Kerberos V5 UNIX User’s Guide

Release: beta 7 Document Edition: b7-1

Last updated: 10 September 1996

MIT

Copyright \begin{matrix}c \\ ◯ \end{matrix} 1990, 1991, 1992, 1993, 1994, 1995, 1996 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. 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.

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

Copyright \begin{matrix}c \\ ◯ \end{matrix} 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.

2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

3. All advertising materials mentioning features or use of this software must display the following

acknowledgement:

This product includes software developed by the University of California, Berkeley and its contributors.

4. Neither the name of the University nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notices and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions.

1 Introduction

Kerberos V5 is based on the Kerberos V5 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 nework security.

The Kerberos V5 package is designed to be easy to use. Most of the commands are nearly identical to UNIX network programs you are already used to. 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.

1.1 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.

1.2 What is a Kerberos Principal?

A Kerberos principal is a unique identity to which Kerberos can assign tickets. By convention, 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.mit.edu would be in the realm

    ATHENA.MIT.EDU.

2 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 fubar.org. When you see one of these, substitute your username, hostname, or domain name accordingly.

2.1 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/krb5 throughout this guide to refer to the top-level directory Kerberos V5 directory. We will therefor use /usr/krb5/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.

2.2 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.

2.2.1 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, FUBAR.ORG.^{1} He would type:

     shell% kinit david@FUBAR.ORG

Password for david@FUBAR.ORG: <– [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 Section 2.2.2 [Viewing Your Tickets with klist], page 6).

1 Note: the realm FUBAR.ORG must be listed in your computer’s Kerberos configuration file,

   /etc/krb5.conf.

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@FUBAR.ORG that would be good for three hours, you would type:

     shell% kinit -f -l 3h david@FUBAR.ORG

Password for david@FUBAR.ORG: <– [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.

2.2.2 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 Section 1.1 [What is a Ticket?], page 2.) The listing would look like this:

shell% klist

     Ticket cache: /tmp/krb5cc_ttypa
     Default principal: jennifer@ATHENA.MIT.EDU
     Valid starting     Expires            Service principal
     06/07/96 19:49:21  06/08/96 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 Section 1.2 [What is a Kerberos Principal?], page 2)

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
     06/07/96 19:49:21  06/08/96 05:49:19  krbtgt/ATHENA.MIT.EDU@ATHENA.MIT.EDU
     06/07/96 20:22:30  06/08/96 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.fubar.org, which is also in another Kerberos realm, FUBAR.ORG. If you telnet to this host, you will receive a ticket-granting ticket for the realm FUBAR.ORG, plus the new host

ticket for trillium.fubar.org. klist will now show:

shell% klist

     Ticket cache: /tmp/krb5cc_ttypa
     Default principal: jennifer@ATHENA.MIT.EDU
     Valid starting     Expires            Service principal
     06/07/96 19:49:21  06/08/96 05:49:19  krbtgt/ATHENA.MIT.EDU@ATHENA.MIT.EDU
     06/07/96 20:22:30  06/08/96 05:49:19  host/daffodil.mit.edu@ATHENA.MIT.EDU
     06/07/96 20:24:18  06/08/96 05:49:19  krbtgt/FUBAR.ORG@ATHENA.MIT.EDU
     06/07/96 20:24:18  06/08/96 05:49:19  host/trillium.fubar.org@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

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 Section 3.1 [kinit Reference], page 28 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 Jul 96 19:06:25 31 Jul 96 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@FUBAR.ORG

Valid starting Expires Service principal 07/31/96 11:52:29 07/31/96 21:11:23 krbtgt/FUBAR.ORG@FUBAR.ORG

Flags: Ff 07/31/96 12:03:48 07/31/96 21:11:23 host/trillium.fubar.org@FUBAR.ORG

Flags: Ff shell%

2.2.3 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 Ticket cache NOT destroyed! shell%

2.3 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 Section 2.3.2 [Password Advice], page 11), and keep it secret. If you need to give access to your account to someone else, you can do so through Kerberos. (See Section 2.3.3 [Granting Access to Your Account], page 13.) 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.

2.3.1 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

Old password for david: <- Type your old password. New Password for david: <- Type your new password. Verifying, please re-enter New Password for david: <- Type the new password again. Password changed. shell%

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

shell% kpasswd

Old password for david: <- Type the incorrect old password. Incorrect old password. 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

Old password for david: <- Type the old password. New Password for david: <- Type the new password. Verifying, please re-enter New Password for david: <- Type a different new password. Mismatch - try again New Password for david: <- Type the new password. Verifying, please re-enter New Password for david: <- Type the same new password. Password changed. 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.

2.3.2 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 whatever criteria they choose. For example, if the user jennifer chose a bad password, Kerberos would give an error message like the following:

shell% kpasswd

Old password for jennifer: <- Type your old password here. New Password for jennifer: <- Type an insecure new password. Verifying, please re-enter New Password for jennifer: <- Type it again.

ERROR: Insecure password not accepted.  Please choose another.
kpasswd: Insecure password rejected while attempting to change password.
Please choose another password.

New Password for jennifer: <- Type a good password here. Verifying, please re-enter New Password for david: <- 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.

2.3.3 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 Section 1.2 [What is a Kerberos Principal?], page 2) 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@FUBAR.ORG

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.

2.4 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.

2.4.1 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 Section 2.2.1 [Obtaining Tickets with kinit], page 4), 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.

2.4.2 telnet

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

-f, --forward

forwards a copy of your tickets to the remote host.

--noforward

turns off forwarding of tickets to the remote host. (This option overrides any forwarding specified in your machine’s configuration files.)

-F, --forwardable

forwards a copy of your tickets to the remote host, and marks them re-forwardable from the remote host.

--noforwardable

makes any forwarded tickets nonforwardable. (This option overrides any forwardability specified in your machine’s configuration files.)

-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, --encrypt

turns on encryption.

--noencrypt

turns off 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.mit.edu

Trying 128.0.0.5 ... Connected to daffodil.mit.edu. 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.fubar.org 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.mit.edu 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.fubar.org, 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.fubar.org

Trying 128.0.0.5... Connected to trillium.fubar.org. Escape character is ’^]’. [ Kerberos V5 accepts you as “jennifer@fubar.org” ] [ Kerberos V5 accepted forwarded credentials ] NetBSD 1.1: Tue May 21 00:31:42 EDT 1996

Welcome to NetBSD! shell%

Note that jennifer’s machine used Kerberos to authenticate her to trillium.fubar.org, 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 Section 3.5 [telnet Reference], page 33)

2.4.3 rlogin

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

-f, --forward

forwards a copy of your tickets to the remote host.

--noforward

turns off forwarding of tickets to the remote host. (This option overrides any forwarding specified in your machine’s configuration files.)

-F, --forwardable

forwards a copy of your tickets to the remote host, and marks them re-forwardable from the remote host.

--noforwardable

makes any forwarded tickets nonforwardable. (This option overrides any forwardability specified in your machine’s configuration files.)

-k realm requests tickets for the remote host in the specified realm, instead of determining the

realm itself.

-x, --encrypt

turns on encryption.

--noencrypt

turns off encryption.

For example, if david wanted to use the standard UNIX rlogin to connect to the machine

daffodil.mit.edu, he would type:
     shell% rlogin daffodil.mit.edu -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.mit.edu 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.fubar.org, 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.fubar.org -f -x

This rlogin session is using DES encryption for all data transmissions.

Last login: Thu Jun 20 16:20:50 from daffodil

SunOS Release 4.1.4 (GENERIC) #2: Tue Nov 14 18:09:31 EST 1995 Not checking quotas. Try quota.real if you need them. shell%

Note that jennifer’s machine used Kerberos to authenticate her to trillium.fubar.org, 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 Section 3.6 [rlogin Reference], page 42)

2.4.4 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.

-forward requests that your tickets be forwarded to the remote host. The -forward 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
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 Section 3.7 [FTP Reference], page 44)

2.4.5 rsh

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

-f, --forward

forwards a copy of your tickets to the remote host.

--noforward

turns off forwarding of tickets to the remote host. (This option overrides any forwarding specified in your machine’s configuration files.)

-F, --forwardable

forwards a copy of your tickets to the remote host, and marks them re-forwardable from the remote host.

--noforwardable

makes any forwarded tickets nonforwardable. (This option overrides any forwardability specified in your machine’s configuration files.)

-k realm requests tickets for the remote host in the specified realm, instead of determining the

realm itself.

-x, --encrypt

turns on encryption.

--noencrypt

turns off encryption.

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

     shell% rsh trillium.fubar.org -l root -x date

This rsh session is using DES encryption for all data transmissions.

Fri Jun 21 17:06:12 EDT 1996

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 Section 3.8 [rsh Reference], page 52)

2.4.6 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, --encrypt

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 Section 3.9 [rcp Reference], page 54)

2.4.7 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 Section 2.3.3 [Granting Access to Your Account], page 13) 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 31 Jul 96 21:53:01 01 Aug 96 07:52:53 krbtgt/ATHENA.MIT.EDU@ATHENA.MIT.EDU 31 Jul 96 21:53:39 01 Aug 96 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@FUBAR.ORG

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 FUBAR.ORG 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 FUBAR.ORG.

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 Section 1.1 [What is a Ticket?], page 2.)

-c specifies the location of your Kerberos credentials cache (ticket file). -C specifies the location you want the Kerberos credentials cache (ticket file) to be for the

target user ID.

-k tells ksu not to destroy your Kerberos tickets when ksu is finished. -f requests forwardable tickets. (See Section 2.2.1 [Obtaining Tickets with kinit], page 4.)

This is only applicable if ksu needs to obtain tickets.

-l lifetime sets the ticket lifetime. (See Section 2.2.1 [Obtaining Tickets with kinit], page 4.) 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 Section 3.10 [ksu Reference], page 55)

26 Kerberos V5 UNIX User’s Guide

3 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/krb5/man to your MANPATH environment variable. (Remember to replace /usr/krb5 with the top-level directory in which Kerberos V5 is installed.) For example, if you had the the following line in your .login file^{1}:

     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

1 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.

28 Kerberos V5 UNIX User’s Guide

3.1 kinit Reference

Reference Manual for kinit Chapter 3: Kerberos V5 Reference 29

Reference Manual for kinit 30 Kerberos V5 UNIX User’s Guide

3.2 klist Reference

Reference Manual for klist Chapter 3: Kerberos V5 Reference 31

3.3 kdestroy Reference

Reference Manual for kdestroy 32 Kerberos V5 UNIX User’s Guide

3.4 kpasswd Reference

Reference Manual for kpasswd Chapter 3: Kerberos V5 Reference 33

3.5 telnet Reference

Reference Manual for telnet 34 Kerberos V5 UNIX User’s Guide

Reference Manual for telnet Chapter 3: Kerberos V5 Reference 35

Reference Manual for telnet 36 Kerberos V5 UNIX User’s Guide

Reference Manual for telnet Chapter 3: Kerberos V5 Reference 37

Reference Manual for telnet 38 Kerberos V5 UNIX User’s Guide

Reference Manual for telnet Chapter 3: Kerberos V5 Reference 39

Reference Manual for telnet 40 Kerberos V5 UNIX User’s Guide

Reference Manual for telnet Chapter 3: Kerberos V5 Reference 41

Reference Manual for telnet 42 Kerberos V5 UNIX User’s Guide

3.6 rlogin Reference

Reference Manual for rlogin Chapter 3: Kerberos V5 Reference 43

Reference Manual for rlogin 44 Kerberos V5 UNIX User’s Guide

3.7 FTP Reference

Reference Manual for FTP Chapter 3: Kerberos V5 Reference 45

Reference Manual for FTP 46 Kerberos V5 UNIX User’s Guide

Reference Manual for FTP Chapter 3: Kerberos V5 Reference 47

Reference Manual for FTP 48 Kerberos V5 UNIX User’s Guide

Reference Manual for FTP Chapter 3: Kerberos V5 Reference 49

Reference Manual for FTP 50 Kerberos V5 UNIX User’s Guide

Reference Manual for FTP Chapter 3: Kerberos V5 Reference 51

Reference Manual for FTP 52 Kerberos V5 UNIX User’s Guide

3.8 rsh Reference

Reference Manual for rsh Chapter 3: Kerberos V5 Reference 53

Reference Manual for rsh 54 Kerberos V5 UNIX User’s Guide

3.9 rcp Reference

Reference Manual for rcp Chapter 3: Kerberos V5 Reference 55

3.10 ksu Reference

Reference Manual for ksu 56 Kerberos V5 UNIX User’s Guide

Reference Manual for ksu Chapter 3: Kerberos V5 Reference 57

Reference Manual for ksu 58 Kerberos V5 UNIX User’s Guide

Reference Manual for ksu Chapter 3: Kerberos V5 Reference 59

Reference Manual for ksu 60 Kerberos V5 UNIX User’s Guide

Appendix A 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 generally has three parts:

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.

62 Kerberos V5 UNIX User’s Guide

Table of Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1 What is a Ticket? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.2 What is a Kerberos Principal? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Kerberos V5 Tutorial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1 Setting Up to Use Kerberos V5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2 Ticket Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2.1 Obtaining Tickets with kinit. . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2.2 Viewing Your Tickets with klist. . . . . . . . . . . . . . . . . . . . . . . . . 6 2.2.3 Destroying Your Tickets with kdestroy . . . . . . . . . . . . . . . . . . 8

2.3 Password Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.3.1 Changing Your Password . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.3.2 Password Advice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.3.3 Granting Access to Your Account . . . . . . . . . . . . . . . . . . . . . . 13

2.4 Kerberos V5 Applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.4.1 Overview of Additional Features . . . . . . . . . . . . . . . . . . . . . . . 14 2.4.2 telnet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.4.3 rlogin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.4.4 FTP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.4.5 rsh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.4.6 rcp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.4.7 ksu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3 Kerberos V5 Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.1 kinit Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 3.2 klist Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3.3 kdestroy Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 3.4 kpasswd Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 3.5 telnet Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 3.6 rlogin Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 3.7 FTP Reference. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 3.8 rsh Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 3.9 rcp Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 3.10 ksu Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Appendix A Kerberos Glossary. . . . . . . . . . . . . . . . . . . . . 61

ii Kerberos V5 UNIX User’s Guide