[Docs] [txt|pdf] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 RFC 2941

Network Working Group                                    T. Ts'o, Editor
Internet-Draft                                          VA Linux Systems
draft-tso-telnet-auth-enc-04.txt                          September 1999

                      Telnet Authentication Option

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.

0.  Abstract

   This document describes the authentication option to the telnet[1]
   protocol as a generic method for negotiating an authentication type
   and mode including whether encryption should be used and if creden-
   tials should be for- warded.  While this document summarizes current-
   ly utilized commands and types it does not define a specific authen-
   tication type.  Separate documents are to be published defining each
   authentication type.

   This document updates a previous specification of the telnet authen-
   tication option, RFC 1409 [2], so that it can be used to its use can
   be used to securely enable the telnet encryption option[3].

1.  Command Names and Codes

   AUTHENTICATION          37

                           Expires March 2000                   [Page 1]

Internet-Draft        Telnet Authentication Option        September 1999

       Authentication Commands
       IS                       0
       SEND                     1
       REPLY                    2
       NAME                     3

       Authentication Types
       NULL                     0
       KERBEROS_V4              1
       KERBEROS_V5              2
       SRP                      5
       KEA_SJ                  12
       KEA_SJ_INTEG            13
       DSS                     14

       Following historical practice, future authentication type numbers
       will be assigned by the IANA under a First Come First Served pol-
       icy as outlined by RFC 2434 [4].  Despite the fact that authenti-
       cation type numbers are allocated out of an 8-bit number space
       (as are most values in the telnet specification) it is not antic-
       ipated that the number space is or will become in danger of being
       exhausted.  However, if this should become an issue, when over
       50% of the number space becomes allocated, the IANA shall refer
       allocation requests to either the IESG or a designated expert for
       approval.

       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        20
       ENCRYPT_OFF              0
       ENCRYPT_USING_TELOPT     4
       ENCRYPT_AFTER_EXCHANGE  16
       ENCRYPT_RESERVED        20

       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

                           Expires March 2000                   [Page 2]

Internet-Draft        Telnet Authentication Option        September 1999

   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 must send this command if it receives a DO AUTHENTICA-
      TION 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 must send this command if it receives a WILL AUTHENTI-
      CATION 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 "authentica-
      tion-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 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

                           Expires March 2000                   [Page 3]

Internet-Draft        Telnet Authentication Option        September 1999

      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.  The au-
   thentication type may or may not include built-in encryption.  For
   instance, when the Kerberos 4 authentication type is negotiated en-
   cryption must be negotiated with the telnet ENCRYPT option.  However,
   the SSL and KEA_SJ authentication types provide an encrypted channel
   as part of a successful telnet AUTH option negotiation.

   There are currently five one bit fields defined in the modifier.  The
   first two of these bits 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

         The server will authenticate itself to the client.  If the ne-
         gotiation 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-

                           Expires March 2000                   [Page 4]

Internet-Draft        Telnet Authentication Option        September 1999

         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 and fifth bits in the modifier are the ENCRYPT_MASK
      bits.  These bits are used to determine if and how encryption
      should be enabled.  Of the four possible combinations only three
      are currently defined:

         ENCRYPT_OFF

            Encryption will not be used for this session.  TELOPT EN-
            CRYPT SHOULD NOT be negotiated.  This mode MUST be used with
            all AUTH types that do not provide a shared secret to be
            used as a session key.

         ENCRYPT_USING_TELOPT

            Encryption will be negotiated via the use of TELOPT ENCRYPT.
            Immediately after authentication has completed TELOPT EN-
            CRYPT MUST be negotiated in both directions.  This is re-
            quired to occur before credentials forwarding; other telnet
            options are negotiated; or any user data is transmitted.  A
            failure to successfully negotiate TELOPT ENCRYPT in either
            direction MUST result in immediate session termination.

         ENCRYPT_AFTER_EXCHANGE

            Encryption will be activated in both directions immediately
            after the successful exchange of the shared secret to be
            used as the session key.  The encryption algorithm to be
            used MUST be implied by the AUTH type.

      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.
      This bit is set by the client to advise the server to expect for-
      warded credentials from the client.

         INI_CRED_FWD_OFF

            The client will not be forwarding credentials to the server.
            This mode must be used if the selected authentication method
            does not support credentials forwarding.

         INI_CRED_FWD_ON

            Once authentication, and perhaps encryption, completes, the
            client will immediately forward authentication credentials
            to the server.

                           Expires March 2000                   [Page 5]

Internet-Draft        Telnet Authentication Option        September 1999

      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 mecha-
      nism does not support credentials forwarding.

      Credentials forwarding MUST NOT be performed if
      AUTH_CLIENT_TO_SERVER|AUTH_HOW_ONE_WAY was used since the identity
      of the server can not be assured.  Credentials SHOULD NOT be for-
      warded if the telnet connection is not protected using some en-
      cryption or integrity protection services.

      Note that older implementations of the telnet authentication op-
      tion will not understand the ENCRYPT_MASK and INI_CRED_FWD_MASK
      bits.  Hence an implementation wishing to offer these bits should
      offer authentication type pairs with these bits both set and not
      set if backwards compatibility is required.

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 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 or via subsequent use of the
   telnet ENCRYPT option.  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 succeeded, the data
   stream can be encrypted to provide protection against active attacks.

   It is intended that the AUTHENTICATION option be general enough that

                           Expires March 2000                   [Page 6]

Internet-Draft        Telnet Authentication Option        September 1999

   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.

   If the authentication type requires that encryption be enabled as a
   separate optional negotiation (the ENCRYPT option), it will provide a
   window of vulnerability from when the authentication completes, up to
   and including the negotiation to turn on encryption by an active at-
   tacker.  An active attack is one where the underlying TCP stream can
   be modified or taken over by the active attacker.  If the server only
   offers authentication type pairs that include the ENCRYPT_US-
   ING_TELOPT set in the ENCRYPT_MASK field, this will avoid the window
   of vulnerability, since both parties will agree that telnet ENCRYPT
   option must be successfully negotiated immediately following the suc-
   cessful completion of telnet AUTH.

   Other authentication types link the enabling of encryption as a side
   effect of successful authentication.  This will also provide protec-
   tion against the active attacker.  The ENCRYPT_AFTER_EXCHANGE bit al-
   lows these authentication types to negotiate encryption so that it
   can be made optional.

   Another opportunity for active attacks is presented when encryption
   may be turned on and off without re-authentication.  Once encryption
   is disabled, an attacker may hijack the telnet stream, and interfere
   with attempts to restart encryption.  Therefore, a client SHOULD NOT
   support the ability to turn off encryption.  Once encryption is dis-
   abled, if an attempt to re-enable encryption fails, the client MUST
   terminate the telnet connection.

   It is important that in both cases the authentication type pair be
   integrity protected at the end of the authentication exchange.  This
   must be specified for each authentication type to ensure that the re-
   sult of the telnet authentication option negotiation is agreed to by
   both the client and the server.  Some authentication type suboptions
   may wish to include all of the telnet authentication negotiation ex-
   changes in the integrity checksum, to fully protect the entire ex-
   change.

   Each side MUST verify the consistency of the auth-type-pairs in each
   message received.  Any variation in the auth-type-pair MUST be treat-

                           Expires March 2000                   [Page 7]

Internet-Draft        Telnet Authentication Option        September 1999

   ed as a fatal protocol error.
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 direction; the server
   must send the "DO", and the client must send the "WILL".  This re-
   striction 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 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.
   When the server has concluded that authentication cannot be nego-
   tiated with the client it should send IAC DONT AUTH to the client.

   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.

   As long as the server is WILL AUTH it may request authentication in-
   formation at any time.  This is done by sending a new list of sup-
   ported authentication types.  Requesting authentication information
   may be done as a way of verifying the validity of the client's cre-
   dentials after an extended period of time or to negotiate a new ses-
   sion key for use during encryption.

7.  User Interface

   Normally protocol specifications do not address user interface speci-
   fications.  However, due to the fact that the user will probably want

                           Expires March 2000                   [Page 8]

Internet-Draft        Telnet Authentication Option        September 1999

   to be able to specify the things about authentication and encryption
   and also know whether or not things succeeded, some guidance needs to
   be given to implementors to provide some minimum level of user con-
   trol.

   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-
   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

                           Expires March 2000                   [Page 9]

Internet-Draft        Telnet Authentication Option        September 1999

       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

   The following is an example of use of the option with encryption ne-
   gotiated via telnet ENCRYPT:

       Client                           Server
                                        IAC DO AUTHENTICATION
       IAC WILL AUTHENTICATION
       [ The server is now free to request authentication information.
         ]
                                        IAC SB AUTHENTICATION SEND
                                        KERBEROS_V4
                                        ENCRYPT_USING_TELOPT|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.  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_USING_TELOPT|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

                           Expires March 2000                  [Page 10]

Internet-Draft        Telnet Authentication Option        September 1999

       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|ENCRYPT_USING_TELOPT
                                        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|ENCRYPT_USING_TELOPT
       CHALLENGE xx xx xx xx xx xx xx
       xx IAC SE
       [ The server sends across a RESPONSE to prove that it really is
         the right server.  ]
                                        IAC SB AUTHENTICATION REPLY
                                        KERBEROS_V4
                                        CLIENT|MUTUAL|ENCRYPT_USING_TELOPT
                                        RESPONSE yy yy yy yy yy yy yy yy
                                        IAC SE
       [ At this point, the client and server begin to negotiate the
         telnet ENCRYPT option in each direction for a secure channel.
         If the option fails in either direction for any reason the
         connection must be immediately terminated.  ]

   The following is an example of use of the option with integrated en-
   cryption:

       Client                           Server
                                        IAC DO AUTHENTICATION
       IAC WILL AUTHENTICATION
       [ The server is now free to request authentication information.
         ]
                                        IAC SB AUTHENTICATION SEND
                                        KEA_SJ
                                        CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE
                                        IAC SE
       [ The server has requested mutual KEA authentication with
         SKIPJACK encryption.  The client will now respond with the name
         of the user that it wants to log in as and the KEA cert.  ]
       IAC SB AUTHENTICATION NAME "joe"
       IAC SE IAC SB AUTHENTICATION IS
       KEA_SJ
       CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE
       '1' CertA||Ra IAC SE

                           Expires March 2000                  [Page 11]

Internet-Draft        Telnet Authentication Option        September 1999

       [ The server responds with its KEA Cert.  ]
                                        IAC SB AUTHENTICATION REPLY
                                        KEA_SJ
                                        CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE
                                        '2'
                                        CertB||Rb||IVb||Encrypt(NonceB)
                                        IAC SE
       [ Next, the client sends across a CHALLENGE to verify that it is
         really talking to the right server.  ]
       IAC SB AUTHENTICATION IS KEA_SJ
       CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE
       '3' IVa||Encrypt( NonceB xor
       0x0C18 || NonceA ) 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
                                        KEA_SJ
                                        CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE
                                        '4' Encrypt( NonceA xor 0x0C18 )
                                        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 mecha-
   nism is described in the authentication 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.

11.  Acknowledgements

   Many people have worked on this document over the span of many years.

                           Expires March 2000                  [Page 12]

Internet-Draft        Telnet Authentication Option        September 1999

   Dave Borman was a document editor and author of much of the original
   text.  Other folks who have contributed ideas and suggestions to this
   text include: David Carrel, Jeff Schiller, and Richard Basch.

10.  References

   [1] Reynolds, Joyce, and Postel, Jon, "Telnet Protocol Specifica-
       tion", RFC 854, May 1983.
   [2] D. Borman, "Telnet Authentication Option", RFC 1409, January
       1993.
   [3] Internet Engineering Task Force, "Telnet Data Encryption Option",
       draft-tso-telnet-encryption-04.txt, T. Ts'o, Editor, VA Linux
       Systems, September 1999.
   [4] Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA
       Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

Author's Address

   Theodore Ts'o, Editor
   VA Linux Systems
   43 Pleasant St.
   Medford, MA 02155
   Phone: (781) 391-3464
   EMail: tytso@valinux.com

   Jeffrey Altman
   Columbia University
   Watson Hall Room 716
   612 West 115th Street
   New York NY 10025
   Phone: +1 (212) 854-1344
   EMail: jaltman@columbia.edu

   Mailing List: telnet-wg@BSDI.COM

                           Expires March 2000                  [Page 13]


Html markup produced by rfcmarkup 1.111, available from https://tools.ietf.org/tools/rfcmarkup/