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IETF RFC 4643
Network News Transfer Protocol (NNTP) Extension for Authentication
Last modified on Thursday, October 19th, 2006
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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 © 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.
Vinocur, et al. Standards Track PAGE 1 
RFC 4643 NNTP Authentication October 2006
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
Vinocur, et al. Standards Track PAGE 2
RFC 4643 NNTP Authentication October 2006
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].
Vinocur, et al. Standards Track PAGE 3
RFC 4643 NNTP Authentication October 2006
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
Vinocur, et al. Standards Track PAGE 4
RFC 4643 NNTP Authentication October 2006
(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.
Vinocur, et al. Standards Track PAGE 5
RFC 4643 NNTP Authentication October 2006
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].
Vinocur, et al. Standards Track PAGE 6
RFC 4643 NNTP Authentication October 2006
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
Vinocur, et al. Standards Track PAGE 7
RFC 4643 NNTP Authentication October 2006
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.
Vinocur, et al. Standards Track PAGE 8
RFC 4643 NNTP Authentication October 2006
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.
Vinocur, et al. Standards Track PAGE 9
RFC 4643 NNTP Authentication October 2006
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.
Vinocur, et al. Standards Track PAGE 10
RFC 4643 NNTP Authentication October 2006
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
Vinocur, et al. Standards Track PAGE 11
RFC 4643 NNTP Authentication October 2006
[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
Vinocur, et al. Standards Track PAGE 12
RFC 4643 NNTP Authentication October 2006
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.
Vinocur, et al. Standards Track PAGE 13
RFC 4643 NNTP Authentication October 2006
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=
Vinocur, et al. Standards Track PAGE 14
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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[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
Vinocur, et al. Standards Track PAGE 15
RFC 4643 NNTP Authentication October 2006
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
Vinocur, et al. Standards Track PAGE 16
RFC 4643 NNTP Authentication October 2006
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)
Vinocur, et al. Standards Track PAGE 17
RFC 4643 NNTP Authentication October 2006
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.
Vinocur, et al. Standards Track PAGE 18
RFC 4643 NNTP Authentication October 2006
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.
Vinocur, et al. Standards Track PAGE 19
RFC 4643 NNTP Authentication October 2006
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.
Vinocur, et al. Standards Track PAGE 20
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.
Vinocur, et al. Standards Track PAGE 21
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.
Vinocur, et al. Standards Track PAGE 22
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
Vinocur, et al. Standards Track PAGE 23
RFC 4643 NNTP Authentication October 2006
Full Copyright Statement
Copyright © 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
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This document and the information contained herein are provided on an
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Acknowledgement
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Vinocur, et al. Standards Track PAGE 24
Network News Transfer Protocol (NNTP) Extension for Authentication
RFC TOTAL SIZE: 51411 bytes
PUBLICATION DATE: Thursday, October 19th, 2006
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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