Network Working Group J. Vinocur
INTERNET DRAFT Cornell University
Document: draft-ietf-nntpext-tls-nntp-00.txt C. Newman
Sun Microsystems
February 2003
Using TLS with NNTP
Status of this memo
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
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
authentication are also described.
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Table of Contents
1. Introduction ............................................. 2
1.1. Conventions Used in this Document ................... 2
2. Advertising Capabilities with the Extensions Mechanism ... 3
3. Authentication Response Codes ............................ 3
4. STARTTLS Command ......................................... 4
4.1. STARTTLS Responses .................................. 5
4.2. Processing After the STARTTLS Command ............... 5
4.3. Result of the STARTTLS Command ...................... 6
4.4. STARTTLS Formal Syntax .............................. 6
5. MULTIDOMAIN Extension .................................... 7
6. Security Considerations .................................. 8
7. Acknowledgements ......................................... 9
8. Normative References ..................................... 9
9. Informative References ................................... 10
10. Authors' Addresses ...................................... 10
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 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].
This document assumes you are familiar with NNTP [NNTP] and TLS
[TLS].
In the examples, commands from the client are indicated with [C],
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and responses from the server are indicated with [S].
2. Advertising Capabilities with the Extensions Mechanism
The "LIST EXTENSIONS" command, documented in section 8 of [NNTP],
provides a mechanism for clients to discover what extensions are
available.
A server supporting the STARTTLS command as defined in section 4
will advertise the "STARTTLS" capability in response to the "LIST
EXTENSIONS" command issued when no TLS layer is active (see section
4.3).
A server supporting multiple domains as defined in section 5 will
advertise the "MULTIDOMAIN" capability in response to the "LIST
EXTENSIONS" command.
Example:
[C] LIST EXTENSIONS
[S] 202 Extensions supported:
[S] OVER
[S] PAT
[S] LISTGROUP
[S] STARTTLS
[S] MULTIDOMAIN
[S] .
Note that the STARTTLS command constitutes a mode changes and thus
clients MUST wait for completion prior to sending additional
commands.
3. Authentication Response Codes
An NNTP server MAY respond to any client command with a 483
response indicating that a strong encryption layer is required; in
general this response will be given to commands which may send
authentication data as plaintext. A client MAY react to a 483
response by establishing an encryption layer (for example, via
STARTTLS), though a 483 response is not required prior to
initiating encryption. The client also MAY try a different command
(for example, a type of authentication that would not risk password
compromise, QUIT, or any other command).
A server SHOULD only respond to a particular command as indicated
in this document, however a client MUST support unexpected response
codes by handling them based on the first digit as specified in
[NNTP].
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4. STARTTLS Command
STARTTLS [domain]
A client issues the STARTTLS command to request negotiation of TLS.
The client MUST NOT send any additional commands on the socket
until after it has received the server response to the command.
The STARTTLS command is usually used to request session encryption,
although it can be used for client certificate authentication. The
optional argument to STARTTLS is only permitted if the MULTIDOMAIN
extension is implemented as described in section 5.
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.
An NNTP server MAY require the client to perform a TLS negotiation
before accepting any commands. In this case, the server SHOULD
return the 483 encryption-required response code to every command
other than HELP, LIST EXTENSIONS, QUIT, and any commands that
establish encryption, such as STARTTLS; the server MUST NOT return
483 in response to those commands.
After receiving a 382 response to a STARTTLS command, the client
MUST start the TLS negotiation before giving any other NNTP
commands. 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.
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].
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4.1. STARTTLS Responses
382 Continue with TLS negotiation
403 TLS temporarily not available
501 Command not supported or command syntax error
580 Security layer already active
Clients MUST support other response codes by processing them based
on the first digit.
4.2. 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. The NNTP client and server may
decide to move ahead even if the TLS negotiation ended with no
authentication and/or no 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.
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 do at least one of
(1) close the connection, being aware that the client may interpret
this behavior as a network problem and immediately reconnect and
issue the same command sequence, or (2) keep the connection open
and reply to NNTP commands from the client with the 483 response
code (with a possible text string such as "Command refused due to
lack of security"), however this behavior may tie up resources
unacceptably.
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 is probably unwise unless the
MULTIDOMAIN extension defined in section 5 has been used), 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
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using the client certificate (perhaps in combination with the
SASL EXTERNAL mechanism [SASL-NNTP], although an implementation
supporting STARTTLS is not required to support that mechanism).
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).
4.3. Result of the STARTTLS Command
Upon completion of the TLS handshake, the NNTP protocol is reset to
the initial state (the state in NNTP directly after the connection
is established). The server MUST discard any knowledge obtained
from the client, such as the result of a previous authentication,
which was not obtained from the TLS negotiation itself; immediately
after the TLS handshake, the server MUST issue a welcome banner
(response code 200 or 201) without the client issuing any further
command. 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 extensions returned in response to a LIST EXTENSIONS command
received after the TLS handshake MAY be different than the list
returned before the TLS handshake. For example, an NNTP server
supporting SASL [SASL-NNTP] 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 in response to a LIST EXTENSIONS command received after a
TLS handshake has completed, and a server MUST respond with a 580
response code if a STARTTLS command is received while a TLS session
is already active.
4.4. STARTTLS Formal Syntax
This amends the formal syntax for NNTP [NNTP] to add the STARTTLS
command. The syntax is defined using ABNF [ABNF], including the
core rules from section 6 of [ABNF].
An optional domain argument is available. The MULTIDOMAIN
extension defined in section 5 describes when this argument may and
may not be sent. The syntax for the domain element is as defined
in section 4.1.2 of the revised SMTP specification [SMTP].
command /= starttls-command
starttls-command = "STARTTLS" [1*WSP domain] *WSP CRLF
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; domain is defined in sec. 4.1.2 of [SMTP]
; WSP and CRLF are defined in sec. 13 of [NNTP]
5. MULTIDOMAIN Extension
Note to implementors of this draft specification:
A facility analagous to the one described below may be
provided in a future extension to the TLS specification
[TLS-EXT]. Standardization of that facility would obsolete
the extension described below, meaning that MULTIDOMAIN may
be entirely removed in a future revision of this draft in
favor of the protocol-independent implementation.
Many modern Internet servers host several domain names using the
same IP address, and each domain might have its own TLS server
certificate. Unless the client communicates the domain name it is
using to the server, the server can only use the IP address to
which the client connected to determine the appropriate server
certificate to present; this interferes with the client's ability
to compare the domain name it expects to the one listed in the
certificate. (The HTTP protocol [HTTP] added the "Host" request-
header in order to resolve this issue.)
If the MULTIDOMAIN extension is advertised, then clients SHOULD
send the domain name used to connect to the server as the argument
to the STARTTLS command. Clients MAY send the domain argument
without checking the result of LIST EXTENSIONS, but if the server
does not implement this extension it may respond with a 501 error
code (even if a STARTTLS command without argument would have been
accepted). The server MAY decline to enter TLS negotation if it
supports this extension and the domain argument is not given. The
client SHOULD send a fully qualified domain whenever that
information is available.
The server MAY use the domain argument to select an appropriate
server certificate to present to the client during TLS, though the
method by which the server selects a certificate is beyond the
scope of this document. However, the server should be prepared to
receive STARTTLS commands that lack the domain argument.
The domain argument MUST be discarded following successful
negotiation as discussed in section 4.3 and therefore cannot be
used later to determine how to authenticate a client. However,
usernames of the form user@host provide a viable alternative for
this functionality.
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6. Security Considerations
In general, the security considerations of the TLS protocol [TLS]
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 14 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
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 382
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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 capability, 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 at a later time, 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.
7. 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.
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.
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[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, Harvard University, March 1997.
[NNTP] Barber, S., "Network News Transport Protocol"
(draft-ietf-nntpext-base-16.txt).
[SMTP] Klensin, J., "Simple Mail Transport Protocol", RFC 2821, AT&T
Laboratories, April 2001.
[TLS] Dierks, T., Allen, C., "The TLS Protocol Version 1.0", RFC 2246,
Certicom, January 1999.
[TLS-EXT] Blake-Wilson, S., Nystrom, M., "Transport Layer Security (TLS)
Extensions" (draft-ietf-tls-extensions-06.txt).
[TLS-IMAPPOP] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
2595, Innosoft, June 1999.
9. Informative References
[HTTP] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter,
L., Leach, P., Berners-Lee, T., "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.
[SASL-NNTP] Vinocur, J., Newman, C., "Using SASL with NNTP", Work
in Progress.
10. Authors' Addresses
Jeffrey M. Vinocur
Department of Computer Science
Upson Hall
Cornell University
Ithaca, NY 14853
EMail: vinocur@cs.cornell.edu
Chris Newman
Sun Microsystems
1050 Lakes Drive, Suite 250
West Covina, CA 91790
EMail: cnewman@iplanet.com
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