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Network Working Group                                  D. Borman, EditorRequest for Comments: 1416                           Cray Research, Inc.Obsoletes:1409                                            February 1993Telnet Authentication OptionStatus 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   ThisRFC 1416 replacesRFC 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          2Telnet Working Group                                            [Page 1]

RFC 1416              Telnet Authentication Option         February 19932.  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.Telnet Working Group                                            [Page 2]

RFC 1416              Telnet Authentication Option         February 1993   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 inSection 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.Telnet Working Group                                            [Page 3]

RFC 1416              Telnet Authentication Option         February 1993      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.Telnet Working Group                                            [Page 4]

RFC 1416              Telnet Authentication Option         February 19936.  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 isTelnet Working Group                                            [Page 5]

RFC 1416              Telnet Authentication Option         February 1993         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 inSection 5.Telnet Working Group                                            [Page 6]

RFC 1416              Telnet Authentication Option         February 1993Author'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.COMChair'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.comTelnet Working Group                                            [Page 7]