Internet DRAFT - draft-klensin-smtp-521code
draft-klensin-smtp-521code
Network Working Group J. Klensin
Internet-Draft May 1, 2015
Updates: 1846, 5321 (if approved)
Intended status: Standards Track
Expires: November 2, 2015
SMTP 521 and 556 Reply Codes
draft-klensin-smtp-521code-07.txt
Abstract
This memo defines two Simple Mail Transfer Protocol (SMTP) reply
codes, 521 and 556. The 521 code was originally described in an
Experimental RFC in 1995 and is in wide use, but has not previously
been formally incorporated into SMTP. The 556 code was created for
RFC-nullMX. These codes are used to indicate that an Internet host
does not accept incoming mail at all (not just under particular
circumstances).
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 2, 2015.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Discussion List . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The 521 Reply Code . . . . . . . . . . . . . . . . . . . . . 4
4. The 556 Reply Code . . . . . . . . . . . . . . . . . . . . . 4
5. Small details to avoid loose ends . . . . . . . . . . . . . . 5
5.1. Specific changes to RFC 5321 . . . . . . . . . . . . . . 5
5.2. The RFC 1846 Experiment . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 7
A.1. Changes from -00 to -01 . . . . . . . . . . . . . . . . . 7
A.2. Changes from -01 to -02 . . . . . . . . . . . . . . . . . 7
A.3. Changes from -02 to -03 . . . . . . . . . . . . . . . . . 7
A.4. Changes from -03 to -04 . . . . . . . . . . . . . . . . . 7
A.5. Changes from -04 to -05 . . . . . . . . . . . . . . . . . 7
A.6. Changes from -05 to -06 . . . . . . . . . . . . . . . . . 8
A.7. Changes from -06 to -07 . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
[[ RFC Editor: the string RFC-nullMX is produced by an XML entity
named &RFCnullMX. When this document is edited for RFC publication,
the value of that entity should be replaced by the appropriate RFC
number and this note deleted. ]]
The SMTP specification [2] (referred to, along with its various
updates, as "SMTP" below) contains a list and discussion of reply
codes. This document updates that list with a new code, 521, for use
in response to an initial connection. In that context, it
specifically denotes a system that does not receive email or
otherwise handle SMTP mail or inquiry transactions. That code
differs from the use of reply code 554, recommended by RFC 5321,
because the latter code can be used in a larger variety of
situations, including mail that is not accepted for, or from,
particular sources, destinations, or addresses. It also introduces a
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second reply code, 556, for use when an SMTP client encounters a
domain in a forward-pointing address that it can determine (e.g.,
from the DNS "null MX" convention [5]) does not support receipt of
email and has to report that condition to a host that delivered the
message to it for further processing.
This specification updates RFC 5321 to add the new codes. The 521
code was first formally proposed in the Experimental RFC 1846 [4];
this document updates that specification to standardize the code and
provide more specific treatment of it.
1.1. Terminology
The reader of this document is expected to have reasonable
familiarity with the SMTP specification in RFC 5321, particularly its
discussion of reply codes and their use and theory.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
1.2. Discussion List
[[CREF1: RFC Editor: please remove this subsection.]]
Discussion of the SMTP aspects and relationships of this
specification should occur on the ietf-smtp list,
https://www.ietf.org/mailman/listinfo/ietf-smtp. Discussions of
"null MX" and the relationship of this specification to it occur on
the apps-discuss list, https://www.ietf.org/mailman/listinfo/apps-
discuss.
2. Background
Many Internet hosts are not in a position -- whether technically,
operationally, or administratively-- to offer email service. If an
SMTP client (sender) attempts to open a mail connection to a system
that does not have an SMTP server, the connection attempt will time
out. SMTP requires that timeouts result in the client queuing the
message and retrying it for an extended period. That behavior will
result in wasted resources and long delays in getting an error
message back to its originator.
One alternative is to run a dummy SMTP server on hosts that do not
receive mail under any circumstances, having that dummy server return
a fatal error reply code in response to any connection-opening
attempt. Another is to determine, from a separate source such as a
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DNS record, that the host does not receive mail. This document
specifies reply codes to be used for those purposes.
3. The 521 Reply Code
This specification adds the 521 reply code to the repertoire
specified in SMTP, reserving it for use at connection-opening time to
indicate that the host does not accept email under any circumstances.
It SHOULD be used for dummy SMTP servers whose sole purpose is to
notify systems that attempt to open mail connections that the host
never accepts mail. It MAY be used in other situations where the
intent is to indicate that the host never accepts email. It SHOULD
NOT be used for situations in which the server rejects mail from
particular hosts or addresses or in which mail for a particular
destination host is not accepted. As discussed in SMTP, reply code
554 is more appropriate for most of those conditions; an additional
case, in which the determination that mail is not accepted is
determined outside the mail system, is covered in the next section
(Section 4).
"Server does not accept mail" (or a variant such as "Host", "Domain",
or a related term) is an acceptable message to accompany a 521 code
used for this purpose.
Once the 521 reply code is returned instead of the usual 220, the
SMTP session proceeds normally. If the SMTP client attempts to send
additional commands other than QUIT, the server MAY either continue
sending 521 reply codes or simply close the connection. If the
purpose of running a dummy SMTP server that returns a 521 code is to
conserve resources, the latter will usually be preferable.
4. The 556 Reply Code
This specification adds the 556 reply code to the repertoire
specified in SMTP. When an intermediate SMTP system (typically a
relay) that would normally attempt to open a mail connection to a
host referred to in a forward-pointing address can determine that the
host does not accept mail connections, and do so without attempting
to open a connection to that target host, it is appropriate for it to
return a reply with a 556 code to the system that sent it the message
for outbound transmission. Interpretation of a special DNS record,
found when a lookup is performed in conjunction with a RCPT command
[5], is one such method (and the only standardized one at the time
this specification was written).
When an SMTP server returns a 556 reply code after receiving a
command (such as RCPT, which contains a forward-pointing address)
because it has information (such as discussed above) that the mail
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will not be accepted, the SMTP client is expected to handle the
response like any other permanent negative completion reply to the
command. This is consistent with the SMTP specification.
5. Small details to avoid loose ends
5.1. Specific changes to RFC 5321
This document adds the 521 code, with message "Host does not accept
mail", and the 556 code, with message "Domain does not accept mail",
to the function group and numerical lists (Sections 4.2.2 and 4.2.3
respectively) of RFC 5321. It also adds the 521 code to the
"CONNECTION ESTABLISHMENT" portion of Section 4.3.2 ("Command-Reply
Sequences"), preceding the 554 code, and the 556 code to the "RCPT"
portion of that same section.
5.2. The RFC 1846 Experiment
By formalizing reply code 521, this specification ends the experiment
proposed in RFC 1846. That document also discusses general
strategies for hosts that do not accept mail directly. That
discussion is out of scope for the present document.
6. IANA Considerations
This document updates RFC 5321 to add descriptions and text for two
reply codes, but there is no registry for those codes. IANA should
update the SMTP Enhanced Status Code Registry [3] to include these
codes, specifically:
o Add 521 to the list of codes associated with the enhanced code
entry for X.3.2, referencing this document.
o Add this document to the references associated with the enhanced
code entry for X.1.10 and reply code 556. Note that, if a use for
556 arises that is not associated with null MX [5], it may be
necessary to add an additional enhanced code, but such action is
outside the scope of this document.
7. Security Considerations
Not running any SMTP server, or running an SMTP server which simply
emits fixed strings in response to incoming connections, should
provide significantly fewer opportunities for security problems than
running a complete SMTP implementation. See the Security
Considerations section of RFC-nullMX [5] for a discussion of security
issues with that approach. Use of the specific codes provided here
provides more information to the client than a generic or
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arbitrarily-chosen 5yz code but should have no other effect on
security.
8. Acknowledgments
Alain Durand and Francis Dupont proposed the 521 code in RFC 1846
[4]. They also participated in an extended conversation and provided
many useful comments that led to this document. The document also
contains, with their permission, some text copied from that early
specification.
Discussion of the "null MX" approach and proposal [5] inspired the
creation of this specification. Specific comments and suggestions
from John Levine (co-author of that document) were also helpful.
Martin Duerst and Tom Petch identified significant issues in the
initial draft of the current form of the document.
Dilyan Palauzov did a careful reading and identified an editorial
error.
Ned Freed, Tony Hansen, and Rolf Sonneveld suggested textual
improvements that were incorporated. Tony Hansen also acted as
document shepherd and made several contributions in conjunction with
that role.
9. References
9.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", March 1997.
[2] Klensin, J., "Simple Mail Transfer Protocol", October
2008.
[3] IANA, "Simple Mail Transfer Protocol (SMTP) Enhanced
Status Codes Registry: Enumerated Status Codes",
<http://www.iana.org/assignments/smtp-enhanced-status-
codes/smtp-enhanced-status-codes.xhtml#smtp-enhanced-
status-codes-3>.
9.2. Informative References
[4] Durand, A. and F. Dupont, "SMTP 521 Reply Code", September
1995.
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[5] Levine, J. and M. Delany, "A "Null MX" No Service Resource
Record for Domains that Accept No Mail", September 2014,
<https://datatracker.ietf.org/doc/draft-ietf-appsawg-
nullmx/>.
Appendix A. Change Log
RFC Editor: Please remove this appendix before publication..
This Internet Draft is the successor to draft-klensin-rfc1846bis-00.
That document was an attempt to completely update and replace RFC
1846. That effort led to the conclusion that it would be better to
focus narrowly on the 521 code, leaving a more general treatment of
hosts that do not receive mail to a separate replacement for RFC 1846
and/or an update to RFC 5321.
A.1. Changes from -00 to -01
Revised abstract and cleaned up "error code" terminology.
A.2. Changes from -01 to -02
o Added provision for code 556 and associated discussion.
o Several editorial corrections and cleanups.
A.3. Changes from -02 to -03
o Updated reference to nullMX to -10
o Changed text describing preferred/acceptable message text.
o Additional editorial improvements.
A.4. Changes from -03 to -04
Corrected errors in previous Change Log sections.
Added an IANA Considerations section and provision for the SMTP
Enhanced Status Code Registry.
A.5. Changes from -04 to -05
o Several small editorial corrections, most suggested by Rolf
Sonneveld.
o Changed sample text for code 521 to use "server" rather than
"host" for consistence with RFC 5321.
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o Rationalized use of "responses" and "reply code" terminology.
A.6. Changes from -05 to -06
o Part of Section 4 rewritten to make the client and server
relationships more clear to people who are not SMTP (and RFC 5321)
experts.
o Corrected an error in IANA considerations to change the extended
reply code for code 556 to X.1.10 from X.10.1.
o Minor editorial issues caught during Last Call corrected.
A.7. Changes from -06 to -07
o Added parenthetical note in Section 3 to "acceptable text" for the
521 reply.
Author's Address
John C Klensin
1770 Massachusetts Ave, Ste 322
Cambridge, MA 02140
USA
Phone: +1 617 245 1457
Email: john-ietf@jck.com
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