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Subject: Kerberos Users' Frequently Asked Questions 1.8
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Summary: This document answers Frequently Asked Questions about the
    Kerberos authentication system.  Read this before you post a
    question to comp.protocols.kerberos or [e-mail].
Xref: bloom-beacon.mit.edu comp.protocols.kerberos:932 news.answers:14278

Archive-name: kerberos-faq/user
Version: 1.8

Kerberos Users' Frequently Asked Questions
January 18, 1994
Compiled by: Barry Jaspan, <[e-mail]>
             OpenVision Technologies

     Kerberos; also spelled Cerberus.  n.  The watch dog of
     Hades, whose duty it was to guard the entrance--against
     whom or what does not clearly appear; . . . it is known
     to have had three heads. . .

        -Ambrose Bierce, The Enlarged Devil's Dictionary

This document answers Frequently Asked Questions about the Kerberos
authentication system.  It is freely distributable.  Direct all
responses and questions to [e-mail].  Most of the
information presented here has been collected from postings to the
comp.protocols.kerberos newsgroup (gatewayed to the mailing list
[e-mail]).

DISCLAIMER: OpenVision Technologies makes no representations about the
suitability of this information for any purpose.  It is provided "as
is" without express or implied warranty.  In particular, this document
is not intended as legal advice for exporting Kerberos, DES, or any
other encryption software.

Release Notes: Questions 1.2 and 1.4 have been updated to include the
release of MIT Kerberos V5 beta 3.  The Kerberos ports list, in
question 2.7, has been temporarily removed until it canbe brought up
to date.

Questions addressed in this release:
    (a * indicates that no answer is included)

1.  Kerberos and its Many Incarnations
----------------------------------------------------------------------

(1.1)    What is Kerberos?  What is it good for?
(1.2)    Where can I get Kerberos version 4 or 5?
(1.3)    What is the current status of version 4?
(1.4)    What is the current status of version 5?
(1.5)    Are version 4 and version 5 compatible?
(1.6)    How/why is Transarc's Kerberos different from MIT Kerberos V4?
    Can they interoperate?
(1.7)    How/why is OSF DCE Security different from MIT Kerberos V5?
    Can they interoperate?
(1.8)    How/why is DEC Ultrix Kerberos different from MIT Kerberos V4?
    Can they interoperate?
(1.9)    What is Bones?  What is it for?

2.  Using and Administering Kerberos
----------------------------------------------------------------------

(2.1)    Can I use Kerberos for local password validation?
(2.2)    What is the export status of Kerberos?
(2.3)    How can I delete a principal from the database?
(2.4)    What are the officially assigned Kerberos port numbers?
(2.5)    Are there Kerberos versions of telnet and ftpd?
    What other Kerberos clients exist?
(2.6)    Why does rlogin print "Warning: No Kerberos tickets obtained"?
(2.7)*    What operating systems has Kerberos been ported to?
    What vendors provide commercial support for Kerberos?

3.  Building and Installing Kerberos
----------------------------------------------------------------------

(3.1)    Why do I get an error message from ld when make_commands is
    executed?
(3.2)    Why doesn't KRB5_defs.h exist when I build version 5?
(3.3)    What is libkrb.a, and why can't ld find it?

4.  Miscellaneous
----------------------------------------------------------------------

(4.1)    List references for Kerberos and network security in general.
(4.2)    Where are archives of comp.protocols.kerberos (a.k.a
    [e-mail])?

======================================================================

1.  Kerberos and its Many Incarnations
----------------------------------------------------------------------

(1.1)    What is Kerberos?  What is it good for?

Kerberos is a network authentication system for use on physically
insecure networks, based on the key distribution model presented by
Needham and Schroeder.[3] It allows entities communicating over
networks to prove their identity to each other while preventing
eavsdropping or replay attacks.  It also provides for data stream
integrity (detection of modification) and secrecy (preventing
unauthorized reading) using cryptography systems such as DES.

Kerberos works by providing principals (users or services) with
/tickets/ that they can use to identify themselves to other principals
and secret cryptographic keys for secure communication with other
principals.[1] A ticket is a sequence of a few hundred bytes.  These
ticket can then be embedded in virtually any other network protocol,
thereby allowing the processes implementing that protocol to be sure
about the identity of the principals involved.

Practically speaking, Kerberos is mostly used in application-level
protocols (ISO model level 7), such as TELNET or FTP, to provide user
to host security.  It is also used, though less frequently, as the
implicit authentication system of data stream (such as SOCK_STREAM) or
RPC mechanisms (ISO model level 6).  It could also be used at a lower
level for host to host security, in protocols like IP, UDP, or TCP
(ISO model levels 3 and 4), although such implementations are
currently rare, if they exist at all.

It is important to realize that Kerberos is a one-trick pony.  It
provides for mutual authentication and secure communication between
principals on an open network by manufacturing secret keys for any
requestor and providing a mechanism for these secret keys to be safely
propagated through the network.  Kerberos does not, per se, provide
for authorization or accounting, although applications that wish to
can use their secret keys to perform those functions securely.
Kerberos also does not provide password validation for individual
workstations unless care is taken; see question 2.1.

It is also important to understand the using Kerberos on time-sharing
machines greatly weakens its protections, since a user's tickets
are then only as secure as the 'root' account (read: not very).
Furthermore, dumb terminals and most X terminals do not understand the
Kerberos protocol and as a result their cable connections remain
insecure.

(1.2)    Where can I get Kerberos version 4 or 5?

In the United States and Canada (*), Kerberos is available via
anonymous FTP from athena-dist.mit.edu (18.71.0.38).  For specific
instructions, cd to pub/kerberos and get the file README.KRB4 (for
version 4) or README.KRB5_BETA3 (for version 5).  Note that *YOU WILL
NOT BE ABLE TO RETRIEVE KERBEROS WITHOUT READING THIS FILE*.

Outside the United States, you can get Bones via anonymous ftp from
ftp.funet.fi (128.214.6.100) in pub/unix/security/kerberos.  A DES
library is available from the same place.  See question 1.9 for
information on Bones.

(*) Kerberos and DES can be exported to Canada.  See question 2.2.

(1.3)    What is the current status of version 4?

With the release of patch level 10 on December 10, 1992, MIT Kerberos
4 is now in its final form.  MIT does not anticipate ever making a new
Kerberos 4 release.

Several vendors provide their own versions of Kerberos which may
contain improvements or extensions; see question 2.7.

(1.4)    What is the current status of version 5?

The newest version of MIT Kerberos V5 is beta 3, released 17 January
1994.  It is backwards compatible with the previous beta release;
changes consist mostly of bug fixes.  See question 1.2 for
instructions on acquiring it.

(1.5)    Are version 4 and version 5 compatible?

No.  Versions 4 and 5 are based on completely different protocols.
The MIT Kerberos V5 distribution contains some compatibility code,
however: (a) there is a library which converts Kerberos V4 library
calls into Kerberos V5 requests, so you can run many V4 programs in a
V5 environment by relinking; (b) the Kerberos server can optionally
service V4 requests; (c) there is a program to convert a V4 format
Kerberos database to a V5 format database.  The names used by the V5
library have a prefix "krb5_" so they do not conflict with the V4
library.

(1.6)    How/why is Transarc's Kerberos different from MIT Kerberos V4?
    Can they interoperate?

This is a fairly complex question, and this answer is known to be
incomplete.  The issue is regularly discussed on the
[e-mail] mailing list; send mail to the -request
list for subscriptions.

Years ago, the designers of AFS decided to implement their own
security system based on the Kerberos specification instead of using
the (then, not yet publicly available) MIT Kerberos V4.  As a result,
Transarc's AFS Kerberos speaks a different protocol but also
understands the MIT Kerberos V4 protocol.  They can, in principal,
talk to each other; however, there are a sufficient number of annoying
details and an insufficient number of advantages such that it may not
be practical.

The two versions use a different string-to-key function (the algorithm
that turns a password into a DES key); the AFS version uses the realm
name as part of the computation while the MIT version does not.  A
program that uses a password to acquire a ticket (e.g.  kinit or
login) will only work with one version, unless it is hacked up to try
both string-to-key algorithms.

The two versions also use a different method of finding Kerberos
servers.  MIT Kerberos uses krb.conf and krb.realms to map hostnames
to realms and realms to Kerberos servers.  AFS kaservers for a realm,
by definition, are located on the AFS database servers and can
therefore be located using /usr/vice/etc/CellServDB.  This means that
a program built using the MIT Kerberos libraries will look in one
place for the information while a program built using the AFS Kerberos
libraries will look in another.  You can certainly set up all three
files and use both libraries, but be sure that everything is
consistent.

The two versions have a different password changing protocol, so you
must use the correct 'kpasswd' program for the server you are talking
to.  In general, AFS clients that talk directly to the kaserver use an
Rx-based protocol, instead of UDP as with MIT Kerberos, so those AFS
clients cannot talk to an MIT server.

In summary, AFS Kerberos and MIT Kerberos can interoperate once you've
acquired a ticket granting ticket, which you can do with kinit (MIT)
or klog (AFS).  With a tgt, Kerberos applications such as rlogin can
talk to an MIT or AFS Kerberos server and achieve correct results.
However, it is probably best to pick one implementation and use it
exclusively

(1.7)    How/why is OSF DCE Security different from MIT Kerberos V5?
    Can they interoperate?

DCE Security is based on Kerberos V5, and uses the same wire protocol.
However, applications from two systems use the protocol in different
ways, so the actual interoperability is limited.  Furthermore, DCE
Security started from an early alpha release of V5 and the two
versions have developed independently; there are therefore some minor
incompatibilities that MIT and the OSF have agreed to resolve.  [Time
frame?]

The DCE Security Server, secd, listens on UDP port 88 for standard
Kerberos requests and responds appropriately.  Therefore, clients of
MIT Kerberos can operate in a DCE environment without modification,
assuming the DCE Security database contains the appropriate principals
with the correct keys.

An MIT Kerberos V5 server cannot replace the DCE Security Server,
however.  First, DCE applications communicate with secd via the DCE
RPC.  Second, the DCE layers additional functionality on top of the
standard application request/response protocol exchange and uses
ticket semantics that are nonsensical in a strict Kerberos
environment.  The MIT Kerberos server does not understand either of
these changes, and therefore cannot support them.

As an additional complication, the DCE does not officially export the
Kerberos V5 API; it only exports a DCE Security-specific API.
Individual vendors are capable of providing the Kerberos V5 API if
they wish, but unless they all do (which seems unlikely) Kerberos V5
API applications will not be compile-time portable to pure DCE
environments; only binaries will continue to work.

[Does secd provide all features of the MIT Kerberos server?  Is it
guaranteed to do so?]

(1.8)    How/why is DEC Ultrix Kerberos different from MIT Kerberos V4?
    Can they interoperate?

DEC ULTRIX contains Kerberos for a single reason, namely, to provide
authenticated name service for the ULTRIX enhanced security option.
It does not support user-level authentication in the normal manner.

DEC's version is essentially the same as (and derived from) MIT
Kerberos V4 with a few changes.  The most significant change is that
the ability to perform any kind of end-to-end user data encryption has
been eliminated in order to comply with export restrictions.  Minor
changes include the placement of ticket files (/var/dss/kerberos/tkt
vs. /tmp) and the principal names used by some standard Kerberos
services (e.g.: kprop vs. rcmd); there are probably other minor
changes as well.

These changes make it annoying but not impossible to use DEC ULTRIX
Kerberos in the normal way.  However, there is no reason at all to do
so, because the MIT distribution supports ULTRIX directly.  [This may
not be completely true.  I imagine that using ULTRIX Kerberos for
enhanced security and MIT's Kerberos at the same time would cause
problems.  Has anyone tried this?]

(1.9)    What is Bones?  What is it for?

Bones is a system that provides the Kerberos API without using
encryption and without providing any form of security whatsoever.  It
is a fake that allows the use of software that expects Kerberos to be
present when it cannot be.

Why does it exist?  Kerberos is a network security system which relies
on cryptographic methods for its security.  Since Kerberos' encryption
system, DES, is not exportable, Kerberos itself cannot be exported or
used outside of the United States in its original form.  (See question
2.2 for more information.)

As a partial solution to this problem, the Kerberos source code was
modified by the addition of #ifdef NOENCRYPTION around all calls to
DES functions.  Compiling this version with the symbol NOENCRYPTION
defined results in a system that looks like Kerberos from an
application's point of view but that does not require DES libraries
(and, as a result, does not speak the real Kerberos protocol and does
not provide any security).

The final piece in this puzzle is a program called "piranha" which
takes the Kerberos sources and removes all of the calls to the
encryption routines, replacing it with the code which was under #ifdef
NOENCRYPTION, producing the system known as Bones.  Bones has the
property that there is absolutely no question about whether or not it
is legal to transport its sources across national boundaries, since it
neither has any encryption routines nor any calls to encryption
routines.

#ifdef NOENCRYPTION was not documented, and it was only intended to be
used in the above manner.  Someone who tries compiling Kerberos with
that #define has in some sense "voided the warranty", and will get
something which is both (a) not secure and (b) not Kerberos.

2.  Using and Administering Kerberos
----------------------------------------------------------------------

(2.1)    Can I use Kerberos for local password validation?

Yes, but only under certain circumstances and only if you are
careful.

Requests for Kerberos ticket granting tickets (tgts) (e.g. from kinit
or login) are sent in plaintext to the Kerberos server, which then
responds with credentials encrypted in the requesting principal's
secret key.  The program then attempts to decrypt the data with the
supplied password and considers the authentication "successful" if the
decryption appears to yield meaningful results (such as the correct
principal name).

The problem here is that the requesting program cannot know for sure
whether the decryption succeeded or, more importantly, whether the
response actually came from the Kerberos server.  An attacker could,
for example, walk up to an unattended machine and "log in" as a
non-existent user.  Kerberos will eventually respond with an
appropriate error, but the attacker can arrange for another program to
deliver a fake response to login first; he then types the correct
password (which he knows because he created the fake response in the
first place) and succeeds in spoofing login.

The solution to this problem is for login to verify the tgt by using
it to acquire a service ticket with a known key and comparing the
results.  Typically, this means requesting an rcmd.<hostname> ticket,
where <hostname> is the local hostname, and checking the response
against the key stored in the machine's /etc/srvtab file.  If the keys
match then the original tgt must have come from Kerberos (since the
key only exists in the srvtab and the Kerberos database), and login
can allow the user to log in.

The solution works only so long as the host has a srvtab containing an
rcmd.<hostname> (or any other standard principal) entry.  This is fine
for physically secure or single-user workstations, but does not work
on public workstations in which anyone could access the srvtab file.

(2.2)    What is the export status of Kerberos?

In general, the COCOM treaty, signed by twenty or so countries
including the United States, says that all cryptographic material will
be treated as munitions.  This means that these countries treat
exporting DES the same way they would treat exporting weapons, fighter
planes, and other nasty stuff.  You cannot export such materials to
any other country (except Canada**) without an export license.

However, it *is* possible to get an export license for Kerberos (DEC
apparently has one for ULTRIX) provided it is hacked up in the correct
way.  The correct way appears to include making it impossible to
perform encryption on arbitrary user data; authentication is okay, but
secrecy is not.  Since the Kerberos API provides this functionality,
it must be carefully removed before an export license will be granted.

Of course, I am not a lawyer; this information is merely a collection
of what others (who are also not lawyers) have said and should not be
interpreted as legal advice.  If anyone out there has firm legal
advice, feel free to contribute it.

**Canada has been granted a general exemption to DES export
restrictions; the exemption is detailed in Title #22, Code of Federal
Regulations, "International Traffic in Arms Rgulations," Section 126.5.
You can export DES to Canada providing that it is not re-exported to
another country and that the above regulation is referenced.  This
information has been confirmed with Mr. Alan Sushinsky, Munitions
Control, The State Department, [phone].

(2.3)    How can I delete a principal from the database?

MIT Kerberos V4 does not include a single command to delete a Kerberos
principal.  This was an intentional omission based on the assumption
that by making deletion difficult, accidents were less likely to
happen.  If you want to delete a principal, do "kdb_util dump", edit
the ASCII dump with an editor, and do a "kdb_util load".  Obviously,
you can write a shell script to make this more convenient.

The admin tools for AFS Kerberos, MIT Kerberos V5, and the DCE
Kerberos have a simple delete request.

(2.4)    What are the officially assigned Kerberos port numbers?

The file src/prototypes/services.append in the MIT Kerberos
distribution contains the commonly used port assignments.  This file
is not the whole story, however.

"kerberos" has officially been moved to port 88, although people will
have to listen on port 750 for some time to come, and assume
that many servers won't be converted to listen to port 88 for some
time.

"kerberos_master" and "krb_prop" have not been reserved, but they are
only used for intra-site transactions so having them reserved probably
isn't necessary.  Furthermore, both of their port numbers have already
been assigned to other services, so requesting an official assignment
will force them to change.

"eklogin", "kpop", and "erlogin" have not been officially reserved,
but probably should be.  Their ports are not currently assigned to
other services, so hopefully they will not have to change if an
official assignment is requested.

(2.5)    Are there Kerberos versions of telnet and ftpd?

(a) TELNET.  An experimental Telnet Authentication Option has been
defined, and is described in RFC1416.  A separate document, RFC1411,
describes how that option is to be used with Kerberos V4, but no RFC
exists for its use with Kerberos V5.  These RFC's only define how
/authentication/ is to be performed; the standard for full encryption
is still under development.

An implementation of Kerberos V4 telnet is available via anonymous ftp
from ftp.uu.net, in /networking/telnet.91.03.25.tar.Z, but it predates
both of the above-mentioned RFCs and is therefore almost certainly not
compliant with them.  A Kerberos V5 telnet implementation, based on
the 4.4BSD telnet/telnetd, also exists but has been temporarily
removed from the MIT V5 beta 2 release (probably because it also does
not comply with the proposed standards).

(b) FTP.  The IETF Common Authentication Technology Working Group is
currently defining security extensions for the FTP protocol.  An
Internet Draft describing their work, and the source code for a
modified ftp/ftpd with the extensions, are now available via anonymous
FTP:

    thumper.bellcore.com:pub/lunt/ftp.tar.Z
    net-dist.mit.edu:tytso/ftp-wg/ftp.tar.Z

Please note that the extensions are still in the Draft stage and may
change at any time, in incompatible ways.

(2.6)    Why does rlogin print "Warning: No Kerberos tickets obtained"?

Kerberos rlogin uses a standard Kerberos exchange to prove the
identity of the user to the remote host, after which it uses the
/etc/passwd and a .klogin file to determine whether the user is
authorized to log in.

Since the user never types a password, klogind on the remote host
cannot obtain a new ticket granting ticket.  The user's existing tgt
cannot be used on the remote host, because MIT Kerberos V4 tickets are
host-specific.  Therefore, even though the user has logged in to the
remote host, there is no ticket granting ticket for the user available
on the remote host.  The warning message is merely a reminder of this
fact.

The most recent release of MIT Kerberos V4 (patchlevel 10) contains a
system called "rkinit" that allows a user to obtain a ticket granting
ticket on a remote machine.  Using this system, it is possible first
to obtain a tgt on a machine and then log into it with Kerberos
rlogin, thereby achieving a secure remote login with tickets.
Alternatively, you use Kerberos V5, which has forwardable tickets.

(2.7)    What operating systems has Kerberos been ported to?
    What vendors provide commercial support for Kerberos?

The Kerberos ports list has been temporarily removed because it is
extremely out of date.

3.  Building and Installing Kerberos
----------------------------------------------------------------------

(3.1)    Why do I get an error message from ld when make_commands is
    executed?

The make_commands program (from the file util/ss/make_commands.c,
around line 101) spawns ld as part of its normal operation.  The
arguments to ld are hard-coded into the exec() call and are not
correct for all systems.  To fix the problem, examine the call and
determine the correct arguments for your environment; once you know
the correct arguments, the change to the source code will be obvious.

(3.2)    Why doesn't KRB5_defs.h exist when I build version 5?

KRB5_defs.h, and other files like KRB5_tables.c, KRB5-types.h, and
KRB5_pre_defs.h are created by the ISODE ASN.1 compiler, pepsy, from
KRB5.py.  You therefore need to have ISODE built and you have to
configure your site.def file so that the Makefiles know how to run the
pepsy compiler.

Note that there's a bug in Sun's imake/cpp setup, so the Makefile that
is generated in lib/asn.1 is broken.  KRB5-types.h is generated by the
ISODE program pepsy; look in the makefile just before pepsy is called.
It's fairly obvious where a tab character is missing.

(3.3)    What is libkrb.a, and why can't ld find it?

The MIT Kerberos V5 server (krb5kdc) can operate in a V4 compatibility
mode in which it accepts and responds to standard V4 requests (see
question 1.5).  In order to do so, it needs the V4 Kerberos library,
libkrb.a.  That library is not part of the V5 distribution.  It is
assumed that if you want V4 compatibility you already have V4 built
and installed; see question 1.2 for information on obtaining V4.

To get krb5kdc to link properly, set the KRB4LIB variable in site.def
to the path of your V4 library.  If you do not need V4 backwards
compability, comment out the definition of Krb4KDCCompat in site.def.
(Be sure to run "make clean" after changing anything in site.def to
make sure everything is rebuilt correctly according to the new
parameters.)

4.  Miscellaneous
----------------------------------------------------------------------

(4.1)    List references for Kerberos and network security in general.

See the bibliography at the end of this document.

(4.2)    Where are archives of comp.protocols.kerberos (a.k.a
    [e-mail])?

Archives are available via anonymous FTP from athena-dist.mit.edu in
the directory pub/kerberos/krb-mail.  The [e-mail]
archives prensently extend up to the end of 1992.  Some archives of
the kerberos protocol mailing list are also available.

----------------------------------------------------------------------

BIBLIOGRAPHY

The FTP site for a reference, when known, is listed in square brackets
following the entry.  Yes, I know that these are not in Officially
Blessed Bibliography Format.  Sue me.

[1] Jennifer G. Steiner, Clifford Neuman, Jeffrey I. Schiller.
"Kerberos: An Authentication Service for Open Network Systems", USENIX
Mar 1988.  [athena-dist.mit.edu:pub/kerberos/doc/usenix.PS]

[2] S. P. Miller, B. C. Neuman, J. I. Schiller, and J. H. Saltzer,
"Kerberos Authentication and Authorization System", 12/21/87.

[3] R. M. Needham and M. D.  Schroeder, "Using Encryption for
Authentication in Large Networks of Computers," Communications of the
ACM, Vol.  21(12), pp. 993-999 (December, 1978).

[4] V. L. Voydock and S. T. Kent, "Security Mechanisms in High-Level
Network Protocols," Computing Surveys, Vol.  15(2), ACM (June 1983).

[5] Li Gong, "A Security Risk of Depending on Synchronized Clocks",
Operating Systems Review, Vol 26, #1, pp 49--53.

[6] S.M. Bellovin and M. Merritt, "Limitations of the Kerberos
Authentication System," USENIX Jan 1991.
[research.att.com:dist/internet_security/kerblimit.usenix.ps]

[7] Refik Molva, Gene Tsudik, Els Van Herreweghen, and Stefano Zatti,
"KryptoKnight Authentication and Key Distribution System."
[jerico.usc.edu:pub/gene/kryptoknight.ps.Z]

[8] C. Neumann and J. Kohl, "The Kerberos(tm) Network Authentication
Service (V5)," April 1992.  Currently released as an Internet Draft.

--
Barry Jaspan, [e-mail]
OpenVision Technologies