Network Working Group T. Ts'o, Editor
Internet-Draft Massachusetts Institute of Technology
draft-tso-telnet-auth-enc-01.txt November 1998
Telnet Authentication Option
Status of this Memo
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1. Command Names and Codes
AUTHENTICATION 37
Authentication Commands
IS 0
SEND 1
REPLY 2
NAME 3
Authentication Types
NULL 0
KERBEROS_V4 1
KERBEROS_V5 2
SPX 3
SRP 5
RSA 6
LOKI 10
SSA 11
KEA_SJ 12
KEA_SJ_INTEG 13
DSS 14
NTLM 15
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Note that other authentication types numbers may be assigned by
the IANA as requested.
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
ENCRYPT_MASK 4
ENCRYPT_OFF 0
ENCRYPT_ON 4
INI_CRED_FWD_MASK 8
INI_CRED_FWD_OFF 0
INI_CRED_FWD_ON 8
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 AUTHENTICA-
TION command.
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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 AUTHENTICA-
TION) 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 authentica-
tion 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 au-
thentication type, and the second is a modifier to the type. There
are currently four one bit fields defined in the modifier. Two of
these are processed as a pair, the AUTH_WHO_MASK bit and the
AUTH_HOW_MASK bit. There are four possible combinations of these two
bits:
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
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The server will authenticate itself to the client. If the
negotiation is successful, the client will know that it is con-
nected 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 it-
self 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 negotia-
tion 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.
The third bit field in the modifier is the ENCRYPT_MASK bit. This
bit is either set to ENCRYPT_ON or ENCRYPT_OFF. Setting this bit
to ENCRYPT_ON implies that once authentication completes, the tel-
net ENCRYPT option must be negotiated. If the negotiation for the
ENCRYPT option fails, the data connection must be terminated.
The fourth bit field in the modifier is the INI_CRED_FWD_MASK bit.
This bit is either set to INI_CRED_FWD_ON or INI_CRED_FWD_OFF.
Setting this bit to INI_CRED_FWD_ON implies that once authentica-
tion completes, the client will immediately forward authentication
credentials to the server. This bit is set by the client to ad-
vise the server to expect forwarded credentials from the client.
The motivation for this advisory bit is that the server may wish
to wait until the forwarded credentials have been sent before
starting any operating system specific login procedures which may
depend on these credentials. Note that credentials forwarding may
not be supported by all authentication mechanisms. It is a proto-
col error to set this bit if the underlying authentication mechan-
ism does not support credentials forwarding.
Note that some implementations of the telnet authentication option
may not understand the ENCRYPT_ON and INI_CRED_FWD_ON bits. Hence
an implementation wishing to offer these bits should offer authen-
tication type pairs with these bits both set and not set.
3. Default Specification
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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 pass-
words 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 ses-
sion, and a mechanism to enable encryption of the data stream as a
side effect of successful authentication. This means that: 1) the
users password will not be sent in clear text across the network, 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. 3) once authentication has suc-
ceeded, the data stream can be encrypted to provide protection
against active attacks.
It is intended that the AUTHENTICATION option be general enough that
it can be used to pass information for any authentication and encryp-
tion 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 in-
truder could attempt to negotiate the use of an authentication system
which is either weak, or already compromised by the intruder.
By linking the enabling of encryption as a side effect of successful
authentication, protection is provided against an active attacker.
An active attack is one where the underlying TCP stream can be modi-
fied or taken over by the active attacker. If encryption were en-
abled as a separate negotiation, it would provide a window of vulner-
ability from when the authentication completes, up to and including
the negotiation to turn on encryption. It is because of this that,
while there is a command to end encryption, there is not a command to
restart encryption. The only safe way to restart encryption once it
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has been turned off, is to repeat the entire authentication
processes.
6. Implementation Rules
WILL and DO are used only at the beginning of the connection to ob-
tain 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 res-
triction is due to the nature of authentication; there are three pos-
sible cases; server authenticates client, client authenticates
server, and server and client authenticate each other. By only nego-
tiating 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 re-
quests ("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" com-
mand. Only the server may send replies ("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 if the client responds
with a type of NULL, 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.
7. User Interface
Normally protocol specifications do not address user interface
specifications. However, due to the fact that the user will probably
want to be able to specify the things about authentication and en-
cryption and also know whether or not things succeeded, some guidance
needs to be given to implementors to provide some minimum level of
user control.
The user MUST be able to specify whether or not authentication is to
be used, and whether or not encryption is to used if the authentica-
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tion succeeds. There SHOULD be at least four settings, REQUIRE,
PROMPT, WARN and DISABLE. Setting the authentication switch to RE-
QUIRE means that if the authentication fails, then an appropriate er-
ror message must be displayed and the TELNET connection must be ter-
minated. Setting the authentication switch to PROMPT means that if
the authentication fails, then an appropriate error message must be
displayed and the user must be prompted for confirmation before con-
tinuing the TELNET session. Setting the authentication switch to
WARN means that if the authentication fails, then an appropriate er-
ror message must be displayed before continuing the TELNET session.
Setting the authentication switch to DISABLE means that authentica-
tion will not be attempted. The encryption switch SHOULD have the
same settings as the authentication switch; however its settings are
only used when authentication succeeds. The default setting for both
switches should be WARN. Both of these switches may be implemented
as a single switch, though having them separate gives more control to
the user.
8. Example
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. ]
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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
The following is an example of use of the option with encryption:
Client Server
IAC DO AUTHENTICATION
IAC WILL AUTHENTICATION
[ The server is now free to request authentication information.
]
IAC SB AUTHENTICATION SEND
KERBEROS_V4
ENCRYPT_ON|CLIENT|MUTUAL
KERBEROS_V4
ENCRYPT_ON|CLIENT|ONE_WAY IAC SE
[ The server has requested mutual Kerberos authentication, but is
willing to do just one-way Kerberos authentication. In both
cases it is willing to encrypt the data stream. 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
ENCRYPT_ON|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
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KERBEROS_V4
CLIENT|MUTUAL|ENCRYPT_ON 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
[ At this point, the client begins to encrypt the outgoing data
stream, and the server, after receiving this command, begins to
decrypt the incoming data stream. 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
[ At this point, the server begins to encrypt its outgoing data
stream, and the client, after receiving this command, begins to
decrypt its incoming data stream. ]
It is expected that any implementation that supports the Telnet AU-
THENTICATION option will support all of this specification.
9. Security Considerations
This memo describes a general framework for adding authentication and
encryption to the telnet protocol. The actual authentication mechan-
ism is described in the authentcation suboption specifications, and
the security of the authentication option is dependent on the
strengths and weaknesses of the authentication suboption.
It should also be noted that the negotiation of the authentication
type pair is not protected, thus allowing an attacker to force the
result of the authentication to the weakest mutually acceptable
method. (For example, even if both sides of the negotiation can ac-
cept a "strong" mechanism and a "40-bit" mechanism, an attacker could
force selection of the "40-bit" mechanism.) An implementation should
therefore only accept an authentication mechanism to be negotiated if
it is willing to trust it as being secure.
10. References
[1] Reynolds, Joyce, and Postel, Jon, "Assigned Numbers", RFC 1060,
ISI, March 1990
Author's Address
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Theodore Ts'o, Editor
Massachusetts Institute of Technology
MIT Room E40-343
77 Massachusetts Avenue
Cambridge, MA 02139
Phone: (617) 253-8091
Mailing List: telnet-ietf@CRAY.COM
EMail: tytso@mit.edu
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