Telnet auth: Telnet Authentication Option
A way for Telnet client and server to agree on how to verify the user's identity
Borman, D., Ed., "Telnet Authentication Option", RFC 1409, DOI 10.17487/RFC1409, January 1993, <https://www.rfc-editor.org/info/rfc1409>.
@misc{rfc1409,
series = {Request for Comments},
number = 1409,
howpublished = {RFC 1409},
publisher = {RFC Editor},
doi = {10.17487/RFC1409},
url = {https://www.rfc-editor.org/info/rfc1409},
author = {D. Borman},
title = {{Telnet Authentication Option}},
pagetotal = 7,
year = 1993,
month = jan,
} RFC text
Contents
Network Working Group D. Borman, Editor
Request for Comments: 1409 Cray Research, Inc.
January 1993
Telnet Authentication Option
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 37
IS 0
SEND 1
REPLY 2
NAME 3
Authentication Types
NULL 0
KERBEROS_V4 1
KERBEROS_V5 2
SPX 3
RSA 6
LOKI 10
Modifiers
AUTH_WHO_MASK 1
AUTH_CLIENT_TO_SERVER 0
AUTH_SERVER_TO_CLIENT 1
AUTH_HOW_MASK 2
AUTH_HOW_ONE_WAY 0
AUTH_HOW_MUTUAL 2
2. Command Meanings
This document makes reference to a "server" and a "client". For the
purposes of this document, the "server" is the side of the connection
that did the passive TCP open (TCP LISTEN state), and the "client" is
the side of the connection that did the active open.
IAC WILL AUTHENTICATION
The client side of the connection sends this command to indicate
that it is willing to send and receive authentication information.
IAC DO AUTHENTICATION
The servers side of the connection sends this command to indicate
that it is willing to send and receive authentication information.
IAC WONT AUTHENTICATION
The client side of the connection sends this command to indicate
that it refuses to send or receive authentication information; the
server side sends this command if it receives a DO AUTHENTICATION
command.
IAC DONT AUTHENTICATION
The server side of the connection sends this command to indicate
that it refuses to send or receive authentication information; the
client side sends this command if it receives a WILL
AUTHENTICATION command.
IAC SB AUTHENTICATION SEND authentication-type-pair-list IAC SE
The sender of this command (the server) requests that the remote
side send authentication information for one of the authentication
types listed in "authentication-type-pair-list". The
"authentication-type-pair-list" is an ordered list of
"authentication-type" pairs. Only the server side (DO
AUTHENTICATION) is allowed to send this.
IAC SB AUTHENTICATION IS authentication-type-pair <auth data> IAC SE
The sender of this command (the client) is sending the
authentication information for authentication type
"authentication-type-pair". Only the client side (WILL
AUTHENTICATION) is allowed to send this.
IAC SB AUTHENTICATION REPLY authentication-type-pair <auth data> IAC
SE
The sender of this command (the server) is sending a reply to the
the authentication information received in a previous IS command.
Only the server side (DO AUTHENTICATION) is allowed to send this.
IAC SB AUTHENTICATION NAME remote-user IAC SE
This optional command is sent to specify the account name on the
remote host that the user wishes to be authorized to use. Note
that authentication may succeed, and the authorization to use a
particular account may still fail. Some authentication mechanisms
may ignore this command.
The "authentication-type-pair" is two octets, the first is the
authentication type (as listed in Section 1, additions to this list
must be registered with the Internet Assigned Numbers Authority
(IANA)), and the second is a modifier to the type. There are
currently two one bit fields defined in the modifier, the
AUTH_WHO_MASK bit and the AUTH_HOW_MASK bit, so there are four
possible combinations:
AUTH_CLIENT_TO_SERVER
AUTH_HOW_ONE_WAY
The client will send authentication information about the local
user to the server. If the negotiation is successful, the
server will have authenticated the user on the client side of
the connection.
AUTH_SERVER_TO_CLIENT
AUTH_HOW_ONE_WAY
The server will authenticate itself to the client. If the
negotiation is successful, the client will know that it is
connected to the server that it wants to be connected to.
AUTH_CLIENT_TO_SERVER
AUTH_HOW_MUTUAL
The client will send authentication information about the local
user to the server, and then the server will authenticate
itself to the client. If the negotiation is successful, the
server will have authenticated the user on the client side of
the connection, and the client will know that it is connected
to the server that it wants to be connected to.
AUTH_SERVER_TO_CLIENT
AUTH_HOW_MUTUAL
The server will authenticate itself to the client, and then the
client will authenticate itself to the server. If the
negotiation is successful, the client will know that it is
connected to the server that it wants to be connected to, and
the server will know that the client is who it claims to be.
3. Default Specification
The default specification for this option is
WONT AUTHENTICATION
DONT AUTHENTICATION
meaning there will not be any exchange of authentication information.
4. Motivation
One of the deficiencies of the Telnet protocol is that in order to
log into remote systems, users have to type their passwords, which
are passed in clear text through the network. If the connections
goes through untrusted networks, there is the possibility that
passwords will be compromised by someone watching the packets as they
go by.
The purpose of the AUTHENTICATION option is to provide a framework
for the passing of authentication information through the TELNET
session. This means that: 1) the users password will not be sent in
clear text across the network, and 2) if the front end telnet process
has the appropriate authentication information, it can automatically
send the information, and the user will not have to type any
password.
It is intended that the AUTHENTICATION option be general enough that
it can be used to pass information for any authentication system.
5. Security Implications
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.
6. Implementation Rules
WILL and DO are used only at the beginning of the connection to
obtain and grant permission for future negotiations.
The authentication is only negotiated in one directions; the server
must send the "DO", and the client must send the "WILL". This
restriction is due to the nature of authentication; there are three
possible cases; server authenticates client, client authenticates
server, and server and client authenticate each other. By only
negotiating the option in one direction, and then determining which
of the three cases is being used via the suboption, potential
ambiguity is removed. If the server receives a "DO", it must respond
with a "WONT". If the client receives a "WILL", it must respond with
a "DONT".
Once the two hosts have exchanged a DO and a WILL, the server is free
to request authentication information. In the request, a list of
supported authentication types is sent. Only the server may send
requests ("IAC SB AUTHENTICATION SEND authentication-type-pair-list
IAC SE"). Only the client may transmit authentication information
via the "IAC SB AUTHENTICATION IS authentication-type ... IAC SE"
command. Only the server may send replys ("IAC SB AUTHENTICATION
REPLY authentication-type ... IAC SE"). As many IS and REPLY
suboptions may be exchanged as are needed for the particular
authentication scheme chosen.
If the client does not support any of the authentication types listed
in the authentication-type-pair-list, a type of NULL should be used
to indicate this in the IS reply. Note that in this case, the server
may choose to close the connection.
The order of the authentication types MUST be ordered to indicate a
preference for different authentication types, the first type being
the most preferred, and the last type the least preferred.
The following is an example of use of the option:
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 Kerberos authentication, but is
willing to do just one-way Kerberos 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 "joe"
IAC SE
IAC SB AUTHENTICATION IS
KERBEROS_V4 CLIENT|MUTUAL AUTH 4
7 1 67 82 65 89 46 67 7 9 77 0
48 24 49 244 109 240 50 208 43
35 25 116 104 44 167 21 201 224
229 145 20 2 244 213 220 33 134
148 4 251 249 233 229 152 77 2
109 130 231 33 146 190 248 1 9
31 95 94 15 120 224 0 225 76 205
70 136 245 190 199 147 155 13
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 REPLY
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
It is expected that any implementation that supports the Telnet
AUTHENTICATION option will support all of this specification.
7. References
[1] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
USC/Information Sciences Institute, July 1992.
Security Considerations
Security issues are discussed in Section 5.
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
§1The Security Problem
The Telnet protocol has a significant deficiency: when users log into remote systems, they must type their passwords, which are passed across the network in clear text. If the connection passes through untrusted networks, passwords can be compromised by someone watching the network packets.
The AUTHENTICATION option solves this problem in two ways. First, users' passwords are no longer sent in clear text across the network. Second, if the front-end Telnet process has the appropriate authentication information, it can automatically send the information, and the user will not have to type any password at all.
The AUTHENTICATION option is designed to be general enough to work with any authentication system, whether Kerberos, RSA, SPX, or others. This flexibility allows organizations to choose the authentication mechanism that best fits their security requirements.
Typing your password over the network is risky because bad people can read it. A better way is to use a secret key that both computers know about, so the password never travels across the network.
§2Authentication Types and Modifiers
The AUTHENTICATION option defines several authentication types that can be used. NULL (type 0) indicates no authentication. Kerberos V4 (type 1) and Kerberos V5 (type 2) use the Kerberos authentication system. SPX (type 3), RSA (type 6), and LOKI (type 10) represent other authentication mechanisms. Additional types can be registered with the Internet Assigned Numbers Authority (IANA).
Each authentication type is paired with a modifier specifying the direction and mode. The AUTH_WHO_MASK bit indicates direction: AUTH_CLIENT_TO_SERVER (0) means the client authenticates to the server, while AUTH_SERVER_TO_CLIENT (1) means the server authenticates to the client.
The AUTH_HOW_MASK bit indicates mode: AUTH_HOW_ONE_WAY (0) is one-way authentication, where only one side proves its identity. AUTH_HOW_MUTUAL (2) is mutual authentication, where both sides prove their identities to each other.
This gives four possible authentication scenarios: client authenticates to server with one-way authentication, server authenticates to client with one-way authentication, client authenticates to server with mutual authentication, or server authenticates to client with mutual authentication (each side then proves itself).
Different authentication systems (Kerberos, RSA, etc.) can work with Telnet. Each one is paired with a direction (who is proving their identity) and a mode (one way or mutual).
§3Option Negotiation and Commands
The AUTHENTICATION option uses Telnet's standard negotiation commands. The client sends IAC WILL AUTHENTICATION to indicate willingness to send and receive authentication information. The server sends IAC DO AUTHENTICATION to indicate it is willing to send and receive authentication information.
If either side refuses, they send IAC WONT AUTHENTICATION (client) or IAC DONT AUTHENTICATION (server). Authentication negotiation is only performed in one direction: the server must send DO, and the client must send WILL. This removes potential ambiguity.
Once both sides have exchanged DO and WILL, the server is free to request authentication information using IAC SB AUTHENTICATION SEND authentication-type-pair-list IAC SE. The authentication-type-pair-list is an ordered list of authentication-type pairs, with the first type being most preferred and the last being least preferred.
The client responds with IAC SB AUTHENTICATION IS authentication-type-pair <auth data> IAC SE, sending authentication data for one of the types listed by the server. If the client does not support any of the listed types, it responds with NULL to indicate this. The server may send multiple REPLY commands as part of the authentication exchange.
An optional IAC SB AUTHENTICATION NAME remote-user IAC SE command specifies the account name on the remote host that the user wishes to use. Not all authentication mechanisms support this command.
The client and server first agree to use authentication. The server suggests which authentication methods it likes, in order of preference. The client picks one and sends the required credentials, and they exchange messages until the authentication succeeds or fails.
§4Implementation Requirements and Security
- IAC DO AUTHENTICATION
- IAC WILL AUTHENTICATION
- IAC SB AUTHENTICATION SEND [types] IAC SEServer lists preferred authentication types
- IAC SB AUTHENTICATION IS [type] [data] IAC SEClient sends authentication credentials
- IAC SB AUTHENTICATION REPLY [type] ACCEPT IAC SEServer accepts if authentication succeeds
WILL and DO commands are used only at the beginning of the connection to obtain permission for future negotiations. Once permission is granted, the server is the only side allowed to request authentication information via SEND. Only the client may transmit authentication information via IS commands. Only the server may send REPLY commands. As many IS and REPLY suboptions may be exchanged as needed for the particular authentication scheme chosen.
The server MUST order the authentication types offered to indicate preference: first type is most preferred, last type is least preferred. If the client does not support any offered type, it should respond with NULL, but the server may then choose to close the connection.
A critical security implication must be noted: during authentication negotiation, no authentication has yet occurred. Therefore, each system has no way of knowing whether it is talking to the system it intends. An intruder could attempt to negotiate the use of an authentication system that is weak or already compromised. Systems must be designed to mitigate this risk, for example by offering only strong authentication types and refusing to fall back to weaker types.
Every implementation supporting the Telnet AUTHENTICATION option is expected to support the full specification as described in the RFC, including all command types and the ability to exchange multiple authentication suboptions as needed.
The server suggests and picks which method to use. Both sides exchange messages back and forth, but neither can be sure they are talking to the real other side until authentication is complete. Systems should only offer strong authentication methods.