HomeDocsIETF standardsRFCRFC 2900 — 2999
Text version
Network Working Group
Request for Comments: 2942
Category: Standards Track
T. Ts'o
VA Linux Systems
September 2000

Telnet Authentication: Kerberos Version 5

Status of this Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The Internet Society (2000). All Rights Reserved.

Abstract

   This document describes how Kerberos Version 5 [1] is used with the
   telnet protocol.   It describes an telnet authentication suboption to
   be used with the telnet authentication option [2].   This mechanism
   can also used to provide keying material to provide data
   confidentiality services in conjunction with the telnet encryption
   option [3].

1. Command Names and Codes

      Authentication Types
      
         KERBEROS_V5    2
      
      Sub-option Commands
      
         AUTH               0
         REJECT             1
         ACCEPT             2
         RESPONSE           3
         FORWARD            4
         FORWARD_ACCEPT     5
         FORWARD_REJECT     6

2. Command Meanings

IAC SB AUTHENTICATION IS <authentication-type-pair> AUTH <Kerberos V5 KRB_AP_REQ message> IAC SE

This is used to pass the Kerberos V5 [1] KRB_AP_REQ message to the remote side of the connection. The first octet of the <authentication-type-pair> value is KERBEROS_V5, to indicate that Version 5 of Kerberos is being used. The Kerberos V5 authenticator in the KRB_AP_REQ message must contain a Kerberos V5 checksum of the two-byte authentication type pair. This checksum must be verified by the server to assure that the authentication type pair was correctly negotiated. The Kerberos V5 authenticator must also include the optional subkey field, which shall be filled in with a randomly chosen key. This key shall be used for encryption purposes if encryption is negotiated, and shall be used as the negotiated session key (i.e., used as keyid 0) for the purposes of the telnet encryption option; if the subkey is not filled in, then the ticket session key will be used instead.

If data confidentiality services is desired the ENCRYPT_US- ING_TELOPT flag must be set in the authentication-type-pair as specified in [2].

IAC SB AUTHENTICATION REPLY <authentication-type-pair> ACCEPT IAC SE

This command indicates that the authentication was successful.

If the AUTH_HOW_MUTUAL bit is set in the second octet of the authentication-type-pair, the RESPONSE command must be sent before the ACCEPT command is sent.

IAC SB AUTHENTICATION REPLY <authentication-type-pair> REJECT

<optional reason for rejection> IAC SE

This command indicates that the authentication was not successful, and if there is any more data in the sub-option, it is an ASCII text message of the reason for the rejection.

IAC SB AUTHENTICATION REPLY <authentication-type-pair> RESPONSE <KRB_AP_REP message> IAC SE

This command is used to perform mutual authentication. It is only used when the AUTH_HOW_MUTUAL bit is set in the second octet of the authentication-type-pair. After an AUTH command is verified, a RESPONSE command is sent which contains a Kerberos V5 KRB_AP_REP message to perform the mutual authentication.

IAC SB AUTHENTICATION <authentication-type-pair> FORWARD <KRB_CRED message> IAC SE

This command is used to forward kerberos credentials for use by the remote session. The credentials are passed as a Kerberos V5 KRB_CRED message which includes, among other things, the forwarded Kerberos ticket and a session key associated with the ticket. Part of the KRB_CRED message is encrypted in the key previously exchanged for the telnet session by the AUTH suboption.

IAC SB AUTHENTICATION <authentication-type-pair> FORWARD_ACCEPT IAC SE

This command indicates that the credential forwarding was successful. 

   IAC SB AUTHENTICATION <authentication-type-pair> FORWARD_REJECT
   
      <optional reason for rejection> IAC SE

This command indicates that the credential forwarding was not successful, and if there is any more data in the suboption, it is an ASCII text message of the reason for the rejection.

3. Implementation Rules

If the second octet of the authentication-type-pair has the AUTH_WHO bit set to AUTH_CLIENT_TO_SERVER, then the client sends the initial AUTH command, and the server responds with either ACCEPT or REJECT. In addition, if the AUTH_HOW bit is set to AUTH_HOW_MUTUAL, the server will send a RESPONSE before it sends the ACCEPT.

If the second octet of the authentication-type-pair has the AUTH_WHO bit set to AUTH_SERVER_TO_CLIENT, then the server sends the initial AUTH command, and the client responds with either ACCEPT or REJECT. In addition, if the AUTH_HOW bit is set to AUTH_HOW_MUTUAL, the client will send a RESPONSE before it sends the ACCEPT.

The Kerberos principal used by the server will generally be of the form "host/<hostname>@realm". That is, the first component of the Kerberos principal is "host"; the second component is the fully qualified lower-case hostname of the server; and the realm is the Kerberos realm to which the server belongs.

Any Telnet IAC characters that occur in the KRB_AP_REQ or KRB_AP_REP messages, the KRB_CRED structure, or the optional rejection text string must be doubled as specified in [4]. Otherwise the following byte might be mis-interpreted as a Telnet command.

4. Examples

User "joe" may wish to log in as user "pete" on machine "foo". If "pete" has set things up on "foo" to allow "joe" access to his account, then the client would send IAC SB AUTHENTICATION NAME "pete" IAC SE IAC SB AUTHENTICATION IS KERBEROS_V5 AUTH <KRB_AP_REQ_MESSAGE> IAC SE

The server would then authenticate the user as "joe" from the KRB_AP_REQ_MESSAGE, and if the KRB_AP_REQ_MESSAGE was accepted by Kerberos, and if "pete" has allowed "joe" to use his account, the server would then continue the authentication sequence by sending a RESPONSE (to do mutual authentication, if it was requested) followed by the ACCEPT.

If forwarding has been requested, the client then sends IAC SB AUTHENTICATION IS KERBEROS_V5 CLIENT|MUTUAL FORWARD <KRB_CRED structure with credentials to be forwarded> IAC SE. If the server succeeds in reading the forwarded credentials, the server sends FORWARD_ACCEPT else, a FORWARD_REJECT is sent back. 

       Client                           Server
                                        IAC DO AUTHENTICATION
       IAC WILL AUTHENTICATION

[ The server is now free to request authentication information. 

         ]

IAC SB AUTHENTICATION SEND KERBEROS_V5 CLIENT|MUTUAL KERBEROS_V5 CLIENT|ONE_WAY IAC SE

[ The server has requested mutual Version 5 Kerberos

authentication. If mutual authentication is not supported, then the server is willing to do one-way 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
"pete" IAC SE
IAC SB AUTHENTICATION IS
KERBEROS_V5 CLIENT|MUTUAL AUTH
<KRB_AP_REQ message> IAC SE

[ Since mutual authentication is desired, the server sends across

a RESPONSE to prove that it really is the right server. ]

IAC SB AUTHENTICATION REPLY KERBEROS_V5 CLIENT|MUTUAL RESPONSE <KRB_AP_REP message> IAC SE

[ The server responds with an ACCEPT command to state that the

authentication was successful. ]

IAC SB AUTHENTICATION REPLY KERBEROS_V5 CLIENT|MUTUAL ACCEPT IAC SE

[ If so requested, the client now sends the FORWARD command to

forward credentials to the remote site. ]

IAC SB AUTHENTICATION IS KER-
BEROS_V5 CLIENT|MUTUAL
FORWARD <KRB_CRED message> IAC
SE

[ The server responds with a FORWARD_ACCEPT command to state that

the credential forwarding was successful. ]

IAC SB AUTHENTICATION REPLY KERBEROS_V5 CLIENT|MUTUAL FORWARD_ACCEPT IAC SE

5. Security Considerations

The selection of the random session key in the Kerberos V5 authenticator is critical, since this key will be used for encrypting the telnet data stream if encryption is enabled. It is strongly advised that the random key selection be done using cryptographic techniques that involve the Kerberos ticket's session key. For example, using the current time, encrypting it with the ticket session key, and then correcting for key parity is a strong way to generate a subsession key, since the ticket session key is assumed to be never disclosed to an attacker.

Care should be taken before forwarding a user's Kerberos credentials to the remote server. If the remote server is not trustworthy, this could result in the user's credentials being compromised. Hence, the user interface should not forward credentials by default; it would be far safer to either require the user to explicitly request credentials forwarding for each connection, or to have a trusted list of hosts for which credentials forwarding is enabled, but to not enable credentials forwarding by default for all machines.

The IAC SB AUTHENTICATION NAME name IAC SE message is unprotected in this protocol. Its contents should be verified by a secure method after authentication completes before it is used.

6. IANA Considerations

The authentication type KERBEROS_V5 and its associated suboption values are registered with IANA. Any suboption values used to extend the protocol as described in this document must be registered with IANA before use. IANA is instructed not to issue new suboption values without submission of documentation of their use.

7. Acknowledgments

This document was originally written by Dave Borman of Cray Research, Inc. Theodore Ts'o of MIT revised it to reflect the latest implementation experience. Cliff Neuman and Prasad Upasani of USC's Information Sciences Institute developed the credential forwarding support.

In addition, the contributions of the Telnet Working Group are also gratefully acknowledged.

8. References

   [1] Kohl, J. and B. Neuman, "The Kerberos Network Authentication
       System (V5)", RFC 1510, September 1993.
   
   [2] Ts'o, T. and J. Altman, "Telnet Authentication Option", RFC 2941,
       September 2000.
   
   [3] Ts'o, T., "Telnet Data Encryption Option", RFC 2946, September
       2000.
   
   [4] Postel, J. and J. Reynolds, "Telnet Option Specifications", STD
       8, RFC 855, May 1983.

9. Editor's Address

Theodore Ts'o
VA Linux Systems
43 Pleasant St.
Medford, MA 02155 

   Phone: (781) 391-3464
   EMail: tytso@mit.edu

10. Full Copyright Statement

Copyright © The Internet Society (2000). All Rights Reserved.

This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English.

The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

Funding for the RFC Editor function is currently provided by the Internet Society.