.MIT.EDU
         07/31/04 21:53:39  08/01/04 07:52:53  host/daffodil.mit.edu@ATHENA.MIT.EDU
         david%

If `jennifer` had not appeared in `david`'s `.k5login` file (and the system was configured to ask for a password), the exchange would have looked like this (assuming `david` has taken appropriate precautions in protecting his password):

         jennifer% ksu david
         WARNING: Your password may be exposed if you enter it here and are logged
                  in remotely using an unsecure (non-encrypted) channel.
         Kerberos password for david@ATHENA.MIT.EDU:  <-  jennifer types the wrong password here.
         ksu: Password incorrect
         Authentication failed.
         jennifer%

Now, suppose `david` did not want to give `jennifer` full access to his account, but wanted to give her permission to list his files and use the "more" command to view them. He could create a `.k5users` file giving her permission to run only those specific commands.

The `.k5users` file is like the `.k5login` file, except that each principal is optionally followed by a list of commands. `ksu` will let those principals execute only the commands listed, using the `-e` option. `david`'s `.k5users` file might look like the following:

         jennifer@ATHENA.MIT.EDU       /bin/ls /usr/bin/more
         joeadmin@ATHENA.MIT.EDU         /bin/ls
         joeadmin/admin@ATHENA.MIT.EDU   *
         david@EXAMPLE.COM

The above `.k5users` file would let `jennifer` run only the commands `/bin/ls` and `/usr/bin/more`. It would let `joeadmin` run only the command `/bin/ls` if he had regular tickets, but if he had tickets for his `admin` instance, `joeadmin/admin@ATHENA.MIT.EDU`, he would be able to execute any command. The last line gives `david` in the realm EXAMPLE.COM permission to execute any command. (I.e., having only a Kerberos principal on a line is equivalent to giving that principal permission to execute `*`.) This is so that david can allow himself to execute commands when he logs in, using Kerberos, from a machine in the realm EXAMPLE.COM.

Then, when `jennifer` wanted to list his home directory, she would type:

         jennifer% ksu david -e ls ~david
         Authenticated jennifer@ATHENA.MIT.EDU
         Account david: authorization for jennifer@ATHENA.MIT.EDU for execution of
                        /bin/ls successful
         Changing uid to david (3382)
         Mail            News            Personal        misc            bin
         jennifer%

If `jennifer` had tried to give a different command to `ksu`, it would have prompted for a password as with the previous example.

Note that unless the `.k5users` file gives the target permission to run any command, the user must use `ksu` with the `-e` command option.

The `ksu` options you are most likely to use are:

`-n` principal``
  specifies which Kerberos principal you want to use for `ksu`. (e.g., the user `joeadmin` might want to use his `admin` instance. See What is a Ticket?.)
`-c`
  specifies the location of your Kerberos credentials cache (ticket file).
`-k`
  tells `ksu` not to destroy your Kerberos tickets when `ksu` is finished.
`-f`
  requests forwardable tickets. (See Obtaining Tickets with kinit.) This is only applicable if `ksu` needs to obtain tickets.
`-l` lifetime``
  sets the ticket lifetime. (See Obtaining Tickets with kinit.) This is only applicable if `ksu` needs to obtain tickets.
`-z`
  tells `ksu` to copy your Kerberos tickets only if the UID you are switching is the same as the Kerberos primary (either yours or the one specified by the `-n` option).
`-Z`
  tells `ksu` not to copy any Kerberos tickets to the new UID.
`-e` command``
  tells `ksu` to execute command and then exit. See the description of the `.k5users` file above.
`-a` text``
  (at the end of the command line) tells `ksu` to pass everything after `-a` to the target shell.

The full set of options to Kerberos V5 `ksu` are discussed in the Reference section of this manual. (see ksu Reference)

## Kerberos V5 Reference

This section will include copies of the manual pages for the Kerberos V5 client programs. You can read the manual entry for any command by typing `man` command, where command is the name of the command for which you want to read the manual entry. For example, to read the `kinit` manual entry, you would type:

         shell% man kinit

Note: To be able to view the Kerberos V5 manual pages on line, you may need to add the directory `/usr/local/man` to your MANPATH environment variable. (Remember to replace `/usr/local` with the top-level directory in which Kerberos V5 is installed.) For example, if you had the the following line in your `.login` file2:

         setenv MANPATH /usr/local/man:/usr/man

and the Kerberos V5 man pages were in the directory `/usr/krb5/man`, you would change the line to the following:

         setenv MANPATH /usr/krb5/man:/usr/local/man:/usr/man

### kinit Reference

kinit manpage

### klist Reference

klist manpage

### ksu Reference

ksu manpage

### kdestroy Reference

kdestroy manpage

### kpasswd Reference

kpasswd manpage

### telnet Reference

telnet manpage

### FTP Reference

ftp manpage

### rlogin Reference

rlogin manpage

### rsh Reference

rsh manpage

### rcp Reference

rcp manpage

## Kerberos Glossary

client
  an entity that can obtain a ticket. This entity is usually either a user or a host.
host
  a computer that can be accessed over a network.
Kerberos
  in Greek mythology, the three-headed dog that guards the entrance to the underworld. In the computing world, Kerberos is a network security package that was developed at MIT.
KDC
  Key Distribution Center. A machine that issues Kerberos tickets.
keytab
  a key table file containing one or more keys. A host or service uses a keytab file in much the same way as a user uses his/her password.
principal
  a string that names a specific entity to which a set of credentials may be assigned. It can have an arbitrary number of components, but generally has three:
  primary
    the first part of a Kerberos principal. In the case of a user, it is the username. In the case of a service, it is the name of the service.
  instance
    the second part of a Kerberos principal. It gives information that qualifies the primary. The instance may be null. In the case of a user, the instance is often used to describe the intended use of the corresponding credentials. In the case of a host, the instance is the fully qualified hostname.
  realm
    the logical network served by a single Kerberos database and a set of Key Distribution Centers. By convention, realm names are generally all uppercase letters, to differentiate the realm from the internet domain.

  The typical format of a typical Kerberos principal is primary/instance@REALM.

service
  any program or computer you access over a network. Examples of services include "host" (a host, e.g., when you use `telnet` and `rsh`), "ftp" (FTP), "krbtgt" (authentication; cf. ticket-granting ticket), and "pop" (email).
ticket
  a temporary set of electronic credentials that verify the identity of a client for a particular service.
TGT
  Ticket-Granting Ticket. A special Kerberos ticket that permits the client to obtain additional Kerberos tickets within the same Kerberos realm.

## Table of Contents

- Copyright
- Introduction
  - What is a Ticket?
  - What is a Kerberos Principal?
- Kerberos V5 Tutorial
  - Setting Up to Use Kerberos V5
  - Ticket Management
    - Kerberos Ticket Properties
    - Obtaining Tickets with kinit
    - Viewing Your Tickets with klist
    - Destroying Your Tickets with kdestroy
  - Password Management
    - Changing Your Password
    - Password Advice
    - Granting Access to Your Account
  - Kerberos V5 Applications
    - Overview of Additional Features
    - telnet
    - rlogin
    - FTP
    - rsh
    - rcp
    - ksu
- Kerberos V5 Reference
  - kinit Reference
  - klist Reference
  - ksu Reference
  - kdestroy Reference
  - kpasswd Reference
  - telnet Reference
  - FTP Reference
  - rlogin Reference
  - rsh Reference
  - rcp Reference
- Kerberos Glossary

#### Footnotes

1. Note: the realm EXAMPLE.COM must be listed in your computer's Kerberos configuration file, `/etc/krb5.conf`.

2. The MANPATH variable may be specified in a different initialization file, depending on your operating system. Some of the files in which you might specify environment variables include `.login`, `.profile`, or `.cshrc`.