NNTP Extensions Working Group J. Vinocur
Internet Draft Cornell University
Expires: April 2005 K. Murchison
Oceana Matrix Ltd.
C. Newman
Sun Microsystems
October 2004
Using TLS with NNTP
draft-ietf-nntpext-tls-nntp-03.txt
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Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This memo defines an extension to the Network News Transport
Protocol [NNTP] to provide connection-based encryption (via
Transport Layer Security [TLS]). The primary goal is to provide
encryption for single-link confidentiality purposes, but data
integrity and (optional) certificate-based peer entity
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authentication are also possible.
Table of Contents
0. Changes from Previous Version ............................ 2
1. Introduction ............................................. 3
1.1. Conventions Used in this Document ................... 3
2. The STARTTLS Extension ................................... 3
2.1. Advertising the STARTTLS Extension .................. 3
2.2. STARTTLS Command .................................... 4
2.2.1. Usage .......................................... 4
2.2.2. Description .................................... 4
2.2.2.1. Processing After the STARTTLS Command ..... 5
2.2.2.2. Result of the STARTTLS Command ............ 6
2.2.3. Examples ....................................... 7
3. Augmented BNF Syntax for the STARTTLS Extension .......... 8
3.1. Commands ............................................ 8
3.2. LIST EXTENSIONS responses ........................... 8
4. Summary of Response Codes ................................ 8
5. Security Considerations .................................. 9
6. IANA Considerations ...................................... 10
7. References ............................................... 11
7.1. Normative References ................................ 11
7.2. Informative References .............................. 12
8. Authors' Addresses ....................................... 12
9. Acknowledgements ......................................... 12
10. Intellectual Property Rights ............................ 13
11. Copyright ............................................... 13
0. Changes from Previous Version
Changed:
o Reorganized sections to be more like AUTHINFO document.
o Replaced occurrences of "capability" with "extension label".
Clarified:
o 580 is an immediate response only. No response codes are sent
after TLS negotiation.
o Server behavior if it doesn't like the result of the TLS
negotiation.
o Man-in-the-middle attack can be launched by removing STARTTLS
extension label from the LIST EXTENSIONS response (382 response has
nothing to do with MiM attacks).
Other:
o Assorted updates of phrasing and typographical varieties
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1. Introduction
Historically, unencrypted NNTP [NNTP] connections were satisfactory
for most purposes. However, sending passwords unencrypted over the
network is no longer appropriate, and sometimes strong encryption
is desired for the entire connection.
The STARTTLS extension provides a way to use the popular TLS [TLS]
service with the existing NNTP protocol. The current
(unstandardized) use of TLS for NNTP is most commonly on a
dedicated TCP port; this practice is discouraged for the reasons
documented in section 7 of "Using TLS with IMAP, POP3 and ACAP"
[TLS-IMAPPOP]. Therefore, this specification formalizes and
extends the STARTTLS command already in occasional use by the
installed base.
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].
In the examples, commands from the client are indicated with [C],
and responses from the server are indicated with [S].
2. The STARTTLS Extension
This extension provides a new STARTTLS command and has the
extension label STARTTLS.
2.1. Advertising the STARTTLS Extension
A server supporting the STARTTLS command as defined in this
document will advertise the "STARTTLS" extension label in response
to the LIST EXTENSIONS command. However, this extension label is
not advertised after successful authentication [NNTP-AUTH], nor is
it advertised once a TLS layer is active (see section 2.2.2.2).
This extension label may be advertised both before and after any
use of MODE READER, with the same semantics.
As the STARTTLS command is related to security, cached results of
LIST EXTENSIONS from a previous session MUST NOT be relied on, as
per section 11.6 of [NNTP].
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Example:
[C] LIST EXTENSIONS
[S] 202 Extensions supported:
[S] OVER
[S] HDR
[S] LISTGROUP
[S] STARTTLS
[S] .
2.2. STARTTLS Command
2.2.1. Usage
This command MUST NOT be pipelined.
Syntax
STARTTLS
Responses
382 Continue with TLS negotiation
502 Command unavailable [1]
580 Can not initiate TLS negotiation
[1] If a TLS layer is already active, or authentication has
occurred, STARTTLS is not a valid command (see section 2.2.2.2).
NOTE: Notwithstanding section 3.2.1 of [NNTP], the server MUST NOT
return 483 in response to STARTTLS.
2.2.2. Description
A client issues the STARTTLS command to request negotiation of TLS.
The STARTTLS command is usually used to request session encryption,
although it can be used for client certificate authentication.
An NNTP server returns the 483 response to indicate that a secure
or encrypted connection is required for the command sent by the
client. Use of the STARTTLS command as described below is one way
to establish a connection with these properties. The client MAY
therefore send STARTTLS after receiving a 483 response; the client
also MAY decide to send STARTTLS without previously receiving a 483
response. Additionally, the server MUST NOT return 483 in response
to the STARTTLS command.
If the server is unable to initiate the TLS negotiation for any
reason (e.g. a server configuration or resource problem), the
server MUST reject the STARTTLS command with a 580 response.
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Otherwise, the server issues a 382 response and TLS negotiation
begins.
If the client receives a failure response to STARTTLS, the client
must decide whether or not to continue the NNTP session. Such a
decision is based on local policy. For instance, if TLS was being
used for client authentication, the client might try to continue
the session, in case the server allows it to do so even with no
authentication. However, if TLS was being negotiated for
encryption, a client that gets a failure response needs to decide
whether to continue without TLS encryption, to wait and try again
later, or to give up and notify the user of the error.
After receiving a 382 response to a STARTTLS command, the client
MUST start the TLS negotiation before giving any other NNTP
commands. The TLS negotiation begins with the first octet
following the CRLF of the 382 response. If, after having issued
the STARTTLS command, the client finds out that some failure
prevents it from actually starting a TLS handshake, then it SHOULD
immediately close the connection.
Servers MUST be able to understand backwards-compatible TLS Client
Hello messages (provided that client_version is TLS 1.0 or later),
and clients MAY use backwards-compatible Client Hello messages.
Neither clients or servers are required to actually support Client
Hello messages for anything other than TLS 1.0. However, the TLS
extension for Server Name Indication [TLS-EXT] SHOULD be
implemented by all clients; it also SHOULD be implemented by any
server implementing STARTTLS that is known by multiple names
(otherwise it is not possible for a server with several hostnames
to present the correct certificate to the client).
Although current use of TLS most often involves the dedication of
port 563 for NNTP over TLS, the continued use of TLS on a separate
port is discouraged for the reasons documented in section 7 of
"Using TLS with IMAP, POP3 and ACAP" [TLS-IMAPPOP].
2.2.2.1. Processing After the STARTTLS Command
After the TLS handshake has been completed, both parties MUST
immediately decide whether or not to continue based on the
authentication and privacy achieved (if any). The NNTP client and
server may decide to move ahead even if the TLS negotiation ended
with without authentication and/or without privacy because NNTP
services are often performed without authentication or privacy, but
some NNTP clients or servers may want to continue only if a
particular level of authentication and/or privacy was achieved.
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If the NNTP client decides that the level of authentication or
privacy is not high enough for it to continue, it SHOULD issue a
QUIT command immediately after the TLS negotiation is complete. If
the NNTP server decides that the level of authentication or privacy
is not high enough for it to continue, it SHOULD either reject
further NNTP commands from the client (other than a QUIT command)
with a 483 response code (possibly with a text string such as
"Command refused due to lack of security"), or reject a command
with a 400 response code (possibly with a text string such as
"Connection closing due to lack of security") and close the
connection.
The decision of whether or not to believe the authenticity of the
other party in a TLS negotiation is a local matter. However, some
general rules for the decisions are:
o The client MAY check that the identity presented in the server's
certificate matches the intended server hostname or domain.
This check is not required (and may fail in the absence of the
TLS server_name extension [TLS-EXT], as described above), but if
it is implemented and the match fails, the client SHOULD either
request explicit user confirmation, or terminate the connection
but allow the user to disable the check in the future.
o Generally an NNTP server would want to accept any verifiable
certificate from a client, however authentication can be done
using the client certificate (perhaps in combination with the
SASL EXTERNAL mechanism [NNTP-AUTH], although an implementation
supporting STARTTLS is not required to support SASL in general
or that mechanism in particular). The server MAY use
information about the client certificate for identification of
connections or posted articles (either in its logs or directly
in posted articles).
2.2.2.2. Result of the STARTTLS Command
Upon successful completion of the TLS handshake, the NNTP protocol
is reset to the state immediately after the initial greeting
response (see 5.1 of [NNTP]) has been sent. In this case, as no
greeting is sent, the next step is for the client to send a
command. The server MUST discard any knowledge obtained from the
client, such as the current newsgroup and article number, that was
not obtained from the TLS negotiation itself. The client MUST
discard any knowledge obtained from the server, such as the list of
NNTP service extensions, which was not obtained from the TLS
negotiation itself. The client SHOULD send a LIST EXTENSIONS
command as the first command after a successful TLS negotiation.
The extensions returned in response to a LIST EXTENSIONS command
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received after the TLS handshake MAY be different than the list
returned before the TLS handshake. For example, an NNTP server
supporting SASL [NNTP-AUTH] might not want to advertise support for
a particular mechanism unless a client has sent an appropriate
client certificate during a TLS handshake.
Both the client and the server MUST know if there is a TLS session
active. A client MUST NOT attempt to start a TLS session if a TLS
session is already active. A server MUST NOT return the STARTTLS
extension label in response to a LIST EXTENSIONS command received
after a TLS handshake has completed, and a server MUST respond with
a 502 response code if a STARTTLS command is received while a TLS
session is already active.
2.2.3. Examples
Example of a client being prompted to use encryption and
negotiating it successfully (showing the removal of STARTTLS from
the extensions list once a TLS layer is active), followed by a
successful selection of the group and an (inappropriate) attempt by
the client to initiate another TLS negotiation:
[C] LIST EXTENSIONS
[S] 202 Extensions supported:
[S] STARTTLS
[S] OVER
[S] .
[C] GROUP local.confidential
[S] 483 Encryption or stronger authentication required
[C] STARTTLS
[S] 382 Continue with TLS negotiation
[TLS negotiation occurs here]
[Following successful negotiation, traffic is via the TLS layer]
[C] LIST EXTENSIONS
[S] 202 Extensions supported:
[S] OVER
[S] .
[C] GROUP local.confidential
[S] 211 1234 3000234 3002322 local.confidential
[C] STARTTLS
[S] 502 STARTTLS not allowed with active TLS layer
Example of a request to begin TLS negotiation declined by the
server:
[C] STARTTLS
[S] 580 Can not initiate TLS negotiation
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Example of a failed attempt to negotiate TLS, followed by two
attempts at selecting groups only available under a security layer
(in the first case the server allows the session to continue, in
the second it closes the connection):
[C] STARTTLS
[S] 382 Continue with TLS negotiation
[TLS negotiation is attempted here]
[Following failed negotiation, traffic resumes without TLS]
[C] GROUP local.confidential
[S] 483 Encryption or stronger authentication required
[C] GROUP local.private
[S] 400 Closing connection due to lack of security
3. Augmented BNF Syntax for the STARTTLS Extension
This section describes the syntax of the STARTTLS 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 =/ starttls-command
starttls-command = "STARTTLS"
3.2. LIST EXTENSIONS responses
This syntax defines the specific LIST EXTENSIONS responses for the
STARTTLS extension.
extension-descriptor =/ starttls-extension
starttls-extension = %x53.54.41.52.54.54.4C.53 ; "STARTTLS"
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
generate it, what arguments it has, and what its meaning is.
Response code 382
Generated by: STARTTLS
Meaning: continue with TLS negotiation
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Response code 580
Generated by: STARTTLS
Meaning: can not initiate TLS negotiation
5. Security Considerations
In general, the security considerations of the TLS protocol [TLS]
and any implemented extensions [TLS-EXT] 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 11 of [NNTP]; those considerations remain
relevant to any NNTP implementation.
Use of STARTTLS cannot protect protocol exchanges conducted prior
to authentication. For this reason, the LIST EXTENSIONS command
SHOULD be re-issued after successful negotiation of a security
layer, and other protocol state SHOULD be re-negotiated as well.
It should be noted that NNTP is not an end-to-end mechanism. Thus,
if an NNTP client/server pair decide to add TLS privacy, they are
securing the transport only for that link. Further, because
delivery of a single piece of news may go between more than two
NNTP servers, adding TLS privacy to one pair of servers does not
mean that the entire NNTP chain has been made private. Further,
just because an NNTP server can authenticate an NNTP client, it
does not mean that the articles from the NNTP client were
authenticated by the NNTP client when the client received them.
Both the NNTP client and server must check the result of the TLS
negotiation to see whether an acceptable degree of authentication
and privacy was achieved. Ignoring this step completely
invalidates using TLS for security. The decision about whether
acceptable authentication or privacy was achieved is made locally,
is implementation-dependent, and is beyond the scope of this
document.
The NNTP client and server should note carefully the result of the
TLS negotiation. If the negotiation results in no privacy, or if
it results in privacy using algorithms or key lengths that are
deemed not strong enough, or if the authentication is not good
enough for either party, the client may choose to end the NNTP
session with an immediate QUIT command, or the server may choose
not to accept any more NNTP commands.
The client and server should also be aware that the TLS protocol
permits privacy and security capabilities to be renegotiated mid-
connection (see section 7.4.1 of [TLS]). For example, one of the
parties may desire minimal encryption after any authentication
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steps have been performed. This underscores the fact that security
is not present simply because TLS has been negotiated; the nature
of the established security layer must be considered.
A man-in-the-middle attack can be launched by deleting the STARTTLS
extension label in the LIST EXTENSIONS response from the server.
This would cause the client not to try to start a TLS session.
Another man-in-the-middle attack is to allow the server to announce
its STARTTLS extension label, but to alter the client's request to
start TLS and the server's response. An NNTP client can partially
protect against these attacks by recording the fact that a
particular NNTP server offers TLS during one session and generating
an alarm if it does not appear in the LIST EXTENSIONS response for
a later session (of course, the STARTTLS extension would not be
listed after a security layer is in place).
If the TLS negotiation fails or if the client receives a 483
response, the client has to decide what to do next. The client has
to choose among three main options: to go ahead with the rest of
the NNTP session, to retry TLS later in the session, or to give up
and postpone newsreading activity. If a failure or error occurs,
the client can assume that the server may be able to negotiate TLS
in the future, and should try to negotiate TLS in a later session.
However, if the client and server were only using TLS for
authentication and no previous 480 response was received, the
client may want to proceed with the NNTP session, in case some of
the operations the client wanted to perform are accepted by the
server even if the client is unauthenticated.
Before the TLS handshake has begun, any protocol interactions are
performed in the clear and may be modified by an active attacker.
For this reason, clients and servers MUST discard any sensitive
knowledge obtained prior to the start of the TLS handshake upon
completion of the TLS handshake.
6. IANA Considerations
This section gives a formal definition of the STARTTLS extension as
required by Section 8 of [NNTP] for the IANA registry.
o The STARTTLS extension provides connection-based encryption via
Transport Layer Security (TLS).
o The extension-label is "STARTTLS".
o The extension-label has no arguments.
o The extension defines one new command, STARTTLS, whose
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behaviour, arguments, and responses are defined in Section 2.2.
o The extension does not associate any new responses with pre-
existing NNTP commands.
o The extension does affect the overall behaviour of both server
and client, in that after successful use of the new command, all
communication is transmitted with the TLS layer as an
intermediary.
o The extension does not affect the maximum length of commands and
initial response lines.
o The extension does not alter pipelining, but the STARTTLS
command cannot be pipelined.
o Use of this extension does alter the output from LIST
EXTENSIONS; once the new command has been used successfully,
this extension can no longer be advertised by LIST EXTENSIONS.
o The extension does not cause any pre-existing command to produce
a 401, 480, or 483 response.
o The STARTTLS command can be used before or after the MODE READER
command, with the same semantics.
o Published Specification: This document.
o Author, Change Controller, and Contact for Further Information:
Author of this document.
7. References
7.1. Normative References
[ABNF] Crocker, D., Overell, P., "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[NNTP] Feather, C., "Network News Transport Protocol",
draft-ietf-nntpext-base-*.txt, Work in Progress.
[TLS] Dierks, T., Allen, C., "The TLS Protocol Version 1.0", RFC 2246,
January 1999.
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[TLS-EXT] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
Wright, T., "Transport Layer Security (TLS) Extensions", RFC 3546, June
2003.
[TLS-IMAPPOP] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
2595, June 1999.
7.2. Informative References
[NNTP-AUTH] Vinocur, J., Newman, C., Murchison, K., "NNTP Extension
for Authentication", draft-ietf-nntpext-auth-*.txt, Work in
Progress.
8. Authors' Addresses
Jeffrey M. Vinocur
Department of Computer Science
Upson Hall
Cornell University
Ithaca, NY 14853
EMail: vinocur@cs.cornell.edu
Kenneth Murchison
Oceana Matrix Ltd.
21 Princeton Place
Orchard Park, NY 14127 USA
Email: ken@oceana.com
Chris Newman
Sun Microsystems
1050 Lakes Drive, Suite 250
West Covina, CA 91790
EMail: cnewman@iplanet.com
9. Acknowledgements
A significant amount of the STARTTLS text was lifted from RFC 3207
by Paul Hoffman.
Special acknowledgement goes also to the people who commented
privately on intermediate revisions of this document, as well as
the members of the IETF NNTP Working Group for continual insight in
discussion.
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10. 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
pertain 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
proprietary 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
Executive Director.
11. Copyright
Copyright (C) The Internet Society (2004). 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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