





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 <iesg@ietf.org>.



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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: 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

























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Full Copyright Statement

Copyright (C) The Internet Society (2006).

   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
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Acknowledgement

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).







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