RFC5248
Network Working Group T. Hansen Request for Comments: 5248 AT&T Laboratories BCP: 138 J. Klensin Updates: 3463, 4468, 4954 June 2008 Category: Best Current Practice
A Registry for SMTP Enhanced Mail System Status Codes
Status of This Memo
This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Distribution of this memo is unlimited.
Abstract
The specification for enhanced mail system status codes, RFC 3463, establishes a new code model and lists a collection of status codes. While it anticipated that more codes would be added over time, it did not provide an explicit mechanism for registering and tracking those codes. This document specifies an IANA registry for mail system enhanced status codes, and initializes that registry with the codes so far established in published standards-track documents, as well as other codes that have become established in the industry.
Contents
Introduction
Enhanced Status Codes for SMTP were first defined in RFC1893, which was subsequently replaced by RFC3463. While it anticipated that more codes would be added over time (see section 2 of RFC3463), it did not provide an explicit mechanism for registering and tracking those codes. Since then, various RFCs have been published and internet drafts proposed that define additional status codes. However, without an IANA registry, conflicts in definitions have begun to appear.
This RFC defines such an IANA registry and was written to help prevent further conflicts from appearing in the future. It initializes the registry with the established standards-track enhanced status codes from RFC3463, RFC3886, RFC4468, and RFC4954. In addition, this document adds several codes to the registry that were established by various internet drafts and have come into common use, despite the expiration of the documents themselves.
As specified in RFC3463, an enhanced status code consists of a three-part code, with each part being numeric and separated by a period character. The three portions are known as the class sub- code, the subject sub-code, and the detail sub-code. In the tables, a wildcard for the class sub-code is represented by an X, a wildcard for a subject sub-code is represented by an XXX, and a wildcard for a detail sub-code is represented by a YYY. For example, 3.XXX.YYY has an unspecified subject sub-code and an unspecified status code, and X.5.0 is has an unspecified class sub-code. (This is a change from RFC3463, which uses XXX for both the subject sub-code and detail sub-code wildcards.)
IANA Considerations
SMTP Enhanced Status Codes Registry
IANA has created the registry "SMTP Enhanced Status Codes". The SMTP Enhanced Status Codes registry will have three tables:
o Class Sub-Codes
Each of the entries in this table represent class sub-codes and all have an unspecified subject sub-code and an unspecified detail sub-code.
o Subject Sub-Codes
Each of the entries in this table represent subject sub-codes and all have an unspecified class sub-code and an unspecified detail sub-code.
o Enumerated Status Codes
Each of the entries in this table represent the combination of a subject sub-code and a detail sub-code. All entries will have an unspecified class sub-code, a specified subject sub-code, and a specified detail sub-code.
Each entry in the tables will include the following. (The sub-code tables will not have the Associated Basic Status Code entries.)
Code: The status code. For example,
3.XXX.YYY is a class sub-code with an
unspecified subject sub-code and an
unspecified detail sub-code, and X.5.0
is an enumerated status code with an
unspecified class sub-code.
Summary: or Sample Text: For class and subject sub-codes, this
is the summary of the use for the sub-
code shown in section 2 of RFC3463.
For enumerated status codes, this is an
example of a message that might be sent
along with the code.
Associated Basic Status Code: For enumerated status codes, the basic
status code(s) of RFC2821 with which it is usually associated. This may also have a value such as "Any" or "Not given". NOTE: This is a non-exclusive list. In particular, the entries that list some basic status codes for an Enhanced Status Code might allow for other basic status codes, while the entries denoted "Not given" can be filled in by updating the IANA registry through updates to this document or at the direction of the IESG.
Description: A short description of the code.
Reference: A reference to the document in which
the code is defined. This reference
should note whether the relevant
specification is standards-track, best
current practice, or neither, using one
of "(Standards track)", "(Best current
practice)" or "(Not standards track)".
Submitter: The identity of the submitter, usually
the document author.
Change Controller: The identity of the change controller
for the specification. This will be
"IESG" in the case of IETF-produced
documents.
An example of an entry in the enumerated status code table would be:
Code: X.0.0 Sample Text: Other undefined Status Associated basic status code: Any Description: Other undefined status is the only undefined
error code. It should be used for all errors for
which only the class of the error is known.
Reference: RFC 3463 (Standards track) Submitter: G. Vaudreuil Change controller: IESG.
Review Process for New Values
Entries in this registry are expected to follow the "Specification Required" model (RFC5226) although, in practice, most entries are expected to derive from standards-track documents. Non-standards- track documents that specify codes to be registered should be readily available. The principal purpose of this registry is to avoid confusion and conflicts among different definitions or uses for the same code.
Registration Updates
Standards-track registrations may be updated if the relevant standards are updated as a consequence of that action. Non- standards-track entries may be updated by the listed change controller. Only the entry's short description or references may be modified in this way, not the code or associated text. In exceptional cases, any aspect of any registered entity may be updated at the direction of the IESG (for example, to correct a conflict).
Initial Values
The initial values for the class and subject sub-code tables are to be populated from section 2 of RFC3463. Specifically, these are the values for 2.XXX.YYY, 4.XXX.YYY, and 5.XXX.YYY for the Class Sub- Code table, and the values X.0.YYY, X.1.YYY, X.2.YYY, X.3.YYY, X.4.YYY, X.5.YYY, X.6.YYY, and X.7.YYY for the Subject Sub-Code table. The code, sample text, and description for each entry are to be taken from RFC3463. Each entry is to use RFC3463 as the reference, submitted by G. Vaudreuil, and change controlled by the IESG. There are no associated detail sub-code values for the class and subject sub-code tables.
The initial values for the Enumerated Status Code table is to be populated from:
1. sections 3.1 through 3.8 of RFC3463, (X.0.0, X.1.0 through
X.1.8, X.2.0 through X.2.4, X.3.0 through X.3.5, X.4.0 through X.4.7, X.5.0 through X.5.5, X.6.0 through X.6.5, and X.7.0 through X.7.7),
2. section 3.3.4 of RFC3886 (X.1.9),
3. X.6.6 found in section 5 of RFC4468, (but not X.7.8 found in
the same section),
4. and X.5.6, X.7.8, X.7.9, X.7.11, and X.7.12, found in section 6
of RFC4954 (using the text from X.5.6, 5.7.8, 5.7.9, 5.7.11, and 4.7.12).
Each entry is to be designated as defined in the corresponding RFC, submitted by the corresponding RFC author, and change controlled by the IESG. Each of the above RFCs is a standards-track document.
The initial values for the Associated Basic Status Code for each of the above initial enhanced status codes is given in the following table.
As noted above, this table is incomplete. In particular, the entries that have some basic status codes might allow for other detail sub- status codes, while the entries denoted "Not given" can be filled in by updating the IANA registry through updates to this document or at the direction of the IESG.
+--------+---------------+--------+----------+--------+-------------+ | Enh. | Assoc. Basic | Enh. | Assoc. | Enh. | Assoc. | | Status | Status Code | Status | Basic | Status | Basic | | Code | | Code | Status | Code | Status Code | | | | | Code | | | +--------+---------------+--------+----------+--------+-------------+ | X.0.0 | Any | X.1.0 | Not | X.1.1 | 451, 550 | | | | | given | | | | X.1.2 | Not given | X.1.3 | 501 | X.1.4 | Not given | | X.1.5 | 250 | X.1.6 | Not | X.1.7 | Not given | | | | | given | | | | X.1.8 | 451, 501 | X.1.9 | Not | X.2.0 | Not given | | | | | given | | | | X.2.1 | Not given | X.2.2 | 552 | X.2.3 | 552 | | X.2.4 | 450, 452 | X.3.0 | 221, | X.3.1 | 452 | | | | | 250, | | | | | | | 421, | | | | | | | 451, | | | | | | | 550, 554 | | | | X.3.2 | 453 | X.3.3 | Not | X.3.4 | 552, 554 | | | | | given | | | | X.3.5 | Not given | X.4.0 | Not | X.4.1 | 451 | | | | | given | | | | X.4.2 | 421 | X.4.3 | 451, 550 | X.4.4 | Not given | | X.4.5 | 451 | X.4.6 | Not | X.4.7 | Not given | | | | | given | | | | X.5.0 | 220, 250, | X.5.1 | 430, | X.5.2 | 500, 501, | | | 251, 252, | | 500, | | 502, 550, | | | 253, 451, | | 501, | | 555 | | | 452, 454, | | 503, | | | | | 458, 459, | | 530, | | | | | 501, 502, | | 550, | | | | | 503, 554 | | 554, 555 | | | | X.5.3 | 451 | X.5.4 | 451, | X.5.5 | Not given | | | | | 501, | | | | | | | 502, | | | | | | | 503, | | | | | | | 504, | | | | | | | 550, 555 | | | | X.5.6 | 500 | X.6.0 | Not | X.6.1 | Not given | | | | | given | | | | X.6.2 | Not given | X.6.3 | 554 | X.6.4 | 250 | | X.6.5 | Not given | X.6.6 | 554 | X.7.0 | 220, 235, | | | | | | | 450, 454, | | | | | | | 500, 501, | | | | | | | 503, 504, | | | | | | | 530, 535, | | | | | | | 550 |
| X.7.1 | 451, 454, | X.7.2 | 550 | X.7.3 | Not given | | | 502, 503, | | | | | | | 533, 550, 551 | | | | | | X.7.4 | 504 | X.7.5 | Not | X.7.6 | Not given | | | | | given | | | | X.7.7 | Not given | X.7.8 | 535, 554 | X.7.9 | 534 | | X.7.10 | 523 | X.7.11 | 524, 538 | X.7.12 | 422, 432 | | X.7.13 | 525 | X.7.14 | 535, 554 | | | +--------+---------------+--------+----------+--------+-------------+
Table 1
The following additional definitions have been registered in the enumerated status code table. These entries have been used in the industry without any published specification.
Code: X.7.10 Sample Text: Encryption Needed Associated basic status code: 523 Description: This indicates that an external strong privacy
layer is needed in order to use the requested
authentication mechanism. This is primarily
intended for use with clear text authentication
mechanisms. A client that receives this may
activate a security layer such as TLS prior to
authenticating, or attempt to use a stronger
mechanism.
Reference: RFC 5248 (Best current practice) Submitter: T. Hansen, J. Klensin Change controller: IESG
Code: X.7.13 Sample Text: User Account Disabled Associated basic status code: 525 Description: Sometimes a system administrator will have to
disable a user's account (e.g., due to lack of
payment, abuse, evidence of a break-in attempt,
etc.). This error code occurs after a successful
authentication to a disabled account. This
informs the client that the failure is permanent
until the user contacts their system
administrator to get the account re-enabled. It
differs from a generic authentication failure
where the client's best option is to present the
passphrase entry dialog in case the user simply
mistyped their passphrase.
Reference: RFC 5248 (Best current practice) Submitter: T. Hansen, J. Klensin Change controller: IESG
Code: X.7.14 Sample Text: Trust relationship required Associated basic status code: 535, 554 Description: The submission server requires a configured trust
relationship with a third-party server in order
to access the message content. This value
replaces the prior use of X.7.8 for this error
condition, thereby updating RFC4468.
Reference: RFC 5248 (Best current practice) Submitter: T. Hansen, J. Klensin Change controller: IESG
Security Considerations
As stated in RFC1893, use of enhanced status codes may disclose additional information about how an internal mail system is implemented beyond that available through the SMTP status codes.
Many proposed additions to the response code list are security related. Having these registered in one place to prevent collisions will improve their value. Security error responses can leak information to active attackers (e.g., the distinction between "user not found" and "bad password" during authentication). Documents defining security error codes should make it clear when this is the case so SMTP server software subject to such threats can provide appropriate controls to restrict exposure.
Acknowledgements
While the need for this registry should have become clear shortly after RFC3463 was approved, the growth of the code table through additional documents and work done as part of email internationalization and RFC2821 updating efforts made the requirement much more clear. The comments of the participants in those efforts are gratefully acknowledged, particularly the members of the [email protected] mailing list. Chris Newman and Randy Gellens provided useful comments and some text for early versions of the document.
References
Normative References
RFC2821 Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
RFC3463 Vaudreuil, G., "Enhanced Mail System Status Codes",
RFC 3463, January 2003.
RFC3886 Allman, E., "An Extensible Message Format for Message
Tracking Responses", RFC 3886, September 2004.
RFC4468 Newman, C., "Message Submission BURL Extension", RFC 4468,
May 2006.
RFC4954 Siemborski, R. and A. Melnikov, "SMTP Service Extension
for Authentication", RFC 4954, July 2007.
Informative References
RFC1893 Vaudreuil, G., "Enhanced Mail System Status Codes",
RFC 1893, January 1996.
RFC5226 Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
Authors' Addresses
Tony Hansen AT&T Laboratories 200 Laurel Ave. Middletown, NJ 07748 USA
EMail: [email protected]
John C Klensin 1770 Massachusetts Ave, Ste 322 Cambridge, MA 02140 USA
Phone: +1 617 245 1457 EMail: [email protected]
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