| Independent Submission | J. Élie |
| Internet-Draft | January 4, 2017 |
| Intended status: Standards Track | |
| Expires: July 8, 2017 |
Modernization of RFC 4642
draft-elie-nntp-tls-recommendations-rfc4642bis-01
This document shows the sections that changed between RFC 4642 and draft-elie-nntp-tls-recommendations. The -00 version contains the wording in RFC 4642. The -01 version contains the wording in draft-elie-nntp-tls-recommendations-04.
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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, and
(optional) certificate-based peer entity authentication are also
possible.
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.
TCP port 563 is dedicated to NNTP over TLS, and registered in the
IANA Service Name and Transport Protocol Port Number Registry for
that usage. NNTP implementations using TCP port 563 begin the TLS
negotiation immediately upon connection and then continue with the
initial steps of an NNTP session. This immediate TLS negotiation
on a separate port (referred to in this document as "implicit
TLS") is the preferred way of using TLS with NNTP.
If a host wishes to offer separate servers for transit and reading
clients (Section 3.4.1 of [NNTP]), TCP port 563 SHOULD be used for
implicit TLS with the reading server, and an unused port of its
choice different than TCP port 433 SHOULD be used for implicit TLS
with the transit server. The ports used for implicit TLS should
be clearly communicated to the clients, and specifically that no
plain-text communication occurs before the TLS session is
negotiated.
As some existing implementations negotiate TLS via a dynamic
upgrade from unencrypted to TLS-protected traffic during an NNTP
session on well-known TCP ports 119 or 433, this specification
formalizes the STARTTLS command in use for that purpose. However,
as already mentioned above, implementations SHOULD use implicit
TLS on a separate port.
Note: a common alternative to protect NNTP exchanges with transit
servers that do not implement TLS is the use of IPsec with
encryption [RFC4301].
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.
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,
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.
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
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.
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
described in Section 12 of [NNTP]; those considerations remain
relevant to any NNTP implementation.
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 TLS negotiation, the client MUST verify the server's
identity in order to prevent man-in-the-middle attacks. The
client MUST follow the rules and guidelines defined in [RFC6125],
where the reference identifier MUST be the server hostname that
the client used to open the connection, and that is also specified
in the TLS "server_name" extension [RFC6066]. The following NNTP-
specific consideration applies: DNS domain names in server
certificates MAY contain the wildcard character "*" as the
complete left-most label within the identifier.
If the match fails, the client MUST follow the recommendations in
Section 6.6 of [RFC6125] regarding certificate pinning and
fallback.
Beyond server identity checking, clients also MUST apply the
procedures specified in [RFC5280] for general certificate
validation (e.g., certificate integrity, signing, and path
validation).
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.
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.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 5280, May 2008.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
January 2011.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service
Identity within Internet Public Key Infrastructure
Using X.509 (PKIX) Certificates in the Context of
Transport Layer Security (TLS)", RFC 6125, March 2011.
[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.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[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.