Telnet Kerberos 5: Telnet Authentication: Kerberos Version 5
How a Telnet client and server prove who they are to each other using Kerberos Version 5 tickets.
Ts'o, T., "Telnet Authentication: Kerberos Version 5", RFC 2942, DOI 10.17487/RFC2942, September 2000, <https://www.rfc-editor.org/info/rfc2942>.
@misc{rfc2942,
series = {Request for Comments},
number = 2942,
howpublished = {RFC 2942},
publisher = {RFC Editor},
doi = {10.17487/RFC2942},
url = {https://www.rfc-editor.org/info/rfc2942},
author = {T. Ts'o},
title = {{Telnet Authentication: Kerberos Version 5}},
pagetotal = 7,
year = 2000,
month = sep,
} RFC text
Contents
Network Working Group T. Ts'o
Request for Comments: 2942 VA Linux Systems
Category: Standards Track September 2000
Telnet Authentication: Kerberos Version 5
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document describes how Kerberos Version 5 [1] is used with the
telnet protocol. It describes an telnet authentication suboption to
be used with the telnet authentication option [2]. This mechanism
can also used to provide keying material to provide data
confidentiality services in conjunction with the telnet encryption
option [3].
1. Command Names and Codes
Authentication Types
KERBEROS_V5 2
Sub-option Commands
AUTH 0
REJECT 1
ACCEPT 2
RESPONSE 3
FORWARD 4
FORWARD_ACCEPT 5
FORWARD_REJECT 6
2. Command Meanings
IAC SB AUTHENTICATION IS <authentication-type-pair> AUTH <Kerberos V5
KRB_AP_REQ message> IAC SE
This is used to pass the Kerberos V5 [1] KRB_AP_REQ message to the
remote side of the connection. The first octet of the
<authentication-type-pair> value is KERBEROS_V5, to indicate that
Version 5 of Kerberos is being used. The Kerberos V5
authenticator in the KRB_AP_REQ message must contain a Kerberos V5
checksum of the two-byte authentication type pair. This checksum
must be verified by the server to assure that the authentication
type pair was correctly negotiated. The Kerberos V5 authenticator
must also include the optional subkey field, which shall be filled
in with a randomly chosen key. This key shall be used for
encryption purposes if encryption is negotiated, and shall be used
as the negotiated session key (i.e., used as keyid 0) for the
purposes of the telnet encryption option; if the subkey is not
filled in, then the ticket session key will be used instead.
If data confidentiality services is desired the ENCRYPT_US-
ING_TELOPT flag must be set in the authentication-type-pair as
specified in [2].
IAC SB AUTHENTICATION REPLY <authentication-type-pair> ACCEPT IAC SE
This command indicates that the authentication was successful.
If the AUTH_HOW_MUTUAL bit is set in the second octet of the
authentication-type-pair, the RESPONSE command must be sent before
the ACCEPT command is sent.
IAC SB AUTHENTICATION REPLY <authentication-type-pair> REJECT
<optional reason for rejection> IAC SE
This command indicates that the authentication was not successful,
and if there is any more data in the sub-option, it is an ASCII
text message of the reason for the rejection.
IAC SB AUTHENTICATION REPLY <authentication-type-pair> RESPONSE
<KRB_AP_REP message> IAC SE
This command is used to perform mutual authentication. It is only
used when the AUTH_HOW_MUTUAL bit is set in the second octet of
the authentication-type-pair. After an AUTH command is verified,
a RESPONSE command is sent which contains a Kerberos V5 KRB_AP_REP
message to perform the mutual authentication.
IAC SB AUTHENTICATION <authentication-type-pair> FORWARD <KRB_CRED
message> IAC SE
This command is used to forward kerberos credentials for use by
the remote session. The credentials are passed as a Kerberos V5
KRB_CRED message which includes, among other things, the forwarded
Kerberos ticket and a session key associated with the ticket.
Part of the KRB_CRED message is encrypted in the key previously
exchanged for the telnet session by the AUTH suboption.
IAC SB AUTHENTICATION <authentication-type-pair> FORWARD_ACCEPT IAC
SE
This command indicates that the credential forwarding was
successful.
IAC SB AUTHENTICATION <authentication-type-pair> FORWARD_REJECT
<optional reason for rejection> IAC SE
This command indicates that the credential forwarding was not
successful, and if there is any more data in the suboption, it is
an ASCII text message of the reason for the rejection.
3. Implementation Rules
If the second octet of the authentication-type-pair has the AUTH_WHO
bit set to AUTH_CLIENT_TO_SERVER, then the client sends the initial
AUTH command, and the server responds with either ACCEPT or REJECT.
In addition, if the AUTH_HOW bit is set to AUTH_HOW_MUTUAL, the
server will send a RESPONSE before it sends the ACCEPT.
If the second octet of the authentication-type-pair has the AUTH_WHO
bit set to AUTH_SERVER_TO_CLIENT, then the server sends the initial
AUTH command, and the client responds with either ACCEPT or REJECT.
In addition, if the AUTH_HOW bit is set to AUTH_HOW_MUTUAL, the
client will send a RESPONSE before it sends the ACCEPT.
The Kerberos principal used by the server will generally be of the
form "host/<hostname>@realm". That is, the first component of the
Kerberos principal is "host"; the second component is the fully
qualified lower-case hostname of the server; and the realm is the
Kerberos realm to which the server belongs.
Any Telnet IAC characters that occur in the KRB_AP_REQ or KRB_AP_REP
messages, the KRB_CRED structure, or the optional rejection text
string must be doubled as specified in [4]. Otherwise the following
byte might be mis-interpreted as a Telnet command.
4. Examples
User "joe" may wish to log in as user "pete" on machine "foo". If
"pete" has set things up on "foo" to allow "joe" access to his
account, then the client would send IAC SB AUTHENTICATION NAME "pete"
IAC SE IAC SB AUTHENTICATION IS KERBEROS_V5 AUTH <KRB_AP_REQ_MESSAGE>
IAC SE
The server would then authenticate the user as "joe" from the
KRB_AP_REQ_MESSAGE, and if the KRB_AP_REQ_MESSAGE was accepted by
Kerberos, and if "pete" has allowed "joe" to use his account, the
server would then continue the authentication sequence by sending a
RESPONSE (to do mutual authentication, if it was requested) followed
by the ACCEPT.
If forwarding has been requested, the client then sends IAC SB
AUTHENTICATION IS KERBEROS_V5 CLIENT|MUTUAL FORWARD <KRB_CRED
structure with credentials to be forwarded> IAC SE. If the server
succeeds in reading the forwarded credentials, the server sends
FORWARD_ACCEPT else, a FORWARD_REJECT is sent back.
Client Server
IAC DO AUTHENTICATION
IAC WILL AUTHENTICATION
[ The server is now free to request authentication information.
]
IAC SB AUTHENTICATION SEND
KERBEROS_V5 CLIENT|MUTUAL
KERBEROS_V5 CLIENT|ONE_WAY IAC
SE
[ The server has requested mutual Version 5 Kerberos
authentication. If mutual authentication is not supported,
then the server is willing to do one-way authentication.
The client will now respond with the name of the user that it
wants to log in as, and the Kerberos ticket. ]
IAC SB AUTHENTICATION NAME
"pete" IAC SE
IAC SB AUTHENTICATION IS
KERBEROS_V5 CLIENT|MUTUAL AUTH
<KRB_AP_REQ message> IAC SE
[ Since mutual authentication is desired, the server sends across
a RESPONSE to prove that it really is the right server. ]
IAC SB AUTHENTICATION REPLY
KERBEROS_V5 CLIENT|MUTUAL
RESPONSE <KRB_AP_REP message>
IAC SE
[ The server responds with an ACCEPT command to state that the
authentication was successful. ]
IAC SB AUTHENTICATION REPLY
KERBEROS_V5 CLIENT|MUTUAL ACCEPT
IAC SE
[ If so requested, the client now sends the FORWARD command to
forward credentials to the remote site. ]
IAC SB AUTHENTICATION IS KER-
BEROS_V5 CLIENT|MUTUAL
FORWARD <KRB_CRED message> IAC
SE
[ The server responds with a FORWARD_ACCEPT command to state that
the credential forwarding was successful. ]
IAC SB AUTHENTICATION REPLY
KERBEROS_V5 CLIENT|MUTUAL
FORWARD_ACCEPT IAC SE
5. Security Considerations
The selection of the random session key in the Kerberos V5
authenticator is critical, since this key will be used for encrypting
the telnet data stream if encryption is enabled. It is strongly
advised that the random key selection be done using cryptographic
techniques that involve the Kerberos ticket's session key. For
example, using the current time, encrypting it with the ticket
session key, and then correcting for key parity is a strong way to
generate a subsession key, since the ticket session key is assumed to
be never disclosed to an attacker.
Care should be taken before forwarding a user's Kerberos credentials
to the remote server. If the remote server is not trustworthy, this
could result in the user's credentials being compromised. Hence, the
user interface should not forward credentials by default; it would be
far safer to either require the user to explicitly request
credentials forwarding for each connection, or to have a trusted list
of hosts for which credentials forwarding is enabled, but to not
enable credentials forwarding by default for all machines.
The IAC SB AUTHENTICATION NAME name IAC SE message is unprotected in
this protocol. Its contents should be verified by a secure method
after authentication completes before it is used.
6. IANA Considerations
The authentication type KERBEROS_V5 and its associated suboption
values are registered with IANA. Any suboption values used to extend
the protocol as described in this document must be registered with
IANA before use. IANA is instructed not to issue new suboption
values without submission of documentation of their use.
7. Acknowledgments
This document was originally written by Dave Borman of Cray Research,
Inc. Theodore Ts'o of MIT revised it to reflect the latest
implementation experience. Cliff Neuman and Prasad Upasani of USC's
Information Sciences Institute developed the credential forwarding
support.
In addition, the contributions of the Telnet Working Group are also
gratefully acknowledged.
8. References
[1] Kohl, J. and B. Neuman, "The Kerberos Network Authentication
System (V5)", RFC 1510, September 1993.
[2] Ts'o, T. and J. Altman, "Telnet Authentication Option", RFC 2941,
September 2000.
[3] Ts'o, T., "Telnet Data Encryption Option", RFC 2946, September
2000.
[4] Postel, J. and J. Reynolds, "Telnet Option Specifications", STD
8, RFC 855, May 1983.
9. Editor's Address
Theodore Ts'o
VA Linux Systems
43 Pleasant St.
Medford, MA 02155
Phone: (781) 391-3464
EMail: tytso@mit.edu
10. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
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or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
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The limited permissions granted above are perpetual and will not be
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Acknowledgement
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AI ELI5
On this page
§1What This RFC Adds to Telnet
RFC 2942 describes how Kerberos Version 5 is used with the Telnet protocol. It defines one authentication suboption that plugs into the Telnet authentication option, which is specified in RFC 2941.
The same mechanism can also supply keying material for data confidentiality, in conjunction with the Telnet encryption option from RFC 2946.
Telnet already has a place to plug in different ways of proving who you are. This RFC adds the Kerberos ticket way.
§2Type and Command Codes
The authentication type is KERBEROS_V5, with the value 2. Seven suboption commands carry the exchange: AUTH (0), REJECT (1), ACCEPT (2), RESPONSE (3), FORWARD (4), FORWARD_ACCEPT (5) and FORWARD_REJECT (6).
Each command sits inside a Telnet subnegotiation of the form IAC SB AUTHENTICATION ... IAC SE and begins with an authentication-type pair. The first octet of the pair is KERBEROS_V5. The second octet holds flags such as who authenticates to whom and whether authentication is one-way or mutual.
Each kind of message has a small number, like a secret code word, so both sides know whether it is a request, a yes or a no.
- 0AUTH — Carries the Kerberos KRB_AP_REQ message.
- 1REJECT — Authentication failed, with optional ASCII reason.
- 2ACCEPT — Authentication succeeded.
- 3RESPONSE — Carries the KRB_AP_REP message for mutual authentication.
- 4FORWARD — Carries a KRB_CRED message with forwarded credentials.
- 5FORWARD_ACCEPT — Credential forwarding succeeded.
- 6FORWARD_REJECT — Credential forwarding failed, with optional ASCII reason.
§3What Each Command Does
AUTH passes a Kerberos V5 KRB_AP_REQ message to the other side. Its authenticator must contain a Kerberos V5 checksum of the two-byte authentication type pair, and the receiving server must verify it so that the negotiated type pair is known to be correct. The authenticator must also include the optional subkey field, filled with a randomly chosen key. That key is used for encryption if encryption is negotiated, and serves as the negotiated session key (keyid 0) for the Telnet encryption option. If no subkey is present, the ticket session key is used instead.
If data confidentiality is wanted, the ENCRYPT_USING_TELOPT flag must be set in the type pair.
ACCEPT means authentication succeeded. REJECT means it failed, and any extra data is an ASCII text reason. RESPONSE carries a KRB_AP_REP message and is used for mutual authentication, only when the AUTH_HOW_MUTUAL bit is set. In that case RESPONSE must be sent before ACCEPT.
FORWARD sends a KRB_CRED message so that Kerberos credentials can be used by the remote session. It includes the forwarded ticket and its session key, and part of it is encrypted in the key already exchanged by AUTH. The reply is FORWARD_ACCEPT on success or FORWARD_REJECT with optional ASCII reason text.
One side shows its ticket, the other says yes or no. If both want to be sure of each other, the second side shows its own ticket too. A separate step can hand over your ticket so you can use it on the far computer.
§4Who Goes First and Other Rules
If the AUTH_WHO bit is AUTH_CLIENT_TO_SERVER, the client sends the first AUTH and the server answers with ACCEPT or REJECT. With AUTH_HOW_MUTUAL, the server sends RESPONSE before ACCEPT. If AUTH_WHO is AUTH_SERVER_TO_CLIENT, the roles are swapped.
The server's Kerberos principal is generally of the form host/<hostname>@realm: the first component is host, the second is the fully qualified lower-case hostname, and the realm is the Kerberos realm of the server.
Any Telnet IAC byte inside the KRB_AP_REQ, KRB_AP_REP, KRB_CRED or rejection text must be doubled, as specified in RFC 855. Otherwise the next byte could be mistaken for a Telnet command.
Whoever is asked to prove themselves goes first. And because one special byte means something to Telnet, it has to be written twice whenever it shows up inside a ticket.
§5A Worked Login
In the RFC's example, user joe wants to log in as user pete on machine foo, and pete has allowed this. The client sends the account name with IAC SB AUTHENTICATION NAME, then an IS KERBEROS_V5 ... AUTH command with the KRB_AP_REQ. The server works out that the user is joe from the Kerberos message and checks that pete permits joe.
In the full sequence, the server first asks for mutual Version 5 Kerberos authentication and says it will accept one-way authentication if mutual is not supported. The client sends the name and ticket, the server sends RESPONSE to prove it is the right server, then ACCEPT. If forwarding was requested, the client sends FORWARD and the server answers FORWARD_ACCEPT or FORWARD_REJECT.
Joe shows his pass and says he wants to enter as Pete. The building checks the pass, shows its own badge back, and then lets him in.
- WILL AUTHENTICATIONReply to the server's DO AUTHENTICATION.
- SEND KERBEROS_V5 CLIENT|MUTUAL, CLIENT|ONE_WAYPrefers mutual, will accept one-way.
- NAME, then IS ... AUTH with KRB_AP_REQAccount name and Kerberos ticket.
- REPLY ... RESPONSE with KRB_AP_REPProves the server is the right one.
- REPLY ... ACCEPT
- IS ... FORWARD with KRB_CREDOnly if forwarding was requested.
- REPLY ... FORWARD_ACCEPT
§6Security Considerations
The random subkey is critical, because it encrypts the Telnet data stream when encryption is enabled. The RFC strongly advises choosing it with cryptographic techniques that involve the ticket's session key. Its example is to take the current time, encrypt it with the ticket session key and correct the key parity.
Forwarding credentials to an untrustworthy server could compromise them. User interfaces should not forward by default. They should require an explicit request for each connection, or use a trusted list of hosts.
The NAME message is unprotected, so its contents should be verified by a secure method after authentication completes before it is used.
Do not hand your pass to a stranger, and pick your secret number in a way nobody can guess. Also double-check the name someone gives, because that part is not locked up.
§7IANA Considerations
The KERBEROS_V5 authentication type and its suboption values are registered with IANA. Any new suboption values that extend the protocol must be registered before use, and IANA is instructed not to issue new values without documentation of their use.
The code numbers are written in an official list, and nobody may add new ones without explaining what they are for.