Network Working Group J. Vinocur
Internet Draft Cornell University
Document: draft-ietf-nntpext-authinfo-01.txt C. Newman
Sun Microsystems
K. Murchison
Oceana Matrix Ltd.
June 2004
NNTP Extension for Authentication
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
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Abstract
This document defines a profile of the Simple Authentication and
Security Layer [SASL] for the Network News Transport Protocol
[NNTP] protocol and updates/deprecates information contained in
Section 3.1 of [NNTP-COMMON]. This extension allows an NNTP client
to indicate an authentication mechanism to the server, perform an
authentication protocol exchange, and optionally negotiate a secu-
rity layer for subsequent protocol interactions during the remain-
der of an NNTP session.
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Table of Contents
1. Introduction ............................................. 2
1.1. Conventions Used in this Document ................... 3
2. The AUTHINFO Extension ................................... 3
2.1. AUTHINFO USER/PASS .................................. 5
2.1.2. Description .................................... 6
2.1.3. Examples ....................................... 7
2.2. AUTHINFO SASL ....................................... 8
2.2.1. Usage .......................................... 8
2.2.2. Description .................................... 8
2.2.3. Examples ....................................... 11
3. Augmented BNF Syntax for the AUTHINFO Extension .......... 13
3.1. Commands ............................................ 13
3.2. Command Continuation ................................ 14
3.3. Responses ........................................... 14
3.4. LIST EXTENSIONS responses ........................... 14
3.5. General non-terminals ............................... 14
4. Summary of Response Codes ................................ 15
5. Authentication Tracking/Logging .......................... 15
6. Security Considerations .................................. 16
7. IANA Considerations ...................................... 16
7.1. IANA Considerations for SASL/GSSAPI services ........ 17
7.2. IANA Considerations for NNTP extensions ............. 17
8. Normative References ..................................... 18
9. Informative References ................................... 19
10. Authors' Addresses ...................................... 19
11. Acknowledgments ......................................... 19
12. Intellectual Property Rights ............................ 20
13. Copyright ............................................... 20
1. Introduction
Although NNTP [NNTP] has traditionally provided public access to
newsgroups, authentication is often useful, 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 insecurity and
ubiquity they are formalized in this specification, but only for
use in combination with appropriate security layers.
The ad-hoc AUTHINFO GENERIC command, also documented in [NNTP-COM-
MON] 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
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NNTP can benefit from authentication mechanisms developed for other
SASL-enabled application protocols including SMTP, POP, IMAP, LDAP,
and 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 conjunc-
tion 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 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 Authen-
tication" [AUTH].
In the examples, commands from the client are indicated with [C],
and responses from the server are indicated with [S].
2. The AUTHINFO Extension
A server MAY provide this extension, independently of any other
extension defined elsewhere. If the server provides the extension,
it MUST include the AUTHINFO extension label in the response to
LIST EXTENSIONS. If it does not provide it, it MUST NOT include
the extension label. The remainder of this specification is written
as if the extension is provided.
This extension provides three new commands: AUTHINFO USER, AUTHINFO
PASS, and AUTHINFO SASL. At least one of the AUTHINFO USER or
AUTHINFO SASL commands MUST be implemented in order to advertise
the AUTHINFO extension label. The AUTHINFO extension label con-
tains an argument list detailing which authentication commands are
available.
The "USER" argument indicates that AUTHINFO USER/PASS is supported
as defined by Section 2.1 of this document. The "USER" argument
MUST NOT be advertised, and the AUTHINFO USER/PASS commands SHOULD
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NOT be provided, unless a strong encryption layer (e.g. TLS [NNTP-
TLS]) is in use or backward compatibility dictates otherwise.
The "SASL:" argument indicates that AUTHINFO SASL is supported as
defined by Section 2.2 of this document. The "SASL:" argument is
followed immediately (no intervening whitespace) by a comma-sepa-
rated list of available SASL mechanism names, and the colon (":")
is always included even if no mechanisms are available.
The server may list the AUTHINFO capability with no arguments,
which indicates that it complies with this draft and does not per-
mit 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 to do so (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 SHOULD be ignored or brought to the atten-
tion of the user.
Example:
[C] LIST EXTENSIONS
[S] 202 Extensions supported:
[S] STARTTLS
[S] AUTHINFO SASL:CRAM-MD5,DIGEST-MD5,GSSAPI
[S] .
[C] STARTTLS
[S] 382 Continue with TLS negotiation
[TLS negotiation proceeds, further commands protected by TLS layer]
[C] LIST EXTENSIONS
[S] 202 Extensions supported:
[S] AUTHINFO USER SASL:CRAM-MD5,DIGEST-MD5,GSSAPI,PLAIN,EXTERNAL
[S] .
The AUTHINFO extension is used to authenticate a user. Note that
authorization is a matter of site policy, not network protocol, and
is therefore not discussed in this document. The server determines
authorization in whatever manner is defined by its implementation
as configured by the site administrator.
When an NNTP server responds to a client command with a 480
response, this indicates the client MUST authenticate using the
AUTHINFO command in order to use that command or access the indi-
cated resource. A client intending to use AUTHINFO may issue the
LIST EXTENSIONS command to obtain the available authentication com-
mands and mechanisms before attempting authentication.
A client MAY attempt the first step of authentication at any time
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during a session to acquire additional privileges without having
received a 480 response. The client MUST NOT under any circum-
stances continue with any steps of authentication beyond the first,
unless the response code from the server indicates that the authen-
tication exchange is welcomed. In particular, anything other than
a 38x response code indicates that the client MUST NOT continue the
authentication exchange.
Servers are not required to accept unsolicited authentication
information from the client, therefore clients MUST accommodate
servers that reject such authentication information. Additionally,
servers may accept authentication information and yet still deny
access to some or all resources; the permanent 502 response indi-
cates a resource is unavailable even though authentication has been
performed (this is in contrast to the temporary 480 error indicat-
ing that a resource is unavailable now but may become available
after authentication).
After a successful authentication, the client may retry the origi-
nal command (if any) to which the server replied with the 480
response, or continue with some other command (for example, the
client may wish to re-fetch the list of newsgroups).
After an AUTHINFO command has been successfully completed, no more
AUTHINFO commands may be issued in the same session. After a suc-
cessful AUTHINFO command completes, a server MUST reject any fur-
ther AUTHINFO commands with a 502 response.
A server MUST NOT under any circumstances reply to an AUTHINFO com-
mand with a 480 response.
Note that a successful AUTHINFO command may cause the output of the
LIST EXTENSIONS command to change. Any successful authentication
MAY result in the server listing different arguments (perhaps list-
ing zero arguments) for AUTHINFO, but MUST NOT result in the
AUTHINFO capability being removed entirely from LIST EXTENSIONS (as
this might falsely indicate to clients that they were dealing with
a non-compliant server). Additionally, after a successful AUTHINFO
SASL, the SASL: capability MUST continue to be advertised as
described in section 2.2.2.
2.1. AUTHINFO USER/PASS
This section supersedes the definition of the AUTHINFO USER and
AUTHINFO PASS commands as documented in Section 3.1.1 of [NNTP-COM-
MON].
These commands MUST NOT be pipelined.
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2.1.1. Usage
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
[1] Only valid for AUTHINFO USER.
Parameters
username = UTF-8 string identifying the user/client
password = UTF-8 string representing the user's password
2.1.2. 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 while 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 user-
name until the password is received; it MAY require the password to
be 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;
- MUST NOT return a 381 response to AUTHINFO PASS.
The server MAY determine whether or not a password is needed based
on the username. Thus the same server can respond with both 381 and
other response codes to AUTHINFO USER.
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The AUTHINFO PASS command permits the client to use a clear-text
password to authenticate. A compliant implementation MUST NOT
implement this mechanism without also implementing support for TLS
[NNTP-TLS] or the DIGEST-MD5 SASL [DIGEST-MD5] authentication mech-
anism. Use of this mechanism without an active strong encryption
layer is deprecated as it exposes the user's password to all par-
ties on the network between the client and the server. Any imple-
mentation of this mechanism SHOULD be configurable to disable it
unless a strong encryption layer such as that provided by [NNTP-
TLS] is 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.
Usernames and passwords MUST use the UTF-8 [UTF-8] character set
and client MUST convert any user input to UTF-8 if necessary.
Note that usernames and passwords containing whitespace are quite
likely not to work as desired, due to the command argument syntax
[NNTP]. (A client may wish to scan the username and password for
whitespace, and if detected, warn the user of the likelihood of
problems.) The SASL PLAIN [PLAIN] mechanism is recommended as an
alternative, as it is more robust with regard to character set.
2.1.3. 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
[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
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[S] 481 Authentication failed
Example of AUTHINFO PASS before AUTHINFO USER:
[C] AUTHINFO PASS flintstone
[S] 482 Authentication commands issued out of sequence
2.2. AUTHINFO SASL
2.2.1. Usage
This command MUST NOT be pipelined.
Syntax
AUTHINFO SASL sasl-mech-name [sasl-init-resp]
Responses
281 Authentication accepted
283 sasl-server-chal Authentication accepted (with success data) [1]
383 sasl-server-chal Continue with SASL exchange [1]
481 Authentication failed/rejected
482 SASL protocol error
[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.
Parameters
sasl-mech-name = String identifying a [SASL] authentication mechanism
sasl-init-resp = Optional initial client response. If present, the
response MUST be encoded as specified in Section 3
of [BASE64].
sasl-server-chal = Server challenge. The challenge MUST be encoded as
specified in Section 3 of [BASE64].
2.2.2. Description
This section deprecates the definition of the AUTHINFO GENERIC com-
mand as documented in Section 3.1.3 of [NNTP-COMMON]. (An imple-
mentation MAY support AUTHINFO GENERIC for backward compatibility
and still be compliant with this specification. However, this doc-
ument does not provide a formal specification of AUTHINFO GENERIC,
and does not permit it to be reported in LIST EXTENSIONS.)
The AUTHINFO SASL command initiates a [SASL] authentication
exchange between the client and the server. The client identifies
the SASL mechanism to use with the first parameter of the AUTHINFO
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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 pro-
tocol interactions during this session. If the requested authenti-
cation mechanism is invalid (e.g. is not supported), the server
rejects the AUTHINFO SASL command with a 501 reply. If the
requested authentication mechanism requires an encryption layer,
the server rejects the AUTHINFO SASL command with a 483 reply.
The SASL authentication 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 mecha-
nism. If the server challenge has zero length, it MUST instead be
sent as a single equals sign ("="), making the complete response
"383 =".
A client response consists of a line containing a [BASE64] encoded
string. 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] 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 indepen-
dent 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 [SASL]. In NNTP, a server challenge that contains no data is
defined to be the same as a zero-length challenge as described
above.
Note that the AUTHINFO SASL command is still subject to the line
length limitations defined in [NNTP]. If use of the initial
response argument would cause the AUTHINFO SASL command to exceed
this length, the client MUST NOT use 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,
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it MUST instead transmit the response as a single equals sign
("="). This indicates that the response is present, but 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 send, 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 482 reply. 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 explic-
itly allowed by the BASE64 alphabet, and 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 trans-
mitted as the empty string), the server MUST fail the authentica-
tion with a 481 reply.
Should the client successfully complete the exchange, the server
issues either a 283 or 281 reply. If the server is unable to
authenticate the client, it MUST reject the AUTHINFO SASL command
with a 481 reply. If an AUTHINFO 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 server MUST discard any
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knowledge obtained from the client that was not obtained from the
SASL negotiation itself. Likewise, the client MUST discard any
knowledge obtained from the server, such as the list of NNTP exten-
sions, 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.)
After a security layer is established, the server MUST continue to
advertise the AUTHINFO capability and "SASL:" argument (with the
same mechanism list as before authentication) and SHOULD NOT adver-
tise the STARTTLS [NNTP-TLS] capability.
When both [TLS] and SASL security layers are in effect, the TLS
encoding MUST be applied after the SASL encoding, regardless of the
order in which the layers were negotiated.
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 function-
ality of DIGEST-MD5 is insufficient for some environments (e.g. the
server may need to pass the raw password off to an external authen-
tication 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 on the character set used for usernames and passwords.
The service name specified by this protocol's profile of SASL is
"nntp".
2.2.3. Examples
Example of the [PLAIN] SASL mechanism under a TLS layer, using an
initial client response:
[C] LIST EXTENSIONS
[S] 202 Extensions supported:
[S] STARTTLS
[S] AUTHINFO SASL:CRAM-MD5,DIGEST-MD5,GSSAPI
[S] .
[C] STARTTLS
[S] 382 Continue with TLS negotiation
[TLS negotiation proceeds, further commands protected by TLS layer]
[C] LIST EXTENSIONS
[S] 202 Extensions supported:
[S] AUTHINFO USER SASL:CRAM-MD5,DIGEST-MD5,GSSAPI,PLAIN,EXTERNAL
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[S] .
[C] AUTHINFO SASL PLAIN AHRlc3QAMTIzNA==
[S] 281 Authentication accepted
Example of the EXTERNAL SASL mechanism under a TLS layer, using the
derived authorization ID, and thus a zero-length initial client
response (commands prior to AUTHINFO SASL are the same as the pre-
vious 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 (whitespace has been inserted for
clarity; base64-encoded data is sent as a single line with no
embedded whitespace):
[C] AUTHINFO SASL DIGEST-MD5
[S] 383 bm9uY2U9IlBKUE9GczJKa05VYWhraDNjRmVUN2dZZjFKY0VJakVCSHRK
NzFycmNDMTg9IixyZWFsbT0iZWFnbGUub2NlYW5hLmNvbSIscW9wPSJhdXRo
IixtYXhidWY9NDA5NixjaGFyc2V0PXV0Zi04LGFsZ29yaXRobT1tZDUtc2Vz
cw==
[C] dXNlcm5hbWU9InRlc3QiLHJlYWxtPSJlYWdsZS5vY2VhbmEuY29tIixub25j
ZT0iUEpQT0ZzMkprTlVhaGtoM2NGZVQ3Z1lmMUpjRUlqRUJIdEo3MXJyY0Mx
OD0iLGNub25jZT0iUmVkV2VqM3JNdFY5U09XSE5BNUVtZFNmVWRFajNCMlpL
YTNIeFlHbzJCWT0iLG5jPTAwMDAwMDAxLHFvcD1hdXRoLG1heGJ1Zj0xMDI0
LGRpZ2VzdC11cmk9Im5ld3MvbG9jYWxob3N0IixyZXNwb25zZT0zOTg2NWIy
NTk0Nzk4ZjY4ZmY5ZWEwNDg1NGE2NGQ1ZQ==
[S] 283 cnNwYXV0aD0xYzc0NjdmMTY0OTQ3NmM4ZDJjNzM5ZTY4MjgwMzE2OA==
Example of a failed authentication due to bad [GSSAPI] credentials.
Note that while the mechanism can utilize the initial response, the
client does not send it because of the limitation on command
lengths, resulting in a zero-length server challenge (here whites-
pace has been inserted for clarity; base64-encoded data is sent as
a single line with no embedded whitespace):
[C] AUTHINFO SASL GSSAPI
[S] 383 =
[C] YIICOAYJKoZIhvcSAQICAQBuggInMIICI6ADAgEFoQMCAQ6iBwMFACAAAACj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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 Sequence successfully aborted
Example of attempting to use a mechanism that is not supported by
the server:
[C] AUTHINFO SASL EXAMPLE
[S] 501 Mechanism not recognized
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 (server must start the exchange):
[C] AUTHINFO SASL CRAM-MD5 AHRlc3QAMTIzNA==
[S] 482 SASL protocol error
3. Augmented BNF Syntax for the AUTHINFO Extension
This section describes the syntax of the AUTHINFO extension. It
extends the syntax in [NNTP], and non-terminals not defined in this
document are defined there.
3.1. Commands
This syntax extends the non-terminal "command", which represents an
NNTP command.
command /= authinfo-user-command /
authinfo-pass-command /
authinfo-sasl-command
authinfo-user-command = "AUTHINFO" WS "USER" WS username
authinfo-pass-command = "AUTHINFO" WS "PASS" WS password
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authinfo-sasl-command = "AUTHINFO" WS "SASL" WS sasl-mech-name
[WS sasl-init-resp]
username = 1*P-CHAR
password = 1*P-CHAR
sasl-init-resp = "=" / base64
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-continuation
authinfo-sasl-continuation = *([sasl-client-resp] CRLF)
; client waits for a "383" response from the server before
; sending each line
sasl-client-resp = "*" / base64
3.3. Responses
This syntax extends the non-terminal "simple-response-content" for
the various commands in this specification.
simple-response-content /= response-x83-content
response-x83-content = ("283" / "383") SP sasl-server-chal
sasl-server-chal = "=" / base64
3.4. LIST EXTENSIONS responses
This syntax defines the specific LIST EXTENSIONS responses for the
AUTHINFO extension.
extension-descriptor /= authinfo-extension
authinfo-extension = %x41.55.54.48.49.4E.46.4F ; "AUTHINFO"
*(SPA authinfo-extension-arg)
authinfo-extension-arg = "USER" /
"SASL:" [sasl-mech-name *("," sasl-mech-name)]
3.5. General non-terminals
sasl-mech-name = 1*20sasl-mech-char
sasl-mech-char = %x41-5A / DIGIT / "-" / "_"
; mechanism names restricted to uppercase letters,
; digits, "-" and "_"
base64 = *(4base64-char) [base64-terminal]
base64-terminal = 2base64-char "==" / 3base64-char "="
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base64-char = UPPER / LOWER / DIGIT / "+" / "/"
LOWER = %x61-7A
4. Summary of Response Codes
This section contains a list of every new response code defined in
this document, whether it is multi-line, which commands can gener-
ate 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: sasl-server-chal
Meaning: authentication accepted (with success data)
Response code 381
Generated by: AUTHINFO USER
Meaning: password required
Response code 383
Generated by: AUTHINFO SASL
1 argument: sasl-server-chal
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, and does not specify further network protocol
requirements.
When authentication succeeds, the server will create an "identity"
(the syntax resembles that of an email address) for the client
using a technique such as the following (note that when using SASL,
"username" corresponds to the authorization identity), for example:
(1) Lookup the supplied username in an implementation-specific
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database or directory to determine the primary email address
for that user.
(2) Use the username directly as the email address if it is fully
qualified (i.e., includes "@hostname"), otherwise append a con-
figured "default domain" based on the IP address the client
connected to.
(3) Use the username followed by "@" and then the result of a
reverse DNS lookup on the client's IP address. If the reverse
lookup fails, the domain literal syntax defined in SMTP [SMTP]
is appropriate.
Once authenticated, the server SHOULD be configurable to generate
an audit trail associating the authentication 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 con-
sider it a violation of privacy.
6. Security Considerations
Security issues are discussed throughout this memo.
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 dis-
card any knowledge obtained prior to the start of the SASL negotia-
tion upon the establishment of a security layer.
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].
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 weaker mechanisms.
7. IANA Considerations
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7.1. IANA Considerations for SASL/GSSAPI services
Please register the SASL/GSSAPI service name "nntp". This service
name refers to authenticated use of Usenet news service when pro-
vided via the [NNTP] protocol.
7.2. IANA Considerations for NNTP extensions
Below is a formal definition of the AUTHINFO extension as required
by Section 8 of [NNTP] for the IANA registry.
o This extension provides an extensible mechanism for NNTP authen-
tication via a variety of methods.
o The extension-label is "AUTHINFO".
o The extension-label has two possible optional arguments "USER"
and "SASL:" (as defined in Section 2) indicating which variants
of the AUTHINFO command are supported.
o The extension defines three new commands, AUTHINFO USER,
AUTHINFO PASS, and AUTHINFO SASL, whose behavior, arguments, and
responses are defined in Section 2.
o The extension does not associate any new responses with pre-
existing NNTP commands.
o The extension may affect the overall behavior of both server and
client, in that the AUTHINFO SASL command requires that subse-
quent communication to be transmitted via an intermediary secu-
rity layer.
o The extension does not affect the maximum length of commands or
of initial response lines of pre-existing responses.
o The 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
encoded challenge possible for any of the SASL mechanisms sup-
ported by the implementation.
o The extension does not alter pipelining, but AUTHINFO commands
cannot be pipelined.
o Use of this extension may alter the output from LIST EXTENSIONS.
Once any AUTHINFO command has been used successfully, the server
may alter the list of arguments for the AUTHINFO capability
(although the capability itself must still be listed, even with
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zero arguments). However, if a SASL security layer has been
negotiated, the server SHOULD continue to advertise the "SASL:"
argument with the same list of mechanisms, because the client
may wish to compare the pre- and post-authentication list of
SASL mechanisms in order to detect active down-negotiation
attacks.
o The extension does not cause any pre-existing command to produce
a 401, 480, or 483 response.
o The AUTHINFO commands can be used before or after the MODE
READER command, with the same semantics.
8. Normative References
[ABNF] Crocker, D., Overell, P., "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[AUTH] Haller, N., Atkinson, R., "On Internet Authentication", RFC 1704,
Bell Communications Research, October 1994.
[BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
[DIGEST-MD5] Leach, P., Newman, C., "Using Digest Authentication as a
SASL Mechanism", RFC 2831, May 2000.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, Harvard University, March 1997.
[NNTP] Feather, C., "Network News Transport Protocol",
draft-ietf-nntpext-base-*.txt, Work in Progress.
[NNTP-TLS] Vinocur, J., "Using TLS with NNTP",
draft-ietf-nntpext-tls-nntp-*.txt, Work in Progress.
[SASL] Melnikov, A., "Simple Authentication and Security Layer
(SASL)", draft-ietf-sasl-rfc2222bis-*.txt, Work in Progress.
[SASLprep] Zeilega, K., "SASLprep: Stringprep profile for user names
and passwords", draft-ietf-sasl-saslprep-*.txt, Work in Progress.
[StringPrep] Hoffman, P. and Blanchet, M., "Preparation of
Internationalized Strings ("stringprep")",
draft-hoffman-rfc3454bis-*.txt, Work in Progress.
[UTF-8] Yergeau, F. "UTF-8, a transformation format of ISO 10646", RFC
2279, Alis Technologies, January 1998.
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9. Informative References
[CRAM-MD5] Nerenberg, L., "The CRAM-MD5 SASL Mechanism", draft-
ietf-sasl-crammd5-*.txt, Work in Progress.
[GSSAPI] Melnikov, A., "SASL GSSAPI Mechanisms", draft-ietf-sasl-
gssapi-*.txt, Work in Progress.
[NNTP-COMMON] Barber, S., "Common NNTP Extensions", RFC 2980, Aca-
dem Consulting Services, October 2000.
[PLAIN] Zeilenga, K., "The Plain SASL Mechanism", draft-ietf-sasl-
plain-*.txt, Work in Progress.
[SMTP] Klensin, J., "Simple Mail Transport Protocol", RFC 2821,
AT&T Laboratories, April 2001.
10. Authors' Addresses
Jeffrey M. Vinocur
Department of Computer Science
Upson Hall
Cornell University
Ithaca, NY 14853 USA
Email: vinocur@cs.cornell.edu
Chris Newman
Sun Microsystems
1050 Lakes Drive, Suite 250
West Covina, CA 91790 USA
Email: cnewman@iplanet.com
Kenneth Murchison
Oceana Matrix Ltd.
21 Princeton Place
Orchard Park, NY 14127 USA
Email: ken@oceana.com
11. Acknowledgments
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
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revisions to RFC 1734 and RFC 2554 by Rob Siemborski.
Special acknowledgment 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.
12. Intellectual Property Rights
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to per-
tain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on
the IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances
of licenses to be made available, or the result of an attempt made
to obtain a general license or permission for the use of such pro-
prietary rights by implementers or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Execu-
tive Director.
13. Copyright
Copyright (C) The Internet Society (2004). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain
it or assist in its implementation may be prepared, copied, pub-
lished and distributed, in whole or in part, without restriction of
any kind, provided that the above copyright notice and this para-
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ever, this document itself may not be modified in any way, such as
by removing the copyright notice or references to the Internet
Society or other Internet organizations, except as needed for the
purpose of developing Internet standards in which case the proce-
dures for copyrights defined in the Internet Standards process must
be followed, or as required to translate it into languages other
than English.
The limited permissions granted above are perpetual and will not be
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Internet Draft NNTP Authentication June 2004
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on
an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGI-
NEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED,
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Vinocur, et. al. Expires December 2004 [Page 21]
