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PROPOSED STANDARD
Errata Exist
Network Working Group                                         J. Vinocur
Request for Comments: 4643                            Cornell University
Updates: 2980                                               K. Murchison
Category: Standards Track                     Carnegie Mellon University
                                                            October 2006


                 Network News Transfer Protocol (NNTP)
                      Extension for Authentication

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 (C) The Internet Society (2006).

Abstract

   This document defines an extension to the Network News Transfer
   Protocol (NNTP) that allows a client to indicate an authentication
   mechanism to the server, to perform an authentication protocol
   exchange, and optionally to negotiate a security layer for subsequent
   protocol interactions during the remainder of an NNTP session.

   This document updates and formalizes the AUTHINFO USER/PASS
   authentication method specified in RFC 2980 and deprecates the
   AUTHINFO SIMPLE and AUTHINFO GENERIC authentication methods.
   Additionally, this document defines a profile of the Simple
   Authentication and Security Layer (SASL) for NNTP.
















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Table of Contents

   1. Introduction .............................................  3
      1.1. Conventions Used in This Document ...................  3
   2. The AUTHINFO Extension ...................................  4
      2.1. Advertising the AUTHINFO Extension ..................  4
      2.2. Authenticating with the AUTHINFO Extension ..........  5
      2.3. AUTHINFO USER/PASS Command ..........................  6
           2.3.1. Usage ........................................  7
           2.3.2. Description ..................................  7
           2.3.3. Examples .....................................  9
      2.4. AUTHINFO SASL Command ...............................  9
           2.4.1. Usage ........................................ 10
           2.4.2. Description .................................. 11
           2.4.3. Examples ..................................... 14
   3. Augmented BNF Syntax for the AUTHINFO Extension .......... 16
      3.1. Commands ............................................ 16
      3.2. Command Continuation ................................ 17
      3.3. Responses ........................................... 17
      3.4. Capability Entries .................................. 17
      3.5. General Non-terminals ............................... 18
   4. Summary of Response Codes ................................ 18
   5. Authentication Tracking/Logging .......................... 18
   6. Security Considerations .................................. 19
   7. IANA Considerations ...................................... 20
      7.1. IANA Considerations for SASL/GSSAPI Services ........ 20
      7.2. IANA Considerations for NNTP Extensions ............. 20
   8. Acknowledgements ......................................... 21
   9. References ............................................... 22
      9.1. Normative References ................................ 22
      9.2. Informative References .............................. 22




















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1.  Introduction

   Although NNTP [NNTP] has traditionally been used to provide public
   access to newsgroups, authentication is often useful for several
   purposes; for example, to control resource consumption, to allow
   abusers of the POST command to be identified, and to restrict access
   to "local" newsgroups.

   The ad-hoc AUTHINFO USER and AUTHINFO PASS commands, documented in
   [NNTP-COMMON], provide a very weak authentication mechanism in
   widespread use by the installed base.  Due to their ubiquity, they
   are formalized in this specification but (because of their
   insecurity) only for use in combination with appropriate security
   layers.

   The ad hoc AUTHINFO GENERIC command, also documented in [NNTP-COMMON]
   but much less ubiquitous, provided an NNTP-specific equivalent of the
   generic SASL [SASL] facility.  This document deprecates AUTHINFO
   GENERIC in favor of an AUTHINFO SASL replacement so that NNTP can
   benefit from authentication mechanisms developed for other SASL-
   enabled application protocols, including Simple Mail Transfer
   Protocol (SMTP) [SMTP-AUTH], Post Office Protocol (POP) [POP-AUTH],
   Internet Message Access Protocol (IMAP) [IMAP], Lightweight Directory
   Access Protocol (LDAP) [LDAP-AUTH], and Blocks Extensive Exchange
   Protocol (BEEP) [BEEP].

   This specification is to be read in conjunction with the NNTP base
   specification [NNTP].  Except where specifically stated otherwise, in
   the case of a conflict between these two documents, [NNTP] takes
   precedence over this one.

   It is also recommended that this specification be read in conjunction
   with the SASL base specification [SASL].

1.1.  Conventions Used in This Document

   The notational conventions used in this document are the same as
   those in [NNTP], and any term not defined in this document has the
   same meaning as it does in that one.

   The key words "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD NOT",
   "MAY", and "OPTIONAL" in this document are to be interpreted as
   described in "Key words for use in RFCs to Indicate Requirement
   Levels" [KEYWORDS].

   Terms related to authentication are defined in "On Internet
   Authentication" [AUTH].




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   In the examples, commands from the client are indicated with [C], and
   responses from the server are indicated with [S].

2.  The AUTHINFO Extension

   The AUTHINFO extension is used to authenticate a user.  Note that
   authorization is a matter of site policy, not network protocol, and
   therefore it is not discussed in this document.  The server
   determines authorization in whatever manner is defined by its
   implementation as configured by the site administrator.

   This extension provides three new commands: AUTHINFO USER, AUTHINFO
   PASS, and AUTHINFO SASL.  The capability label for this extension is
   AUTHINFO.

2.1.  Advertising the AUTHINFO Extension

   A server MUST implement at least one of the AUTHINFO USER or AUTHINFO
   SASL commands in order to advertise the "AUTHINFO" capability label
   in response to the CAPABILITIES command ([NNTP] Section 5.2).
   However, this capability MUST NOT be advertised after successful
   authentication (see Section 2.2).  This capability MAY be advertised
   both before and after any use of the MODE READER command ([NNTP]
   Section 5.3), with the same semantics.

   The AUTHINFO capability label contains an argument list detailing
   which authentication commands are available.

   The "USER" argument indicates that AUTHINFO USER/PASS is supported as
   defined by Section 2.3 of this document.  The "USER" argument MUST
   NOT be advertised, and the AUTHINFO USER/PASS commands SHOULD NOT be
   provided, unless a strong encryption layer (e.g., Transport Layer
   Security (TLS) [NNTP-TLS]) is in use or backward compatibility
   dictates otherwise.

   The "SASL" argument indicates that AUTHINFO SASL is supported as
   defined by Section 2.4 of this document.  If the server advertises
   the "SASL" argument, then it MUST also advertise the "SASL"
   capability in response to the CAPABILITIES command.  The SASL
   capability is followed by a whitespace-separated list of available
   SASL mechanism names.

   The server MAY list the AUTHINFO capability with no arguments, which
   indicates that it complies with this specification and does not
   permit any authentication commands in its current state.  In this
   case, the client MUST NOT attempt to utilize any AUTHINFO commands,
   even if it contains logic that might otherwise cause it to do so




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   (e.g., for backward compatibility with servers that are not compliant
   with this specification).

   Future extensions may add additional arguments to this capability.
   Unrecognized arguments MUST be ignored by the client.

   As the AUTHINFO command is related to security, cached results of
   CAPABILITIES from a previous session MUST NOT be relied on, as per
   Section 12.6 of [NNTP].  However, a client MAY use such cached
   results in order to detect active down-negotiation attacks.

   Example of AUTHINFO capabilities before and after the use of the
   STARTTLS [NNTP-TLS] extension:

      [C] CAPABILITIES
      [S] 101 Capability list:
      [S] VERSION 2
      [S] READER
      [S] IHAVE
      [S] STARTTLS
      [S] AUTHINFO SASL
      [S] SASL CRAM-MD5 DIGEST-MD5 GSSAPI
      [S] LIST ACTIVE NEWSGROUPS
      [S] .
      [C] STARTTLS
      [S] 382 Continue with TLS negotiation
      [TLS negotiation proceeds, further commands protected by TLS]
      [C] CAPABILITIES
      [S] 101 Capability list:
      [S] VERSION 2
      [S] READER
      [S] IHAVE
      [S] AUTHINFO USER SASL
      [S] SASL CRAM-MD5 DIGEST-MD5 GSSAPI PLAIN EXTERNAL
      [S] LIST ACTIVE NEWSGROUPS
      [S] .

2.2.  Authenticating with the AUTHINFO Extension

   An NNTP server responds to a client command with a 480 response to
   indicate that the client MUST authenticate and/or authorize in order
   to use that command or access the indicated resource.  Use of the
   AUTHINFO command as described below is one such way that a client can
   authenticate/authorize to the server.  The client MAY therefore use
   an AUTHINFO command after receiving a 480 response.  A client
   intending to use an AUTHINFO command SHOULD issue the CAPABILITIES
   command to obtain the available authentication commands and
   mechanisms before attempting authentication.



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   If a server advertises the AUTHINFO capability, a client MAY attempt
   the first step of authentication at any time during a session to
   acquire additional privileges without having received a 480 response.
   Servers SHOULD accept such unsolicited authentication requests.  A
   server MUST NOT under any circumstances reply to an AUTHINFO command
   with a 480 response.

   A client MUST NOT under any circumstances continue with any steps of
   authentication beyond the first, unless the response code from the
   server indicates that the authentication exchange is welcomed.  In
   particular, anything other than a 38x response code indicates that
   the client MUST NOT continue the authentication exchange.

   After a successful authentication, the client MUST NOT issue another
   AUTHINFO command in the same session.  A server MUST NOT return the
   AUTHINFO capability in response to a CAPABILITIES command, and a
   server MUST reject any subsequent AUTHINFO commands with a 502
   response.  Additionally, the client MUST NOT issue a MODE READER
   command after authentication, and a server MUST NOT advertise the
   MODE-READER capability.

   In agreement with [SASL], the server MUST continue to advertise the
   SASL capability in response to a CAPABILITIES command with the same
   list of SASL mechanisms that it did before authentication (thereby
   enabling the client to detect a possible active down-negotiation
   attack).  Other capabilities returned in response to a CAPABILITIES
   command received after authentication MAY be different from those
   returned before authentication.  For example, an NNTP server may not
   want to advertise support for a specific extension unless a client
   has been authenticated.

   Note that a server may perform a successful authentication exchange
   with a client and yet still deny access to some or all resources; the
   permanent 502 response indicates that a resource is unavailable even
   though authentication has been performed (this is in contrast to the
   temporary 480 error, which indicates that a resource is unavailable
   now but may become available after authentication).

2.3.  AUTHINFO USER/PASS Command

   This section supersedes the definition of the AUTHINFO USER and
   AUTHINFO PASS commands as documented in Section 3.1.1 of
   [NNTP-COMMON].








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2.3.1.  Usage

   These commands MUST NOT be pipelined.

   Syntax
     AUTHINFO USER username
     AUTHINFO PASS password

   Responses
     281 Authentication accepted
     381 Password required [1]
     481 Authentication failed/rejected
     482 Authentication commands issued out of sequence
     502 Command unavailable [2]

     [1] Only valid for AUTHINFO USER.  Note that unlike traditional 3xx
         codes, which indicate that the client may continue the current
         command, the legacy 381 code means that the AUTHINFO PASS
         command must be used to complete the authentication exchange.

     [2] If authentication has already occurred, AUTHINFO USER/PASS are
         not valid commands (see Section 2.2).

         NOTE: Notwithstanding Section 3.2.1 of [NNTP], the server MUST
         NOT return 480 in response to AUTHINFO USER/PASS.

   Parameters
     username = string identifying the user/client
     password = string representing the user's password

2.3.2.  Description

   The AUTHINFO USER and AUTHINFO PASS commands are used to present
   clear text credentials to the server.  These credentials consist of a
   username or a username plus a password (the distinction is that a
   password is expected to be kept secret, whereas a username is not;
   this does not directly affect the protocol but may have an impact on
   user interfaces).  The username is supplied through the AUTHINFO USER
   command, and the password through the AUTHINFO PASS command.

   If the server requires only a username, it MUST NOT give a 381
   response to AUTHINFO USER and MUST give a 482 response to AUTHINFO
   PASS.

   If the server requires both username and password, the former MUST be
   sent before the latter.  The server will need to cache the username
   until the password is received; it MAY require that the password be




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   sent in the immediately next command (in other words, only caching
   the username until the next command is sent).  The server:

   -  MUST return a 381 response to AUTHINFO USER;

   -  MUST return a 482 response to AUTHINFO PASS if there is no cached
      username;

   -  MUST use the argument of the most recent AUTHINFO USER for
      authentication; and

   -  MUST NOT return a 381 response to AUTHINFO PASS.

   The server MAY determine whether a password is needed for a given
   username.  Thus the same server can respond with both 381 and other
   response codes to AUTHINFO USER.

   Should the client successfully present proper credentials, the server
   issues a 281 reply.  If the server is unable to authenticate the
   client, it MUST reject the AUTHINFO USER/PASS command with a 481
   reply.  If an AUTHINFO USER/PASS command fails, the client MAY
   proceed without authentication.  Alternatively, the client MAY try
   another authentication mechanism or present different credentials by
   issuing another AUTHINFO command.

   The AUTHINFO PASS command permits the client to use a clear-text
   password to authenticate.  A compliant implementation MUST NOT
   implement this command without also implementing support for TLS
   [NNTP-TLS].  Use of this command without an active strong encryption
   layer is deprecated, as it exposes the user's password to all parties
   on the network between the client and the server.  Any implementation
   of this command SHOULD be configurable to disable it whenever a
   strong encryption layer (such as that provided by [NNTP-TLS]) is not
   active, and this configuration SHOULD be the default.  The server
   will use the 483 response code to indicate that the datastream is
   insufficiently secure for the command being attempted (see Section
   3.2.1 of [NNTP]).

   Note that a server MAY (but is not required to) allow white space
   characters in usernames and passwords.  A server implementation MAY
   blindly split command arguments at white space and therefore may not
   preserve the exact sequence of white space characters in the username
   or password.  Therefore, a client SHOULD scan the username and
   password for white space and, if any is detected, warn the user of
   the likelihood of problems.  The SASL PLAIN [PLAIN] mechanism is
   recommended as an alternative, as it does not suffer from these
   issues.




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   Also note that historically the username is not canonicalized in any
   way.  Servers MAY use the [SASLprep] profile of the [StringPrep]
   algorithm to prepare usernames for comparison, but doing so may cause
   interoperability problems with legacy implementations.  If
   canonicalization is desired, the SASL PLAIN [PLAIN] mechanism is
   recommended as an alternative.

2.3.3.  Examples

   Example of successful AUTHINFO USER:

      [C] AUTHINFO USER wilma
      [S] 281 Authentication accepted

   Example of successful AUTHINFO USER/PASS:

      [C] AUTHINFO USER fred
      [S] 381 Enter passphrase
      [C] AUTHINFO PASS flintstone
      [S] 281 Authentication accepted

   Example of AUTHINFO USER/PASS requiring a security layer:

      [C] AUTHINFO USER fred@stonecanyon.example.com
      [S] 483 Encryption or stronger authentication required

   Example of failed AUTHINFO USER/PASS:

      [C] AUTHINFO USER barney
      [S] 381 Enter passphrase
      [C] AUTHINFO PASS flintstone
      [S] 481 Authentication failed

   Example of AUTHINFO PASS before AUTHINFO USER:

      [C] AUTHINFO PASS flintstone
      [S] 482 Authentication commands issued out of sequence

2.4.  AUTHINFO SASL Command

   This section defines a formal profile of the Simple Authentication
   and Security Layer [SASL].  The use of the AUTHINFO GENERIC command
   as documented in Section 3.1.3 of [NNTP-COMMON], as a way to perform
   SASL authentication, is deprecated in favor of the AUTHINFO SASL
   command.  A server SHOULD NOT advertise AUTHINFO GENERIC in the list
   of capabilities returned by CAPABILITIES.





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2.4.1.  Usage

   This command MUST NOT be pipelined.

   Syntax
      AUTHINFO SASL mechanism [initial-response]

   This command MAY exceed 512 octets.  The maximum length of this
   command is increased to that which can accommodate the largest
   encoded initial response possible for any of the SASL mechanisms
   supported by the implementation.

   Responses
     281             Authentication accepted
     283 challenge   Authentication accepted (with success data) [1]
     383 challenge   Continue with SASL exchange [1]
     481             Authentication failed/rejected
     482             SASL protocol error
     502             Command unavailable [2]

     [1] These responses MAY exceed 512 octets.  The maximum length of
         these responses is increased to that which can accommodate the
         largest encoded challenge possible for any of the SASL
         mechanisms supported by the implementation.

     [2] If authentication has already occurred, AUTHINFO SASL is not a
         valid command (see Section 2.2).

         NOTE: Notwithstanding Section 3.2.1 of [NNTP], the server MUST
         NOT return 480 in response to AUTHINFO SASL.

   Parameters
     mechanism         = String identifying a [SASL] authentication
                         mechanism.
     initial-response  = Optional initial client response.
                         If present, the response MUST be encoded as
                         specified in Section 4 of [BASE64]. [3]
     challenge         = Server challenge.
                         The challenge MUST be encoded as specified
                         in Section 4 of [BASE64].

     [3] This argument MAY exceed 497 octets.  The maximum length of
         this argument is increased to that which can accommodate the
         largest encoded initial response possible for any of the SASL
         mechanisms supported by the implementation.






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2.4.2.  Description

   The AUTHINFO SASL command initiates a [SASL] exchange between the
   client and the server.  The client identifies the SASL mechanism to
   be used with the first parameter of the AUTHINFO SASL command.  If
   the server supports the requested authentication mechanism, it
   performs the SASL exchange to authenticate the user.  Optionally, it
   also negotiates a security layer for subsequent protocol interactions
   during this session.  If the requested authentication mechanism is
   invalid (e.g., is not supported), the server rejects the AUTHINFO
   SASL command with a 503 reply (see Section 3.2.1 of [NNTP]).  If the
   requested authentication mechanism requires an encryption layer, the
   server rejects the AUTHINFO SASL command with a 483 reply (see
   Section 3.2.1 of [NNTP]).

   The service name specified by this protocol's profile of SASL is
   "nntp".

   The SASL exchange consists of a series of server challenges and
   client responses that are specific to the chosen [SASL] mechanism.

   A server challenge is sent as a 383 reply with a single argument
   containing the [BASE64]-encoded string supplied by the SASL
   mechanism.  A server challenge that has zero length MUST be sent as a
   single equals sign ("=") and MUST be included (in order to comply
   with the [NNTP] requirement that responses always have the same
   number of arguments).

   A client response consists of a line containing a [BASE64]-encoded
   string.  A client response that has zero length MUST be sent as a
   single equals sign ("=") and MUST be included (for consistency with
   the server challenge format).  If the client wishes to cancel the
   authentication exchange, it issues a line with a single "*".  If the
   server receives such a response, it MUST reject the AUTHINFO SASL
   command by sending a 481 reply.

   Note that these [BASE64]-encoded strings can be much longer than
   normal NNTP responses.  Clients and servers MUST be able to handle
   the maximum encoded size of challenges and responses generated by
   their supported authentication mechanisms.  This requirement is
   independent of any line length limitations the client or server may
   have in other parts of its protocol implementation.

   The optional initial response argument to the AUTHINFO SASL command
   is used to save a round trip when using authentication mechanisms
   that support an initial client response.  If the initial response
   argument is omitted and the chosen mechanism requires an initial
   client response, the server MUST proceed as defined in section 5.1 of



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   [SASL].  In NNTP, a server challenge that contains no data is
   equivalent to a zero-length challenge and is encoded as a single
   equals sign ("=").

   Note that the [BASE64]-encoded initial response argument can exceed
   497 octets, and therefore that the AUTHINFO SASL command can exceed
   512 octets.  Clients SHOULD and servers MUST be able to handle the
   maximum encoded size of initial responses possible for their
   supported authentication mechanisms.  This requirement is independent
   of any command or argument length limitations the client or server
   may have in other parts of its protocol implementation.

   If use of the initial response argument would cause the AUTHINFO SASL
   command to exceed 512 octets, the client MAY choose to omit the
   initial response parameter (and instead proceed as defined in Section
   5.1 of [SASL]).

   If the client is transmitting an initial response of zero length, it
   MUST instead transmit the response as a single equals sign ("=").
   This indicates that the response is present, but that it contains no
   data.

   If the client uses an initial-response argument to the AUTHINFO SASL
   command with a SASL mechanism that does not support an initial client
   response, the server MUST reject the AUTHINFO SASL command with a 482
   reply.

   If the server cannot [BASE64] decode any client response, it MUST
   reject the AUTHINFO SASL command with a 504 reply (see Section 3.2.1
   of [NNTP]).  If the client cannot BASE64 decode any of the server's
   challenges, it MUST cancel the authentication using the "*" response.
   In particular, servers and clients MUST reject (and not ignore) any
   character not explicitly allowed by the BASE64 alphabet, and they
   MUST reject any sequence of BASE64 characters that contains the pad
   character ('=') anywhere other than the end of the string (e.g.,
   "=AAA" and "AAA=BBB" are not allowed).

   The authorization identity generated by this [SASL] exchange is a
   simple username, and both client and server MUST use the [SASLprep]
   profile of the [StringPrep] algorithm to prepare these names for
   transmission or comparison.  If preparation of the authorization
   identity fails or results in an empty string (unless it was
   transmitted as the empty string), the server MUST fail the
   authentication with a 481 reply.

   Should the client successfully complete the exchange, the server
   issues either a 281 or a 283 reply.  If the server is unable to
   authenticate the client, it MUST reject the AUTHINFO SASL command



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   with a 481 reply.  If an AUTHINFO SASL command fails, the client MAY
   proceed without authentication.  Alternatively, the client MAY try
   another authentication mechanism, or present different credentials by
   issuing another AUTHINFO command.

   If the SASL mechanism returns additional data on success (e.g.,
   server authentication), the NNTP server issues a 283 reply with a
   single argument containing the [BASE64]-encoded string supplied by
   the SASL mechanism.  If no additional data is returned on success,
   the server issues a 281 reply.

   If a security layer is negotiated during the SASL exchange, it takes
   effect for the client on the octet immediately following the CRLF
   that concludes the last response generated by the client.  For the
   server, it takes effect immediately following the CRLF of its success
   reply.

   When a security layer takes effect, the NNTP protocol is reset to the
   state immediately after the initial greeting response (see 5.1 of
   [NNTP]) has been sent, with the exception that if a MODE READER
   command has been issued, the effects of it (if any) are not reversed.
   The server MUST discard any knowledge obtained from the client, such
   as the current newsgroup and article number, that was not obtained
   from the SASL negotiation itself.  Likewise, the client SHOULD
   discard and MUST NOT rely on any knowledge obtained from the server,
   such as the capability list, that was not obtained from the SASL
   negotiation itself.  (Note that a client MAY compare the advertised
   SASL mechanisms before and after authentication in order to detect an
   active down-negotiation attack.)

   When both TLS [NNTP-TLS] and SASL security layers are in effect, the
   TLS encoding MUST be applied after the SASL encoding (the cleartext
   data is always SASL encoded first, and then the resultant data is TLS
   encoded).

   To ensure interoperability, client and server implementations of this
   extension MUST implement the [DIGEST-MD5] SASL mechanism.

   If AUTHINFO USER/PASS and AUTHINFO SASL are both implemented, the
   SASL [PLAIN] mechanism SHOULD also be implemented, as the
   functionality of DIGEST-MD5 is insufficient for some environments
   (e.g., the server may need to pass off the plaintext password to an
   external authentication service).  The SASL PLAIN mechanism is
   preferred over AUTHINFO USER, even if there is not a strong
   encryption layer active, because it eliminates limitations that
   AUTHINFO USER/PASS has with regards to the use of white space
   characters being used in usernames and passwords.




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2.4.3.  Examples

   Example of the [PLAIN] SASL mechanism under a TLS layer, using an
   initial client response:

      [C] CAPABILITIES
      [S] 101 Capability list:
      [S] VERSION 2
      [S] READER
      [S] STARTTLS
      [S] AUTHINFO SASL
      [S] SASL CRAM-MD5 DIGEST-MD5 GSSAPI
      [S] LIST ACTIVE NEWSGROUPS
      [S] .
      [C] STARTTLS
      [S] 382 Continue with TLS negotiation
      [TLS negotiation proceeds, further commands protected by TLS]
      [C] CAPABILITIES
      [S] 101 Capability list:
      [S] VERSION 2
      [S] READER
      [S] AUTHINFO USER SASL
      [S] SASL CRAM-MD5 DIGEST-MD5 GSSAPI PLAIN EXTERNAL
      [S] LIST ACTIVE NEWSGROUPS
      [S] .
      [C] AUTHINFO SASL PLAIN AHRlc3QAMTIzNA==
      [S] 281 Authentication accepted

   Example of the EXTERNAL SASL mechanism under a TLS layer, using the
   authorization identity derived from the client TLS certificate, and
   thus a zero-length initial client response (commands prior to
   AUTHINFO SASL are the same as the previous example and have been
   omitted):

      [C] AUTHINFO SASL EXTERNAL =
      [S] 281 Authentication accepted

   Example of the [DIGEST-MD5] SASL mechanism, which includes a server
   challenge and server success data (white space has been inserted for
   clarity; base64-encoded data is actually sent as a single line with
   no embedded white space):

      [C] AUTHINFO SASL DIGEST-MD5
      [S] 383 bm9uY2U9InNheUFPaENFS0dJZFBNSEMwd3RsZUxxT0ljT0kyd1FZSWU0
          enplQXR1aVE9IixyZWFsbT0iZWFnbGUub2NlYW5hLmNvbSIscW9wPSJhdXRo
          LGF1dGgtaW50LGF1dGgtY29uZiIsY2lwaGVyPSJyYzQtNDAscmM0LTU2LHJj
          NCxkZXMsM2RlcyIsbWF4YnVmPTQwOTYsY2hhcnNldD11dGYtOCxhbGdvcml0
          aG09bWQ1LXNlc3M=



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RFC 4643                  NNTP Authentication               October 2006


      [C] dXNlcm5hbWU9InRlc3QiLHJlYWxtPSJlYWdsZS5vY2VhbmEuY29tIixub25j
          ZT0ic2F5QU9oQ0VLR0lkUE1IQzB3dGxlTHFPSWNPSTJ3UVlJZTR6emVBdHVp
          UT0iLGNub25jZT0iMFkzSlFWMlRnOVNjRGlwK08xU1ZDMHJoVmcvLytkbk9J
          aUd6LzdDZU5KOD0iLG5jPTAwMDAwMDAxLHFvcD1hdXRoLWNvbmYsY2lwaGVy
          PXJjNCxtYXhidWY9MTAyNCxkaWdlc3QtdXJpPSJubnRwL2xvY2FsaG9zdCIs
          cmVzcG9uc2U9ZDQzY2Y2NmNmZmE5MDNmOWViMDM1NmMwOGEzZGIwZjI=
      [S] 283 cnNwYXV0aD1kZTJlMTI3ZTVhODFjZGE1M2Q5N2FjZGEzNWNkZTgzYQ==

   Example of a failed authentication due to bad [GSSAPI] credentials.
   Note that although the mechanism can utilize the initial response,
   the client chooses not to use it because of its length, resulting in
   a zero-length server challenge (here, white space has been inserted
   for clarity; base64-encoded data is actually sent as a single line
   with no embedded white space):

      [C] AUTHINFO SASL GSSAPI
      [S] 383 =
      [C] YIICOAYJKoZIhvcSAQICAQBuggInMIICI6ADAgEFoQMCAQ6iBwMFACAAAACj
          ggE/YYIBOzCCATegAwIBBaEYGxZURVNULk5FVC5JU0MuVVBFTk4uRURVoiQw
          IqADAgEDoRswGRsEbmV3cxsRbmV0bmV3cy51cGVubi5lZHWjge8wgeygAwIB
          EKEDAgECooHfBIHcSQfLKC8vm2i17EXmomwk6hHvjBY/BnKnvvDTrbno3198
          vlX2RSUt+CjuAKhcDcj4DW0gvZEqH7t5v9yWedzztlpaThebBat6hQNr9NJP
          ozh1/+74HUwhGWb50KtjuftO/ftQ8q0nTuYKgIq6PM4tp2ddo1IfpjfdNR9E
          95GFi3y1uBT7lQOwtQbRJUjPSO3ijdue9V7cNNVmYsBsqNsaHhvlBJEXf4WJ
          djH8yG+Dw/gX8fUTtC5fDpB5zLt01mkSXh6Wc4UhqQtwZBI2t/+TpX1okbg6
          Hr1ZZupeH6SByjCBx6ADAgEQooG/BIG8GnCmcXWtqhXh48dGTLHQgJ04K5Fj
          RMMq2qPSbiha9lq0osqR2KAnQA6LioWYxU+6yPKpBDSC5WOT441fUfkM8iAL
          kW3uNc+luFCGcnDsacrmoVU7Y6Akcp9m7Fm7orRc+TWSWPpBg3OR2oG3ATW0
          0NAz8TT06VOLVxIMUTINKdYVI/Ja7f3sy+/N4LGkJqScCQOwlo5tfDWn/UQF
          iTWo5Zw435rH8pjy2smQCnqC14v3NMAWTu4j+dzHUNw=
      [S] 481 Authentication error

   Example of a client aborting in the midst of an exchange:

      [C] AUTHINFO SASL GSSAPI
      [S] 383 =
      [C] *
      [S] 481 Authentication aborted as requested

   Example of attempting to use a mechanism that is not supported by the
   server:

      [C] AUTHINFO SASL EXAMPLE
      [S] 503 Mechanism not recognized







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   Example of attempting to use a mechanism that requires a security
   layer:

      [C] AUTHINFO SASL PLAIN
      [S] 483 Encryption or stronger authentication required

   Example of using an initial response with a mechanism that doesn't
   support it (the server must start the exchange when using
   [CRAM-MD5]):

      [C] AUTHINFO SASL CRAM-MD5 AHRlc3QAMTIzNA==
      [S] 482 SASL protocol error

   Example of an authentication that failed due to an incorrectly
   encoded response:

      [C] AUTHINFO SASL CRAM-MD5
      [S] 383 PDE1NDE2NzQ5My4zMjY4MzE3QHRlc3RAZXhhbXBsZS5jb20+
      [C] abcd=efg
      [S] 504 Base64 encoding error

3.  Augmented BNF Syntax for the AUTHINFO Extension

   This section describes the formal syntax of the AUTHINFO extension
   using ABNF [ABNF].  It extends the syntax in Section 9 of [NNTP], and
   non-terminals not defined in this document are defined there.  The
   [NNTP] ABNF should be imported first before attempting to validate
   these rules.

3.1.  Commands

   This syntax extends the non-terminal "command", which represents an
   NNTP command.

   command =/ authinfo-sasl-command /
        authinfo-user-command /
        authinfo-pass-command

   authinfo-sasl-command = "AUTHINFO" WS "SASL" WS mechanism
        [WS initial-response]
   authinfo-user-command = "AUTHINFO" WS "USER" WS username
   authinfo-pass-command = "AUTHINFO" WS "PASS" WS password

   initial-response = base64-opt
   username = 1*user-pass-char
   password = 1*user-pass-char
   user-pass-char = B-CHAR




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   NOTE: a server implementation MAY parse AUTHINFO USER and AUTHINFO
   PASS specially so as to allow white space to be used within the
   username or password.  Such implementations accept the additional
   syntax (making these two items inconsistent with "token" in Section
   9.8 of [NNTP]):

   user-pass-char =/ SP / TAB

   In doing so, the grammar can become ambiguous if the username or
   password begins or ends with white space.  To solve this ambiguity,
   such implementations typically treat everything after the first white
   space character following "USER"/"PASS", up to, but not including,
   the CRLF, as the username/password.

3.2.  Command Continuation

   This syntax extends the non-terminal "command-continuation", which
   represents the further material sent by the client in the case of
   multi-stage commands.

   command-continuation =/ authinfo-sasl-383-continuation

   authinfo-sasl-383-continuation = ("*" / base64-opt) CRLF

3.3.  Responses

   This syntax extends the non-terminal "initial-response-content",
   which represents an initial response line sent by the server.

   initial-response-content =/ response-283-content /
        response-383-content

   response-283-content = "283" SP base64
   response-383-content = "383" SP base64-opt

3.4.  Capability Entries

   This syntax extends the non-terminal "capability-entry", which
   represents a capability that may be advertised by the server.

   capability-entry =/ authinfo-capability /
        sasl-capability

   authinfo-capability = "AUTHINFO" *(WS authinfo-variant)
   authinfo-variant = "USER" / "SASL"
   sasl-capability = "SASL" 1*(WS mechanism)





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3.5.  General Non-terminals

   base64-opt = "=" / base64
   mechanism = 1*20mech-char
   mech-char = UPPER / DIGIT / "-" / "_"

4.  Summary of Response Codes

   This section contains a list of each new response code defined in
   this document and indicates whether it is multi-line, which commands
   can generate it, what arguments it has, and what its meaning is.

   Response code 281
      Generated by: AUTHINFO USER, AUTHINFO PASS, AUTHINFO SASL
      Meaning: authentication accepted

   Response code 283
      Generated by: AUTHINFO SASL
      1 argument: challenge
      Meaning: authentication accepted (with success data)

   Response code 381
      Generated by: AUTHINFO USER
      Meaning: password required via AUTHINFO PASS command.  Note
      that this code is used for backwards compatibility and does
      not conform to the traditional use of 3xx codes.

   Response code 383
      Generated by: AUTHINFO SASL
      1 argument: challenge
      Meaning: continue with SASL exchange

   Response code 481
      Generated by: AUTHINFO USER, AUTHINFO PASS, AUTHINFO SASL
      Meaning: authentication failed/rejected

   Response code 482
      Generated by: AUTHINFO USER, AUTHINFO PASS, AUTHINFO SASL
      Meaning: authentication commands issued out of sequence or
      SASL protocol error

5.  Authentication Tracking/Logging

   This section contains implementation suggestions and notes of best
   current practice; it does not specify further network protocol
   requirements.





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   Once authenticated, the authorization identity presented in the
   AUTHINFO exchange (username when using USER/PASS) SHOULD be included
   in an audit trail associating the identity with any articles supplied
   during a POST operation, and this configuration SHOULD be the
   default.  This may be accomplished, for example, by inserting headers
   in the posted articles or by a server logging mechanism.  The server
   MAY provide a facility for disabling the procedure described above,
   as some users or administrators may consider it a violation of
   privacy.

6.  Security Considerations

   Security issues are discussed throughout this memo.

   In general, the security considerations of [SASL] and any implemented
   SASL mechanisms are applicable here; only the most important are
   highlighted specifically below.  Also, this extension is not intended
   to cure the security considerations described in section 12 of
   [NNTP]; those considerations remain relevant to any NNTP
   implementation.

   Before the [SASL] negotiation has begun, any protocol interactions
   may have been performed in the clear and may have been modified by an
   active attacker.  For this reason, clients and servers MUST discard
   any sensitive knowledge obtained prior to the start of the SASL
   negotiation upon the establishment of a security layer.  Furthermore,
   the CAPABILITIES command SHOULD be re-issued upon the establishment
   of a security layer, and other protocol state SHOULD be re-negotiated
   as well.

   Servers MAY implement a policy whereby the connection is dropped
   after a number of failed authentication attempts.  If they do so,
   they SHOULD NOT drop the connection until at least 3 attempts at
   authentication have failed.

   Implementations MUST support a configuration where authentication
   mechanisms that are vulnerable to passive eavesdropping attacks (such
   as AUTHINFO USER/PASS and SASL [PLAIN]) are not advertised or used
   without the presence of an external security layer such as TLS
   [NNTP-TLS], and this configuration SHOULD be the default.

   When multiple authentication mechanisms are permitted by both client
   and server, an active attacker can cause a down-negotiation to the
   weakest mechanism.  For this reason, both clients and servers SHOULD
   be configurable to forbid use of weak mechanisms.  The minimum
   strength acceptable is a policy decision that is outside the scope of
   this specification.




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7.  IANA Considerations

7.1.  IANA Considerations for SASL/GSSAPI Services

   The IANA has registered the SASL/GSSAPI service name "nntp".  This
   service name refers to authenticated use of Usenet news service when
   it is provided via the [NNTP] protocol.

   o  Published Specification: This document.

   o  Contact for Further Information: Authors of this document.

   o  Change Controller: IESG <iesg@ietf.org>.

7.2.  IANA Considerations for NNTP Extensions

   This section gives a formal definition of the AUTHINFO extension, as
   required by Section 3.3.3 of [NNTP] for the IANA registry.

   o  This extension provides an extensible mechanism for NNTP
      authentication via a variety of methods.

   o  The capability label for this extension is "AUTHINFO".

   o  The "AUTHINFO" capability label has two possible optional
      arguments, "USER" and "SASL" (as defined in Section 2.1),
      indicating which variants of the AUTHINFO command are supported.

   o  This extension also provides the "SASL" capability label, whose
      arguments list the available SASL mechanisms.

   o  This extension defines three new commands, AUTHINFO USER, AUTHINFO
      PASS, and AUTHINFO SASL, whose behavior, arguments, and responses
      are defined in Sections 2.3 and 2.4.

   o  This extension does not associate any new responses with pre-
      existing NNTP commands.

   o  This extension may affect the overall behavior of both server and
      client in that the AUTHINFO SASL command may require that
      subsequent communication be transmitted via an intermediary
      security layer.

   o  The length of the AUTHINFO SASL command (as defined in this
      document) may exceed 512 octets.  The maximum length of this
      command is increased to that which can accommodate the largest
      initial response possible for any of the SASL mechanisms supported
      by the implementation.



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RFC 4643                  NNTP Authentication               October 2006


   o  This extension defines two new responses, 283 and 383, whose
      lengths may exceed 512 octets.  The maximum length of these
      responses is increased to that which can accommodate the largest
      challenge possible for any of the SASL mechanisms supported by the
      implementation.

   o  This extension does not alter pipelining, but AUTHINFO commands
      cannot be pipelined.

   o  Use of this extension may alter the capabilities list; once the
      AUTHINFO command has been used successfully, the AUTHINFO
      capability can no longer be advertised by CAPABILITIES.
      Additionally, the MODE-READER capability MUST NOT be advertised
      after successful authentication.

   o  This extension does not cause any pre-existing command to produce
      a 401, 480, or 483 response.

   o  This extension is unaffected by any use of the MODE READER
      command; however, the MODE READER command MUST NOT be used in the
      same session following successful authentication.

   o  Published Specification: This document.

   o  Contact for Further Information: Authors of this document.

   o  Change Controller: IESG <iesg@ietf.org>.

8.  Acknowledgements

   This RFC originated from a document initially written by Chris
   Newman.

   A significant amount of the authentication text was originally from
   the NNTP revision or common authentication specs written by Stan
   Barber.  A significant amount of the SASL text was lifted from the
   revisions to RFC 1734 and RFC 2554 by Rob Siemborski.

   Special acknowledgement also goes to Russ Allbery, Clive Feather, and
   others who commented privately on intermediate revisions of this
   document, as well as the members of the IETF NNTP Working Group for
   continual (yet sporadic) insight in discussion.









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RFC 4643                  NNTP Authentication               October 2006


9.  References

9.1.  Normative References

   [ABNF]        Crocker, D., Ed. and P. Overell, "Augmented BNF for
                 Syntax Specifications: ABNF", RFC 4234, October 2005.

   [AUTH]        Haller, N. and R. Atkinson, "On Internet
                 Authentication", RFC 1704, October 1994.

   [BASE64]      Josefsson, S., "The Base16, Base32, and Base64 Data
                 Encodings", RFC 4648, October 2006.

   [DIGEST-MD5]  Leach, P. and C. Newman, "Using Digest Authentication
                 as a SASL Mechanism", RFC 2831, May 2000.

   [KEYWORDS]    Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

   [NNTP]        Feather, C., "Network News Transfer Protocol (NNTP)",
                 RFC 3977, October 2006.

   [NNTP-TLS]    Murchison, K., Vinocur, J., and C. Newman, "Using
                 Transport Layer Security (TLS) with Network News
                 Transfer Protocol (NNTP)", RFC 4642, October 2006.

   [SASL]        Melnikov, A. and K. Zeilenga, "Simple Authentication
                 and Security Layer (SASL)", RFC 4422, June 2006.

   [SASLprep]    Zeilenga, K., "SASLprep: Stringprep Profile for User
                 Names and Passwords", RFC 4013, February 2005.

   [StringPrep]  Hoffman, P. and M. Blanchet, "Preparation of
                 Internationalized Strings ("stringprep")", RFC 3454,
                 December 2002.

9.2. Informative References

   [BEEP]        Rose, M., "The Blocks Extensible Exchange Protocol
                 Core", RFC 3080, March 2001.

   [CRAM-MD5]    Nerenberg, L., "The CRAM-MD5 SASL Mechanism", Work in
                 Progress.

   [GSSAPI]      Melnikov, A., "SASL GSSAPI mechanisms", Work in
                 Progress.





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RFC 4643                  NNTP Authentication               October 2006


   [IMAP]        Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
                 VERSION 4rev1", RFC 3501, March 2003.

   [LDAP-AUTH]   Harrison, R., "Lightweight Directory Access Protocol
                 (LDAP): Authentication Methods and Security
                 Mechanisms", RFC 4513, June 2006.

   [NNTP-COMMON] Barber, S., "Common NNTP Extensions", RFC 2980, October
                 2000.

   [PLAIN]       Zeilenga, K., Ed., "The PLAIN Simple Authentication and
                 Security Layer (SASL) Mechanism", RFC 4616, August
                 2006.

   [POP-AUTH]    Myers, J., "POP3 AUTHentication command", RFC 1734,
                 December 1994.

   [SMTP-AUTH]   Myers, J., "SMTP Service Extension for Authentication",
                 RFC 2554, March 1999.

Authors' Addresses

   Jeffrey M. Vinocur
   Department of Computer Science
   Upson Hall
   Cornell University
   Ithaca, NY 14853 USA

   EMail: vinocur@cs.cornell.edu


   Kenneth Murchison
   Carnegie Mellon University
   5000 Forbes Avenue
   Cyert Hall 285
   Pittsburgh, PA  15213 USA

   EMail: murch@andrew.cmu.edu













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Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM 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.

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   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
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   this document or the extent to which any license under such rights
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   Copies of IPR disclosures made to the IETF Secretariat and any
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   attempt made to obtain a general license or permission for the use of
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   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
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   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.

Acknowledgement

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).







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