Telnet Kerberos: Telnet Authentication: Kerberos Version 4
Use Kerberos Version 4 to authenticate users in Telnet sessions.
Borman, D., Ed., "Telnet Authentication: Kerberos Version 4", RFC 1411, DOI 10.17487/RFC1411, January 1993, <https://www.rfc-editor.org/info/rfc1411>.
@misc{rfc1411,
series = {Request for Comments},
number = 1411,
howpublished = {RFC 1411},
publisher = {RFC Editor},
doi = {10.17487/RFC1411},
url = {https://www.rfc-editor.org/info/rfc1411},
author = {D. Borman},
title = {{Telnet Authentication: Kerberos Version 4}},
pagetotal = 4,
year = 1993,
month = jan,
} RFC text
Contents
Network Working Group D. Borman, Editor
Request for Comments: 1411 Cray Research, Inc.
January 1993
Telnet Authentication: Kerberos Version 4
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. Discussion and suggestions for improvement are requested.
Please refer to the current edition of the "IAB Official Protocol
Standards" for the standardization state and status of this protocol.
Distribution of this memo is unlimited.
1. Command Names and Codes
Authentication Types
KERBEROS_V4 1
Suboption Commands
AUTH 0
REJECT 1
ACCEPT 2
CHALLENGE 3
RESPONSE 4
2. Command Meanings
IAC SB AUTHENTICATION IS <authentication-type-pair> AUTH <kerberos
ticket and authenticator> IAC SE
This is used to pass the Kerberos ticket to the remote side of the
connection. The first octet of the <authentication-type-pair>
value is KERBEROS_V4, to indicate the usage of Kerberos version 4.
IAC SB AUTHENTICATION REPLY <authentication-type-pair> ACCEPT IAC SE
This command indicates that the authentication was successful.
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 IS <authentication-type-pair> CHALLENGE
<encrypted challenge> IAC SE
IAC SB AUTHENTICATION REPLY <authentication-type-pair> RESPONSE
<encrypted response> IAC SE
These two commands are used to perform mutual authentication.
They are only used when the AUTH_HOW_MUTUAL bit is set in the
second octet of the authentication-type-pair. After successfully
sending an AUTH and receiving an ACCEPT, a CHALLENGE is sent. The
challenge is a random 8 byte number with the most significant byte
first, and the least significant byte last. When the CHALLENGE
command is sent, the "encrypted challenge" is the 8-byte-challenge
encrypted in the session key. When the CHALLENGE command is
received, the contents are decrypted to get the original 8-byte-
challenge, this value is then incremented by one, re-encrypted
with the session key, and returned as the "encrypted response" in
the RESPONSE command. The receiver of the RESPONSE command
decrypts the "encrypted response", and verifies that the resultant
value is the original 8-byte-challenge incremented by one.
The "encrypted challenge" value sent/received in the CHALLENGE
command is also encrypted with the session key on both sides of
the session, to produce a random 8-byte key to be used as the
default key for the ENCRYPTION option.
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, and the
server responds with ACCEPT, then the client then sends a CHALLENGE,
and the server sends a RESPONSE.
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, and the
client responds with ACCEPT, then the server then sends a CHALLENGE,
and the client sends a RESPONSE.
The authenticator (Kerberos Principal) used is of the form
"rcmd.host@realm".
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_V4 AUTH <joe's kerberos
ticket> IAC SE The server would then authenticate the user as "joe"
from the ticket information, and since "pete" is allowing "joe" to
use his account, the server would send back ACCEPT. If mutual
authentication is being used, the the client would send a CHALLENGE,
and verify the RESPONSE that the server sends back.
Client Server
IAC DO AUTHENTICATION
IAC WILL AUTHENTICATION
[ The server is now free to request authentication information.
]
IAC SB AUTHENTICATION SEND
KERBEROS_V4 CLIENT|MUTUAL
KERBEROS_V4 CLIENT|ONE_WAY IAC
SE
[ The server has requested mutual Version 4 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_V4 CLIENT|MUTUAL AUTH
<kerberos ticket information>
IAC SE
[ The server responds with an ACCEPT command to state that the
authentication was successful. ]
IAC SB AUTHENTICATION REPLY
KERBEROS_V4 CLIENT|MUTUAL ACCEPT
IAC SE
[ Next, the client sends across a CHALLENGE to verify that it is
really talking to the right server. ]
IAC SB AUTHENTICATION IS
KERBEROS_V4 CLIENT|MUTUAL
CHALLENGE xx xx xx xx xx xx xx
xx IAC SE
[ Lastly, the server sends across a RESPONSE to prove that it
really is the right server.
IAC SB AUTHENTICATION REPLY
KERBEROS_V4 CLIENT|MUTUAL
RESPONSE yy yy yy yy yy yy yy yy
IAC SE
Security Considerations
The ability to negotiate a common authentication mechanism between
client and server is a feature of the authentication option that
should be used with caution. When the negotiation is performed, no
authentication has yet occurred. Therefore, each system has no way
of knowing whether or not it is talking to the system it intends. An
intruder could attempt to negotiate the use of an authentication
system which is either weak, or already compromised by the intruder.
Author's Address
David A. Borman, Editor
Cray Research, Inc.
655F Lone Oak Drive
Eagan, MN 55123
Phone: (612) 452-6650
EMail: dab@CRAY.COM
Mailing List: telnet-ietf@CRAY.COM
Chair's Address
The working group can be contacted via the current chair:
Steve Alexander
INTERACTIVE Systems Corporation
1901 North Naper Boulevard
Naperville, IL 60563-8895
Phone: (708) 505-9100 x256
EMail: stevea@isc.com
AI ELI5
On this page
§1Kerberos V4 Authentication for Telnet
Telnet is a protocol for remote login sessions, and it includes an Authentication option that allows client and server to negotiate a common authentication mechanism. This document specifies how Kerberos Version 4 can be used as one such mechanism. The authentication type is identified as KERBEROS_V4 (code 1). The mechanism uses Kerberos tickets and an optional mutual authentication exchange to prove the identity of both the client and server. The authenticator used is a Kerberos principal of the form rcmd.host@realm, following the convention for Kerberos service principals.
Remote login (Telnet) can use Kerberos to check who you are before letting you in. The server asks for your Kerberos ticket, and if it is real, you get access.
§2The Authentication Exchange
The Kerberos V4 authentication in Telnet uses several suboption commands: AUTH transmits the Kerberos ticket to the remote side; ACCEPT indicates successful authentication; REJECT indicates failure (optionally with a reason message); and CHALLENGE and RESPONSE are used for mutual authentication. When mutual authentication is negotiated (AUTH_HOW_MUTUAL bit set), after successful initial authentication, the client sends an encrypted 8-byte challenge, the server decrypts and increments it, re-encrypts and returns it, and the client verifies the response. This challenge is also used as a seed to generate a default encryption key for the Encryption option.
First, the client sends its ticket. The server checks it and says accept or reject. If they both want to check each other, they send encrypted numbers back and forth to make sure they are really talking to each other.
§3How It Works in Practice
The protocol supports two authentication directions. If the client-to-server bit is set, the client sends the initial AUTH command, and the server responds with ACCEPT or REJECT. If the server-to-client bit is set, the direction is reversed. The mutual authentication bits control whether an additional challenge-response exchange occurs after initial authentication succeeds. A user can log in as a different username by first sending a NAME command with the desired username, allowing the server to grant access to that account if the authenticated user is authorized.
The client sends a ticket for user X, and the server can decide to let user X log in as a different user Y if that is allowed. They can also check that they are really talking to each other by passing secret numbers back and forth.