Telnet auth: Telnet Authentication Option
A way to prove your identity over Telnet without sending passwords in the clear
Borman, D., Ed., "Telnet Authentication Option", RFC 1416, DOI 10.17487/RFC1416, February 1993, <https://www.rfc-editor.org/info/rfc1416>.
@misc{rfc1416,
series = {Request for Comments},
number = 1416,
howpublished = {RFC 1416},
publisher = {RFC Editor},
doi = {10.17487/RFC1416},
url = {https://www.rfc-editor.org/info/rfc1416},
author = {D. Borman},
title = {{Telnet Authentication Option}},
pagetotal = 7,
year = 1993,
month = feb,
} RFC text
Contents
Network Working Group D. Borman, Editor
Request for Comments: 1416 Cray Research, Inc.
Obsoletes: 1409 February 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.
Note
This RFC 1416 replaces RFC 1409, which has an important typographical
error in the example on page 6 (one occurance of "REPLY" should be
"IS").
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 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
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 Problem with Clear-Text Passwords
The Telnet protocol lets users log into remote systems, but it has a significant security weakness: users must type their passwords, which are transmitted in clear text across the network. If the connection passes through untrusted networks, an attacker watching the packets can steal the password.
RFC 1416 defines an AUTHENTICATION option for Telnet to solve this problem. The option provides a framework for exchanging authentication information securely, so passwords do not travel in the clear. Additionally, if the Telnet client has appropriate credentials, it can automatically send the authentication information without the user typing anything.
Telnet sends everything you type, including passwords, where anyone watching the network can read it. This option lets you prove who you are with a secret code instead.
§2Commands and Negotiation
The AUTHENTICATION option uses standard Telnet option negotiation. The client sends WILL AUTHENTICATION to indicate it will support authentication. The server sends DO AUTHENTICATION to request it. Once both sides agree, the server becomes free to request authentication information.
The negotiation defines two roles: the client (the side that made the active TCP connection) and the server (the side that listened). Only the server may send a SEND command listing the authentication types it accepts. Only the client may send an IS command with authentication data. The server may send a REPLY command to respond to the client's credentials. The client may also send a NAME command to specify which account on the remote host it wishes to use.
First the client and server agree to talk about authentication. Then the server asks for credentials, the client sends them, and the server says whether it worked.
§3Authentication Types and Modifiers
The RFC defines several authentication types: NULL (no authentication), KERBEROS_V4, KERBEROS_V5, SPX, RSA, and LOKI. The Internet Assigned Numbers Authority (IANA) maintains the list for future additions.
Each authentication type is accompanied by a modifier that specifies the direction and mode of authentication. The AUTH_WHO_MASK bit indicates whether the client authenticates to the server (CLIENT_TO_SERVER) or the server authenticates to the client (SERVER_TO_CLIENT). The AUTH_HOW_MASK bit indicates whether authentication is one-way (HOW_ONE_WAY) or mutual (HOW_MUTUAL), where mutual means both sides prove their identity to each other.
This creates four possible combinations: client authenticates to server one-way, server authenticates to client one-way, both authenticate to each other mutually with client first, or both authenticate mutually with server first. The server lists its preferred authentication types in order of preference when it sends the SEND command.
Different ways to prove who you are (Kerberos, RSA, etc.) can work one-way or both ways. The server picks which methods it trusts and asks the client to use one.
§4Message Exchange Example
- DO AUTHENTICATIONServer offers to support authentication
- WILL AUTHENTICATIONClient accepts
- SEND KERBEROS_V4Server requests Kerberos credentials
- IS KERBEROS_V4 + ticketClient sends username and Kerberos ticket
- REPLY ACCEPTServer verifies ticket and accepts
The RFC provides an example exchange using Kerberos authentication. The server requests KERBEROS_V4 with CLIENT|MUTUAL (mutual authentication with client first) or KERBEROS_V4 with CLIENT|ONE_WAY (one-way authentication) as a fallback.
The client responds with a NAME command specifying the username (e.g., 'joe') and then sends the authentication data in the IS command—in this case, a Kerberos ticket. The server verifies the ticket and replies with ACCEPT. For mutual authentication, the client then sends a CHALLENGE to verify the server's identity, and the server responds with a RESPONSE to prove it is genuine. Multiple exchanges of IS and REPLY commands may occur as needed for the chosen authentication scheme.
If the client does not support any of the offered authentication types, it should respond with NULL to indicate this. The server may then choose to close the connection.
The server asks for credentials. The client sends them with the username. The server checks them and says yes or no. If both sides need to verify each other, they send more messages to do that.
§5Security Implications
The RFC notes an important security consideration: during the authentication negotiation phase, before any authentication has occurred, neither side knows whether it is talking to the intended party. An intruder could attempt to negotiate the use of a weak authentication method, or one already compromised by the intruder. This means the choice of authentication method itself must be protected, or administrators must ensure that only strong methods are offered.
Despite this limitation, the AUTHENTICATION option is a significant improvement over sending passwords in clear text. By moving authentication out of the Telnet command stream and into a negotiated framework, the mechanism allows implementations to use strong cryptographic methods appropriate to the environment.
An attacker could try to trick you into using a weak way of proving who you are. But even so, this is much safer than typing your password where everyone can see it.