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rfc:rfc4642

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.

Murchison, et al. Standards Track [Page 1] RFC 4642 Using TLS with NNTP October 2006

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.

Murchison, et al. Standards Track [Page 2] RFC 4642 Using TLS with NNTP October 2006

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] .

Murchison, et al. Standards Track [Page 3] RFC 4642 Using TLS with NNTP October 2006

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,

Murchison, et al. Standards Track [Page 4] RFC 4642 Using TLS with NNTP October 2006

 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

Murchison, et al. Standards Track [Page 5] RFC 4642 Using TLS with NNTP October 2006

 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

Murchison, et al. Standards Track [Page 6] RFC 4642 Using TLS with NNTP October 2006

    [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

Murchison, et al. Standards Track [Page 7] RFC 4642 Using TLS with NNTP October 2006

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

Murchison, et al. Standards Track [Page 8] RFC 4642 Using TLS with NNTP October 2006

 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:
  1. 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.
  1. If a subjectAltName extension of type dNSName is present in the

certificate, it SHOULD be used as the source of the server's

    identity.
  1. Matching is case-insensitive.

Murchison, et al. Standards Track [Page 9] RFC 4642 Using TLS with NNTP October 2006

  1. 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.
  1. 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.

Murchison, et al. Standards Track [Page 10] RFC 4642 Using TLS with NNTP October 2006

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 <iesg@ietf.org>.

Murchison, et al. Standards Track [Page 11] RFC 4642 Using TLS with NNTP October 2006

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.

Murchison, et al. Standards Track [Page 12] RFC 4642 Using TLS with NNTP October 2006

Authors' Addresses

 Kenneth Murchison
 Carnegie Mellon University
 5000 Forbes Avenue
 Cyert Hall 285
 Pittsburgh, PA  15213 USA
 EMail: murch@andrew.cmu.edu
 Jeffrey M. Vinocur
 Department of Computer Science
 Upson Hall
 Cornell University
 Ithaca, NY  14853
 EMail: vinocur@cs.cornell.edu
 Chris Newman
 Sun Microsystems
 3401 Centrelake Dr., Suite 410
 Ontario, CA  91761
 EMail: Chris.Newman@sun.com

Murchison, et al. Standards Track [Page 13] RFC 4642 Using TLS with NNTP October 2006

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 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Acknowledgement

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Murchison, et al. Standards Track [Page 14]

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