Network Working Group K. Murchison
Request for Comments: 4642 Carnegie Mellon University
Category: Standards Track J. Vinocur
Cornell University
C. Newman
Sun Microsystems
October 2006
Using Transport Layer Security (TLS)
with Network News Transfer Protocol (NNTP)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This memo defines an extension to the Network News Transfer Protocol
(NNTP) that allows an NNTP client and server to use Transport Layer
Security (TLS). The primary goal is to provide encryption for
single-link confidentiality purposes, but data integrity, (optional)
certificate-based peer entity authentication, and (optional) data
compression are also possible.
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Table of Contents
1. Introduction ....................................................2
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
2.2.3. Examples ............................................6
3. Augmented BNF Syntax for the STARTTLS Extension .................8
3.1. Commands ...................................................8
3.2. Capability entries .........................................8
4. Summary of Response Codes .......................................8
5. Security Considerations .........................................8
6. IANA Considerations ............................................11
7. References .....................................................12
7.1. Normative References ......................................12
7.2. Informative References ....................................12
8. Acknowledgements ...............................................12
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 integrity and/or
confidentiality protection are desired for the entire connection.
The TLS protocol (formerly known as SSL) provides a way to secure an
application protocol from tampering and eavesdropping. Although
advanced SASL authentication mechanisms [NNTP-AUTH] can provide a
lightweight version of this service, TLS is complimentary to both
simple authentication-only SASL mechanisms and deployed clear-text
password login commands.
In some existing implementations, TCP port 563 has been dedicated to
NNTP over TLS. These implementations begin the TLS negotiation
immediately upon connection and then continue with the initial steps
of an NNTP session. This 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].
This specification formalizes the STARTTLS command already in
occasional use by the installed base. The STARTTLS command rectifies
a number of the problems with using a separate port for a "secure"
protocol variant; it is the preferred way of using TLS with NNTP.
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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 capability
label STARTTLS.
2.1. Advertising the STARTTLS Extension
A server supporting the STARTTLS command as defined in this document
will advertise the "STARTTLS" capability label in response to the
CAPABILITIES command ([NNTP] Section 5.2). However, this capability
MUST NOT be advertised once a TLS layer is active (see Section 2.2.2)
or after successful authentication [NNTP-AUTH]. This capability MAY
be advertised both before and after any use of the MODE READER
command ([NNTP] Section 5.3), with the same semantics.
As the STARTTLS 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].
Example:
[C] CAPABILITIES
[S] 101 Capability list:
[S] VERSION 2
[S] READER
[S] IHAVE
[S] STARTTLS
[S] LIST ACTIVE NEWSGROUPS
[S] .
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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 if authentication has
occurred, STARTTLS is not a valid command (see Section 2.2.2).
NOTE: Notwithstanding Section 3.2.1 of [NNTP], the server MUST NOT
return either 480 or 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 initiate session security,
although it can also be used for client and/or server certificate
authentication and/or data compression.
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 use the STARTTLS command after receiving a 483 response.
If a server advertises the STARTTLS capability, a client MAY attempt
to use the STARTTLS command at any time during a session to negotiate
TLS without having received a 483 response. Servers SHOULD accept
such unsolicited TLS negotiation requests.
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. Then, it
SHOULD either reject subsequent restricted NNTP commands from the
client with a 483 response code (possibly with a text string such as
"Command refused due to lack of security") or reject a subsequent
restricted command with a 400 response code (possibly with a text
string such as "Connection closing due to lack of security") and
close the connection. Otherwise, the server issues a 382 response,
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and TLS negotiation begins. A server MUST NOT under any
circumstances reply to a STARTTLS command with either a 480 or 483
response.
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.
Upon 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 for both the client and server 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 nor servers are required to actually support Client
Hello messages for anything other than TLS 1.0. However, the TLS
extension for Server Name Indication ("server_name") [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.)
If the TLS negotiation fails, both client and server SHOULD
immediately close the connection. Note that while continuing the
NNTP session is theoretically possible, in practice a TLS negotiation
failure often leaves the session in an indeterminate state;
therefore, interoperability can not be guaranteed.
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, with the exception that if a MODE
READER command has been issued, its effects (if any) are not
reversed. At this point, 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. Likewise, the client SHOULD discard and MUST NOT rely on any
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knowledge obtained from the server, such as the capability list,
which was not obtained from the TLS negotiation itself.
The server remains in the non-authenticated state, even if client
credentials are supplied during the TLS negotiation. The AUTHINFO
SASL command [NNTP-AUTH] with the EXTERNAL mechanism [SASL] MAY be
used to authenticate once TLS client credentials are successfully
exchanged, but servers supporting the STARTTLS command are not
required to support AUTHINFO in general or the EXTERNAL mechanism in
particular. The server MAY use information from the client
certificate for identification of connections or posted articles
(either in its logs or directly in posted articles).
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
capability label in response to a CAPABILITIES 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. Additionally, the client MUST NOT issue a MODE
READER command while a TLS session is active, and a server MUST NOT
advertise the MODE-READER capability.
The capability list returned in response to a CAPABILITIES command
received after a successful TLS handshake MAY be different from 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.
2.2.3. Examples
Example of a client being prompted to use encryption and negotiating
it successfully (showing the removal of STARTTLS from the capability
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] CAPABILITIES
[S] 101 Capability list:
[S] VERSION 2
[S] READER
[S] STARTTLS
[S] LIST ACTIVE NEWSGROUPS OVERVIEW.FMT
[S] OVER
[S] .
[C] GROUP local.confidential
[S] 483 Encryption or stronger authentication required
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[C] STARTTLS
[S] 382 Continue with TLS negotiation
[TLS negotiation occurs here]
[Following successful negotiation, traffic is protected by TLS]
[C] CAPABILITIES
[S] 101 Capability list:
[S] VERSION 2
[S] READER
[S] LIST ACTIVE NEWSGROUPS OVERVIEW.FMT
[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
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). Note that unrestricted commands
such as CAPABILITIES are unaffected by the failure:
[C] STARTTLS
[S] 382 Continue with TLS negotiation
[TLS negotiation is attempted here]
[Following failed negotiation, traffic resumes without TLS]
[C] CAPABILITIES
[S] 101 Capability list:
[S] VERSION 2
[S] READER
[S] STARTTLS
[S] LIST ACTIVE NEWSGROUPS OVERVIEW.FMT
[S] OVER
[S] .
[C] GROUP local.confidential
[S] 483 Encryption or stronger authentication required
[C] GROUP local.private
[S] 400 Closing connection due to lack of security
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3. Augmented BNF Syntax for the STARTTLS Extension
This section describes the formal syntax of the STARTTLS 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 =/ starttls-command
starttls-command = "STARTTLS"
3.2. Capability entries
This syntax extends the non-terminal "capability-entry", which
represents a capability that may be advertised by the server.
capability-entry =/ starttls-capability
starttls-capability = "STARTTLS"
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 382
Generated by: STARTTLS
Meaning: continue with TLS negotiation
Response code 580
Generated by: STARTTLS
Meaning: can not initiate TLS negotiation
5. Security Considerations
Security issues are discussed throughout this memo.
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
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described in Section 12 of [NNTP]; those considerations remain
relevant to any NNTP implementation.
NNTP client and server implementations MUST implement the
TLS_RSA_WITH_RC4_128_MD5 [TLS] cipher suite and SHOULD implement the
TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA [TLS] cipher suite. This is
important, as it assures that any two compliant implementations can
be configured to interoperate. All other cipher suites are OPTIONAL.
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 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.
Note that NNTP is not an end-to-end mechanism. Thus, if an NNTP
client/server pair decide to add TLS confidentiality, they are
securing the transport only for that link. Similarly, because
delivery of a single Netnews article may go between more than two
NNTP servers, adding TLS confidentiality to one pair of servers does
not mean that the entire NNTP chain has been made private.
Furthermore, 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 itself received them
(prior to forwarding them to the server).
During the TLS negotiation, the client MUST check its understanding
of the server hostname against the server's identity as presented in
the server Certificate message, in order to prevent man-in-the-middle
attacks. Matching is performed according to these rules:
- The client MUST use the server hostname it used to open the
connection (or the hostname specified in TLS "server_name"
extension [TLS-EXT]) as the value to compare against the server
name as expressed in the server certificate. The client MUST NOT
use any form of the server hostname derived from an insecure
remote source (e.g., insecure DNS lookup). CNAME canonicalization
is not done.
- If a subjectAltName extension of type dNSName is present in the
certificate, it SHOULD be used as the source of the server's
identity.
- Matching is case-insensitive.
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- A "*" wildcard character MAY be used as the left-most name
component in the certificate. For example, *.example.com would
match a.example.com, foo.example.com, etc., but would not match
example.com.
- If the certificate contains multiple names (e.g., more than one
dNSName field), then a match with any one of the fields is
considered acceptable.
If the match fails, the client SHOULD either ask for explicit user
confirmation or terminate the connection with a QUIT command and
indicate the server's identity is suspect.
Additionally, clients MUST verify the binding between the identity of
the servers to which they connect and the public keys presented by
those servers. Clients SHOULD implement the algorithm in Section 6
of [PKI-CERT] for general certificate validation, but MAY supplement
that algorithm with other validation methods that achieve equivalent
levels of verification (such as comparing the server certificate
against a local store of already-verified certificates and identity
bindings).
A man-in-the-middle attack can be launched by deleting the STARTTLS
capability label in the CAPABILITIES response from the server. This
would cause the client not to try to start a TLS session. Another
man-in-the-middle attack would allow the server to announce its
STARTTLS capability, but 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 CAPABILITIES response for a later session. (Of course,
the STARTTLS capability would not be listed after a security layer is
in place.)
If the client receives a 483 or 580 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 (re)try
TLS later in the session, or to give up and postpone
newsreading/transport activity. If an 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.
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6. IANA Considerations
This section gives a formal definition of the STARTTLS extension as
required by Section 3.3.3 of [NNTP] for the IANA registry.
o The STARTTLS extension provides connection-based security via the
Transport Layer Security (TLS).
o The capability label for this extension is "STARTTLS".
o The capability label has no arguments.
o This extension defines one new command, STARTTLS, whose behavior,
arguments, and responses are defined in Section 2.2.
o This extension does not associate any new responses with pre-
existing NNTP commands.
o This extension does affect the overall behavior of both server and
client, in that after successful use of the STARTTLS command, all
communication is transmitted with the TLS protocol as an
intermediary.
o This extension does not affect the maximum length of commands or
initial response lines.
o This extension does not alter pipelining, but the STARTTLS command
cannot be pipelined.
o Use of this extension does alter the capabilities list; once the
STARTTLS command has been used successfully, the STARTTLS
capability can no longer be advertised by CAPABILITIES.
Additionally, the MODE-READER capability MUST NOT be advertised
after a successful TLS negotiation.
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 a successful TLS negotiation.
o Published Specification: This document.
o Contact for Further Information: Authors of this document.
o Change Controller: IESG <
[email protected]>.
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7. References
7.1. Normative References
[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", RFC 4234, October 2005.
[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.
[PKI-CERT] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[TLS] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.1", RFC 4346, April
2006.
[TLS-EXT] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen,
J., and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, April 2006.
7.2. Informative References
[NNTP-AUTH] Vinocur, J., Murchison, K., and C. Newman, "Network
News Transfer Protocol (NNTP) Extension for
Authentication", RFC 4643, October 2006.
[SASL] Melninov, A., Ed. and K. Zeilenga, Ed, "Simple
Authentication and Security Layer (SASL)", RFC 4422,
June 2006.
[TLS-IMAPPOP] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
2595, June 1999.
8. Acknowledgements
A significant amount of the text in this document was lifted from RFC
2595 by Chris Newman and 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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Authors' Addresses
Kenneth Murchison
Carnegie Mellon University
5000 Forbes Avenue
Cyert Hall 285
Pittsburgh, PA 15213 USA
EMail:
[email protected]
Jeffrey M. Vinocur
Department of Computer Science
Upson Hall
Cornell University
Ithaca, NY 14853
EMail:
[email protected]
Chris Newman
Sun Microsystems
3401 Centrelake Dr., Suite 410
Ontario, CA 91761
EMail:
[email protected]
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Murchison, et al. Standards Track [Page 14]
