Internet DRAFT - draft-ellermann-spf-eai
draft-ellermann-spf-eai
Network Working Group F. Ellermann
Internet-Draft xyzzy
Intended status: Experimental November 12, 2011
Expires: May 15, 2012
Sender Policy Framework: Email Address Internationalization
draft-ellermann-spf-eai-06
Abstract
UTF8SMTP is an extension of SMTP (Simple Mail Transfer Protocol)
allowing the use of UTF-8 in the SMTP envelope for EAI (Email Address
Internationalization) and message headers. This memo discusses the
consequences for SPF (Sender Policy Framework).
Editorial note
This note, the IANA considerations (Section 6), and the document
history (Appendix A) should be removed before publication. For some
imported terms in Section 1.1 the sources have to be fixed. The
draft can be discussed on the <mailto:spf-discuss@listbox.com>
mailing list.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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This Internet-Draft will expire on May 15, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Imported terms . . . . . . . . . . . . . . . . . . . . . . 4
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. SPF HELO identity . . . . . . . . . . . . . . . . . . . . 4
2.2. EAI MAIL FROM identity . . . . . . . . . . . . . . . . . . 5
2.3. Local parts . . . . . . . . . . . . . . . . . . . . . . . 6
3. Details . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Considerations for SPF publishers . . . . . . . . . . . . 8
3.2. Received-SPF . . . . . . . . . . . . . . . . . . . . . . . 8
3.3. Internationalization of explanations . . . . . . . . . . . 9
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Normative References . . . . . . . . . . . . . . . . . . . 11
7.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Document History . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
Readers should be familiar with SMTP as specified in [RFC5321] and
the SPF terminology in [I-D.kitterman-4408bis].
The keywords "MUST NOT" and "SHOULD" in this memo are to be
interpreted as described in [RFC2119].
For an EAI (Email Address Internationalization) overview see
[RFC4952]. UTF-8 is specified in [RFC3629]. An MTA is a Mail
Transfer Agent, e.g., an SMTP relay.
1.1. Imported terms
Some [RFC5234] ABNF terms used in this memo are specified in other
sources:
<domain-spec> = <see 4408bis section 8.1>
<target-name> = <see 4408bis section 4.8>
<explanation> = <see 4408bis section 6.2>
<textstring> = <see RFC5321 section 4.2>
<Dot-string> = <see RFC5321 section 4.1.2>
<Quoted-string> = <see RFC5321 section 4.1.2>
<Atom> = <see RFC5321 section 4.1.2>
<atext> = <see RFC5322 section 3.2.3>
<UTF8-non-ascii> = <see RFC5336 section 2.3>
<uMailbox> = <see RFC5336 section 2.3>
2. Background
SMTP as specified in [RFC5321] supports only ASCII addresses and LDH
(letter, digit, hyphen) domain labels. The letters are ASCII
letters; certain LDH-labels are also known as A-labels in the context
of IDN (Internationalization of Domain Names) and [RFC5890].
2.1. SPF HELO identity
In SMTP sessions after an SMTP EHLO command from the client the
server response can indicate supported SMTP extensions. [RFC5336]
specifies the UTF8SMTP extension.
The SMTP client can accept an offered UTF8SMTP extension by using one
of the specified features, notably by the use of UTF-8 in mailbox
addresses of SMTP commands, by the use of alternative ASCII addresses
in these commands, or by the use of UTF-8 in the message header for
addresses and other purposes, i.e. by sending a "message/global"
instead of a "message/rfc822" as specified in [RFC5335].
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Because UTF8SMTP support is indicated in the response to an EHLO
command it cannot be used after HELO, and the SPF HELO identity is
not affected by EAI: The domain in a HELO or EHLO command consists of
ordinary LDH-labels, or it is a domain literal. For an empty reverse
path, as it is used in non-delivery reports and other auto-replies,
SPF fabricates a MAIL FROM identity based on the HELO identity with a
case insensitive local part "postmaster"; this scenario is also not
affected by EAI.
A domain consisting of LDH-labels including IDN A-labels beginning
with "xn--" is an ordinary LDH-domain as far as DNS (Domain Name
System), SPF, and UTF8SMTP are concerned. Apart from HELO and EHLO
the only relevant SMTP command for SPF is the MAIL FROM command with
the reverse path containing the envelope sender address (if it is not
empty, see above). When the derived MAIL FROM identity is an
ordinary address SPF can handle it as specified in
[I-D.kitterman-4408bis].
2.2. EAI MAIL FROM identity
The interesting UTF8SMTP cases for SPF contain non-ASCII UTF-8
characters in the local part (left hand side) or the domain part
(right hand side) of the MAIL FROM identity. Domain labels
containing non-ASCII UTF-8 characters are also known as U-labels in
IDN.
SPF checks typically make only sense at the "border MTA", and this is
normally an MX (Mail eXchanger) of the receiver talking with a
sender. An MTA wishing to check the SPF sender policy against the IP
of the sender fetches the sender policy for the domain in the HELO or
MAIL FROM identity as DNS client of a server for the alleged sender.
The SPF terminology might be confusing: The border MTA is the SMTP
server, but for the purpose of checking a sender policy it is the SPF
(or rather DNS) client of a name server for the alleged sender with a
given HELO or MAIL FROM identity.
An MTA could "downgrade" EAI MAIL FROM addresses using an optional
alternative address given as UTF8SMTP MAIL-parameter. Where that
happens the resulting new MAIL FROM address is an ordinary [RFC5321]
reverse path and can be handled as usual.
Skipping all ordinary cases as noted above the SPF client confronted
with an EAI address in the MAIL FROM identity is generally an MTA
supporting UTF8SMTP, and supposed to know how to transform U-labels
into corresponding A-labels, e.g., because it might need to send a
non-delivey report to the envelope sender address later; see
[RFC5337].
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For agents trying post-SMTP SPF-checks this might be not the case,
and unsurprisingly attempts to fetch the sender policy of a domain
with U-labels "as is" will fail with SPF result NONE. Arguably this
is a broken setup, the border MTA should not offer and accept
UTF8SMTP mails if critical parts behind it - not limited to the
mailbox of the receiver - don't support EAI.
2.3. Local parts
Top down at this point the remaining SPF clients are supposed to know
how to transform U-labels into A-labels, and fetch the SPF policy of
the alleged sender. SPF implementors and publishers of SPF sender
policies should note that only the domain part of the MAIL FROM
identity is transformed from U-labels into A-labels. The local part
MUST NOT be transformed, it is used "as is" in the construction of a
<target-name> by SPF macro expansion involving local part macros.
SPF allows all octets in labels of a <target-name> excluding dots,
which are supposed to separate labels. Sender policies can directly
talk about any <domain-spec> with labels separated by dots, where
each label consists of 1 to 63 visible ASCII characters except "%"
introducing macros. The macros "%%" and "%_" in a <domain-spec>
expand into "%" or space in the <target-name>, respectively.
Normally sender policies do not use such slightly odd labels, and the
most extravagant case is "_" (underscore) in a label that is
intentionally no LDH-label. Nevertheless implementations have to
support such oddities, because they are needed in the case of a
<target-name> derived from a <domain-spec> using the local part
macro.
SMTP local parts can have two forms, <Dot-string> or <Quoted-string>.
A <Dot-string> consists of dot separated <Atom>s, each <Atom>
consists of one or more <atext> characters. Please note that an
<Atom> is not the same as an LDH-label, it is also not the same as a
domain label, e.g., <Atom>s can be longer than 63 octets.
By definition there are no leading, trailing, or adjacent dots in a
<Dot-string>. [RFC5321] and its predecessor recommend to avoid the
<Quoted-string> form of a local part. Current SPF implementations
are known to strip the quotes from a <Quoted-string> for the purpose
of determining a <target-name> derived from a <domain-spec> using the
local part macro. This can result in an invalid <target-name> with
leading, trailing, or adjacent dots, e.g., for a mail address
"do..ts"@example.org.
Publishers of sender policies using the local part macro SHOULD make
sure that the used pieces of valid local parts in their domain can be
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parsed into non-empty domain labels; one way to achieve this is to
avoid <Quoted-string>. The effect of a <Quoted-string> local part is
not clearly specified in [I-D.kitterman-4408bis]. In theory DNS
supports any octet, even "embedded" dots within a label. In practice
current SPF implementations cannot handle embedded dots, and it is
far from clear that quoted pairs introduced by a "\" (backslash) in a
<Quoted-string> are interpreted as specified in [RFC5321] section
4.1.2.
Publishers of sender policies using the local part macro SHOULD make
sure that the used pieces of valid local parts in their domain result
in 1 to 63 octets per dot separated domain label as mentioned in
[I-D.kitterman-4408bis] section 8.1. Please note that the truncation
of longer labels after macro expansion is not clearly specified: SPF
implementations could truncate longer labels left to right or right
to left, they could also ignore affected directives, or treat this
case as error.
Publishers of sender policies using the local part macro need to be
aware that ASCII letters in the used pieces of valid local parts in
their domain are in essence treated as case-insensitive by DNS as
explained in [RFC4343].
UTF8SMTP extends <atext> by <UTF8-non-ascii>, and it also permits
<UTF8-non-ascii> in quoted strings. As far as SPF is concerned
<UTF8-non-ascii> can result in non-ASCII octets in a <target-name>,
working "as is" in DNS labels with similar caveats as noted above
with respect to the length of labels, case sensitivity, and
normalization.
[RFC5335] suggests to restrict the use of UTF-8 in EAI addresses to
Normalization Form C (NFC) as recommended in [RFC5198]. Publishers
of sender policies using the local part macro need to be aware that
SPF implementations treat local parts "as is". Mapping different
forms of an EAI local part to one mailbox at their border MTAs has no
effect on different forms of EAI local parts in DNS queries. A
straight forward strategy to avoid potential issues with respect to
SPF is to use local part macros only in non-critical explanations and
maybe for logging, if at all.
3. Details
For historical reasons technical SPF [Errata] have been "outsourced".
SPF implementors are hopefully also interested in the SPF test suite
on the same site.
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3.1. Considerations for SPF publishers
Policy publishers should know that this memo does not update
[I-D.kitterman-4408bis], in theory EAI is compatible with SPF. It is
not possible to use U-labels in sender policies directly, they have
to be transformed into the corresponding A-labels. Likewise U-labels
in UTF8SMTP MAIL FROM addresses are transformed into A-labels for the
purposes of SPF by implementations supporting EAI.
SPF implementations not supporting U-labels in MAIL FROM identities
will return NONE instead of the intended result, e.g., PASS or FAIL.
UTF8SMTP senders wishing to avoid this problem could transform MAIL
FROM U-labels into A-labels on their side. They could also hope that
spammers forging MAIL FROM identities will not abuse IDN U-labels in
the near future, and that most SPF implementations will be updated
before this changes. Unfortunately experience has shown that
spammers learn faster than lazy users.
MAIL FROM identities using only A-labels, with or without UTF-8 in
the local part, work "as is" for the purposes of SPF. HELO
identities consist either of A-labels, are domain literals and
irrelevant for SPF, or are syntactically malformed as far as UTF8SMTP
and SPF are concerned. SPF does not specify how receivers should
handle SMTP syntax errors.
UTF8SMTP allows to specify an optional alternative address in the
traditional syntax. Receivers are free to check SPF also or only
based on the alternative address. Obviously a sender policy for the
alternative address should permit the same sending IPs as the sender
policy for the EAI address, and one simple way to achieve this is to
use corresponding A-labels in an alternative address yielding one SPF
sender policy for both addresses. Please note that this is not
required by UTF8SMTP, it permits to use unrelated domains with
different policies. Clearly if some IPs permitted for one address
fail for the other address, or vice versa, the sender will have
problems, if the affected IPs are actually used to send mails.
3.2. Received-SPF
The Received-SPF header field specified in [I-D.kitterman-4408bis]
section 7 is a "message/rfc822" trace header field.
UTF8SMTP transports a "message/global" as specified in [RFC5335]
permitting the use of UTF-8 in header fields. For Received-SPF this
is necessary to record an UTF8SMTP "envelope-from" <uMailbox>
address.
UTF-8 might be also needed in comments and other parts of this header
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field in conjunction with UTF8SMTP. See [RFC5335] for the
corresponding syntax modifications.
SPF implementations could check the EAI MAIL FROM *and* an
alternative address (if given). In this case SPF implementations
SHOULD record both results. An exception could be to omit a "less
interesting" (e.g., equivalent, NONE, or NEUTRAL) result. Receivers
could also adopt the strategy to check the second of two addresses
only if the result of their first check is "not helpful" (e.g., NONE
or NEUTRAL).
3.3. Internationalization of explanations
SMTP replies consist of a reply code and optionally a <textstring> as
explanation. This is the vehicle used by SMTP servers if they reject
a mail after an SPF FAIL, optionally adding an explanation given in
the SPF policy as <explanation>.
The <textstring> is limited to ASCII and hopefully integrated in some
way in any resulting non-delivery report (bounce) created by the SMTP
client. UTF8SMTP does *not* modify this restriction to ASCII
<textstring>s in contexts relevant for SPF.
The optional SPF <explanation> is in essence a pointer to a domain
with a TXT record. The macro-expanded content of this TXT record can
be used in the SMTP reply.
The resulting explanation can contain a URL of a Web page with
internationalized explanations including data based on the sending
IP, sender address, and other details given as SPF macros in the TXT
record.
The upper-case forms of SPF macros trigger URL-encoding for this
purpose. SPF macro expansion does not involve to parse textual
explanations for potential URLs, it percent-encodes any "reserved"
input character. For EAI these characters can be UTF-8 and have to
be percent-encoded as specified in [RFC3987]. This boils down to
"take octets as is, if not unreserved percent-encode".
The procedure outlined above clearly cannot guarantee to produce
valid URLs in an explanation. In fact any use of lower-case macros
could result in non-ASCII explanations, and in that case the SMTP
server cannot use the result in the <textstring>s of its reply after
an SPF FAIL.
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4. Acknowledgements
Various folks on the SPF discuss and devel mailing lists worked on
the SPF [Errata] and test suite. All obscure issues with local parts
discussed in this memo are based on their prior work and indirectly
related discussions on the EAI and SMTP mailing lists.
Thanks to Abel Yang, Alessandro Vesely, Boyd Lynn Gerber, Doug Otis,
Harald T. Alvestrand, John C. Klensin, Julian Mehnle, Kari Hurtta,
Kazunori Fujiwara, Scott Kitterman, Stuart D. Gathman, and
Wayne Schlitt for their encouragement, feedback, or critique.
5. Security Considerations
When UTF8SMTP senders use a different domain in the optional
alternative MAIL FROM address, and the corresponding sender policy is
also different, it is hard to predict which policy will be checked,
if any, depending on the route to the receiver and other factors.
Different results can be hard to diagnose, e.g., if a mail from the
same sender to the same receiver sometimes results in PASS and at
other times in FAIL. One proposal to mitigate this problem is
discussed in Section 3.1.
Not all SPF implementations will already support U-labels as
explained in this memo. Senders could transform U-labels in MAIL
FROM commands to A-labels on their side where this is a problem.
Using the SPF local part macro in conjunction with EAI is not
intuitive, local parts are not transformed to A-labels. This is not
new, but in conjunction with EAI very likely. Various details with
respect to label lengths, quoted strings, adjacent dots, and quoted
pairs are explained in Section 2.3. Not using <Quoted-string>s as
local parts and not using case-sensitive local parts is recommended
in [RFC5321] section 4.1.2; this also covers quoted pairs and
adjacent dots.
A new UTF8SMTP problem is the use of local part macros for the
construction of "per user" policies, when different variations of an
UTF-8 local part correspond to one user mailbox. While the use of
normalized UTF-8 strings is recommended in [RFC5335] section 4.1 this
does not help with case-sensitive <UTF8-non-ascii> variations. One
way to address this problem is to avoid critical uses of the local
part macro as discussed in Section 2.3.
UTF8SMTP servers can be forced to send non-delivery reports to forged
envelope sender addresses, if some receiver mailboxes can handle EAI
mails, while others can't, and the server has no way to "downgrade"
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mails to traditional receivers. Hopefully a future SMTP extension
will allow a kind of "selective reject" mechanism. Publishing SPF
PASS or FAIL policies, and rejecting FAIL at the border MTA, would
eliminate this problem. Similarly non-delivery reports after a PASS
cannot hit innocent bystanders.
Evaluating PRA (Purported Responsible Address) policies as specified
in [RFC4406] with SPF or vice versa can cause havoc, as the
algorithms are semantically different even when the policies are
otherwise syntactically identical. This known problem is discussed
in [I-D.kitterman-4408bis].
SPF local part macros in a <domain-spec> could be abused in an attack
scenario to avoid DNS caching. Compare the SPF processing limits in
[I-D.kitterman-4408bis] section 10.1.
6. IANA Considerations
Keep up the good work, nothing to do here.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[I-D.kitterman-4408bis]
Kitterman, D. and W. Schlitt, "Sender Policy Framework
(SPF) for Authorizing Use of Domains in E-Mail, Version
1", draft-kitterman-4408bis-00 (work in progress),
October 2011.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[RFC5335] Abel, Y., "Internationalized Email Headers", RFC 5335,
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September 2008.
[RFC5336] Yao, J. and W. Mao, "SMTP Extension for Internationalized
Email Addresses", RFC 5336, September 2008.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.
7.2. Informative References
[RFC4343] Eastlake, D., "Domain Name System (DNS) Case Insensitivity
Clarification", RFC 4343, January 2006.
[RFC4406] Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail",
RFC 4406, April 2006.
[RFC4952] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 4952, July 2007.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, March 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5337] Newman, C. and A. Melnikov, "Internationalized Delivery
Status and Disposition Notifications", RFC 5337,
September 2008.
[RFC5451] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451, April 2009.
[Errata] OpenSPF, "RFC 4408 errata", 2008,
<http://www.openspf.net/RFC_4408/Errata>.
Appendix A. Document History
Changes in version 06:
* Fixed broken inline references by using the xml2rfc release
version instead of the experimental version.
* Replaced the openspf.org link for the SPF test suite and [Errata]
by openspf.net temporarily.
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Changes in version 05 (2011):
* Update for UTF8SMTP while UTF8SMTPBIS is not yet approved. Only
the references and boilerplate texts were updated; otherwise the
content is as it was in 2008.
* The IESG put the "outsourced" [Errata] on hold for an SPF document
update; the errata are covered by [I-D.kitterman-4408bis].
* Meanwhile [RFC5321], [RFC5322], [RFC5451], [RFC5890], and
[I-D.kitterman-4408bis] exist.
Changes in version 04 (2008):
* Fixed two typos and the references for the published EAI RFCs.
Changes in version 03:
* Added a terminology Section 1.1. Apart from the added [RFC5322]
defining <atext> all referenced drafts are now approved RFCs.
* Clarified that case-insensitive uses of <UTF8-non-ascii> can cause
havoc in conjunction with local part macros. This is not
mitigated by the recommended normalization.
* Added a pointer to the [I-D.kitterman-4408bis] security
considerations in Section 5. Mentioned the role of local part
macros in an attack scenario with credits to Doug Otis.
* Added the missing credits for the SPF discussions on the EAI list
back in 2006 and 2007.
* Added a link to the "outsourced" [Errata] mentioning the SPF test
suite. Errata and test suite cover some <target-name> issues
discussed in this memo, notably "adjacent dots".
* Added a long Section 3.3 about the wonders of SPF explanations
with the rather trivial conclusion that [RFC3987] percent-encoding
works as expected.
Changes in version 02:
* This memo is about EAI, not wildcard issues elsewhere; replaced
4592 by [RFC4952].
* Noted that only certain LDH-labels are or might be A-labels. The
details are specified in [RFC5890].
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* [RFC5451] not yet added, extending the Authentication-Results
header field to allow UTF-8 values for EAI should be straight
forward.
Changes in version 01:
* Extended the local part discussion in Section 2.3 significantly to
cover known gaps in the SPF specification. Added potential issues
with non-normalized local parts.
* Added a Received-SPF section using the now "Last Called"
[RFC5335]. Added references to [RFC4343], [RFC4952], [RFC5198],
and the approved [RFC5337].
* Removed the discussion of two vs. five SPF "subtypes", this
belongs into another draft. Kept the caveat about using an
algorithm designed for subtype x on a policy with subtype y (or
v.v.), as this could cause hard to debug mail losses.
Initial version:
* In a short discussion on the EAI list Harald Alvestrand and John
Klensin encouraged to collect SPF EAI considerations in a separate
memo.
Author's Address
Frank Ellermann
xyzzy
Hamburg, Germany
Email: hmdmhdfmhdjmzdtjmzdtzktdkztdjz@gmail.com
URI: http://purl.net/xyzzy/
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