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Network Working Group                                   Arnt Gulbrandsen
Internet-Draft                                    Oryx Mail Systems GmbH
Intended Status: Proposed Standard                      December 4, 2008

                          IMAP Response Codes

Status of this Memo

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   IMAP responses consist of a response type (OK, NO, BAD), an optional
   machine-readable response code and a human-readable text.

   This document collects and documents a variety of machine-readable
   response codes, for better interoperation and error reporting.

1.  Conventions Used in This Document

   Formal syntax is defined by [RFC5234] as modified by [RFC3501].

   Example lines prefaced by "C:" are sent by the client and ones
   prefaced by "S:" by the server. "[...]" means elision.

2. Introduction

   [RFC3501] section 7.1 defines a number of response codes which can
   help tell an IMAP client why a command failed. However, experience
   has shown that more codes are useful. For example, it is useful for a
   client to know that an authentication attempt failed because of a
   server problem as opposed to a password problem.

   Currently many IMAP servers use English-language human-readable text
   to describe these errors, and a few IMAP clients attempt to translate
   this text into the user's language.

   This document names a variety of errors as response codes. It is
   based on errors checked and reported in some IMAP server
   implementations, and on needs in some IMAP clients.

   This document doesn't require any servers to test for these errors,
   or any clients to test for these names. It only names errors for
   better reporting and handling.

   [RFC Editor: Please remove this paragraph.] In general, this document
   aims to do that which is widely considered good, and nothing more.
   Several controversial and/or complex features were discussed, but
   just listing the simple and desirable response codes is enough for
   one document.

3. Response Codes

   This section defines all the new response codes. Each definition is
   followed by one or more examples.

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   UNAVAILABLE Temporary failure because a subsystem is down. For
               example, an IMAP server which uses an LDAP or Radius
               server for authentication might use this when the
               LDAP/Radius server is down.

               C: a LOGIN "fred" "foo"
               S: a NO [UNAVAILABLE] User's backend down for maintenance

   AUTHENTICATIONFAILED Authentication failed for some reason which the
               server is not willing to elaborate. Typically this
               includes "unknown user" and "bad password".

               This is the same as not sending any response code, except
               that when a client sees AUTHENTICATIONFAILED, it knows
               that the problem wasn't e.g. UNAVAILABLE, so there's no
               point in trying the same login/password again later.

               C: b LOGIN "fred" "foo"
               S: b NO [AUTHENTICATIONFAILED] Authentication failed

   AUTHORIZATIONFAILED Authentication succeeded, but authorization
               failed. This is only applicable when the authentication
               and authorization identities are different.

               C: c AUTHENTICATE PLAIN
               S: c NO [AUTHORIZATIONFAILED] No such auth-ID

   EXPIRED     Authentication succeeded or the server didn't have the
               necessary data any more, but access is no longer
               permitted using that passphrase. The client or user
               should get a new passphrase.

               C: d login "fred" "foo"
               S: d NO [EXPIRED] That password isn't valid any more

   CONTACTADMIN The user should contact the system administrator or
               support desk.

               C: e login "fred" "foo"
               S: e OK [CONTACTADMIN]

   NOPERM      The access control system (e.g. ACL, see [RFC4314]) does
               not permit this user to carry out an operation, such as
               selecting or creating a mailbox.

               C: f select "/archive/projects/experiment-iv"
               S: f NO [NOPERM] Access denied

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   INUSE       An operation has not been carried out because it involves
               sawing off a branch someone else is sitting on. Someone
               else may be holding an exclusive lock needed for this
               operation, or it may involve deleting a resource someone
               else is using, typically a mailbox.

               The operation may succeed if the client tries again

               C: g delete "/archive/projects/experiment-iv"
               S: g NO [INUSE] Mailbox in use

   EXPUNGEISSUED Someone else has issued an EXPUNGE for the same
               mailbox.  The client may want to issue NOOP soon.
               [RFC2180] discusses this subject in depth.

               C: h search from fred@example.com
               S: * SEARCH 1 2 3 5 8 13 21 42
               S: h OK [EXPUNGEISSUED] Search completed

   CORRUPTION  The server discovered that some relevant data (e.g. the
               mailbox) are corrupt. This response code does not include
               any information about what's corrupt, but the server can
               write that to its logfiles.

               C: i select "/archive/projects/experiment-iv"
               S: i NO [CORRUPTION] Cannot open mailbox

   SERVERBUG   The server encountered a bug in itself or violated one of
               its own invariants.

               C: j select "/archive/projects/experiment-iv"
               S: j NO [SERVERBUG] This should not happen

   CLIENTBUG   The server has detected a client bug. This can accompany
               all of OK, NO and BAD, depending on what the client bug

               C: k1 select "/archive/projects/experiment-iv"
               S: k1 OK [READ-ONLY] Done
               C: k2 status "/archive/projects/experiment-iv" (messages)
               S: k2 OK [CLIENTBUG] Done

   CANNOT      The operation violates some invariant of the server and
               can never succeed.

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               C: l create "///////"
               S: l NO [CANNOT] Adjacent slashes is not supported

   LIMIT       The operation ran up against an implementation limit of
               some kind, such as the number of flags on a single
               message or number of flags used in a mailbox.

               C: m STORE 42 FLAGS f1 f2 f3 f4 f5 ... f250
               S: m NO [LIMIT] At most 32 flags in one mailbox supported

   OVERQUOTA   The user is or would be over quota after the operation.
               (The user may or may not be over quota already.)

               Note that if the server sends OVERQUOTA but doesn't
               support the IMAP QUOTA extension defined by [RFC2087],
               then there is a quota, but the client cannot find out
               what the quota is.

               C: n1 uid copy 1:* oldmail
               S: n1 NO [OVERQUOTA] Sorry

               C: n2 uid copy 1:* oldmail
               S: n2 OK [OVERQUOTA] You are now over your soft quota

   ALREADYEXISTS The operation attempts to create something which
               already exists, such as when the CREATE or RENAME
               directories attempt to create a mailbox and there is one
               of that name.

               C: o RENAME this that
               S: o NO [ALREADYEXISTS] Mailbox "that" already exists

   NONEXISTENT The operation attempts to delete something which does not
               exist. Similar to ALREADYEXISTS.

               C: p RENAME this that
               S: p NO [NONEXISTENT] No such mailbox

4. Formal Syntax

   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) notation as specified in [RFC5234]. [RFC3501] defines the
   non-terminal "resp-text-code".

   Except as noted otherwise, all alphabetic characters are case-
   insensitive.  The use of upper or lower case characters to define
   token strings is for editorial clarity only.

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        resp-text-code =/ "UNAVAILABLE" / "AUTHENTICATIONFAILED" /
                         "AUTHORIZATIONFAILED" / "EXPIRED" /
                         "CONTACTADMIN" / "NOPERM" / "INUSE" /
                         "EXPUNGEISSUED" / "CORRUPTION" / "SERVERBUG" /
                         "CLIENTBUG" / "CANNOT" / "LIMIT" / "OVERQUOTA"
                         / "ALREADYEXISTS" / "NONEXISTENT"

5. Security considerations

   Revealing information about a passphrase to unauthenticated IMAP
   clients has bad karma.

   Response codes are easier to parse than human-readable text. This can
   amplify the consequences of an information leak. For example,
   selecting a mailbox can fail because the mailbox doesn't exist,
   because the user doesn't have the "l" right (right to know the
   mailbox exists) or "r" (right to read the mailbox). If the server
   sent different responses in the first two cases in the past, only
   malevolent clients would discover it. With response codes it's
   possible, perhaps probable, that benevolent clients forward the
   leaked information to the user. Server authors are encouraged to be
   particularly careful with the NOPERM and authentication-related

6. IANA considerations

   The IANA is requested to create a new registry, tentatively named
   imap-response-codes, and populate it as follows:

       REFERRAL             RFC 2221
       ALERT                RFC 3501
       BADCHARSET           RFC 3501
       PARSE                RFC 3501
       PERMANENTFLAGS       RFC 3501
       READ-ONLY            RFC 3501
       READ-WRITE           RFC 3501
       TRYCREATE            RFC 3501
       UIDNEXT              RFC 3501
       UIDVALIDITY          RFC 3501
       UNSEEN               RFC 3501
       UNKNOWN-CTE          RFC 3516
       UIDNOTSTICKY         RFC 4315
       APPENDUID            RFC 4315
       COPYUID              RFC 4315
       URLMECH              RFC 4467
       TOOBIG               RFC 4469

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       BADURL               RFC 4469
       HIGHESTMODSEQ        RFC 4551
       NOMODSEQ             RFC 4551
       MODIFIED             RFC 4551
       CLOSED               RFC 5162
       BADCOMPARATOR        RFC 5255
       ANNOTATE             RFC 5257
       METADATA             RFC (draft-daboo-imap-annotatemore-16.txt)
       UNAVAILABLE          RFC (this)
       EXPIRED              RFC (this)
       CONTACTADMIN         RFC (this)
       NOPERM               RFC (this)
       INUSE                RFC (this)
       EXPUNGEISSUED        RFC (this)
       CORRUPTION           RFC (this)
       SERVERBUG            RFC (this)
       CLIENTBUG            RFC (this)
       CANNOT               RFC (this)
       LIMIT                RFC (this)
       OVERQUOTA            RFC (this)
       ALREADYEXISTS        RFC (this)
       NONEXISTENT          RFC (this)

   The RFC editor is requested to delete this entire text, and insert a
   sentence or two mentioning the registry's URL instead.

   The new registry should only be extended by publishing an RFC. The
   IANA may to add placeholders for internet-drafts at its discretion.

7. Acknowledgements

   Peter Coates, Mark Crispin, Philip Guenther, Philip Van Hoof, Alexey
   Melnikov, Ken Murchison, Chris Newman, Timo Sirainen, Dale Wiggins
   and Sarah Wilkin helped with this document.

8. Normative References

   [RFC3501]  Crispin, "Internet Message Access Protocol - Version
              4rev1", RFC 3501, University of Washington, June 2003.

   [RFC5234]  Crocker, Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 5234, Brandenburg
              Internetworking, THUS plc, January 2008.

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9. Informative References

   [RFC2087]  Myers, "IMAP4 QUOTA extension", RFC 2087, Carnegie Mellon,
              January 1997.

   [RFC2180]  Gahrns, "IMAP4 Multi-Accessed Mailbox Practice", RFC 2180,
              Microsoft, July 1997.

   [RFC4314]  Melnikov, "IMAP4 Access Control List (ACL) Extension", RFC
              4314, December 2005.

10. Author's Address

   Arnt Gulbrandsen
   Oryx Mail Systems GmbH
   Schweppermannstr. 8
   D-81671 Muenchen

   Fax: +49 89 4502 9758

   Email: arnt@oryx.com

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          (RFC Editor: Please delete everything after this point)

Open Issues

   I took TOOWEAK out since it doesn't seem to have real purpose: "The
   server requires a stronger authentication mechanism.  If the
   connection is not encrypted, the client could also try the same
   mechanism via an encrypted connection." But now I remember why it was
   there: The server may offer e.g. AUTH=CRAM-MD5, but not be able to
   carry that out for every user. Maybe it should be returned with a
   better name. I'd like to hear whether anyone actually does this.

   The name, if any, should reflect that the server cannot carry out
   this particular mechanism for this particular authentication-id. This
   may be because it's too weak (a policy decision) or because the
   server lacks data for this (user,mechanism) combination.

Changes since -00


   - ACCESSDENIED renamed ACL to clarify its scope.

   - NOBODYPART scheduled for deletion if noone minds.

   - EXISTS renamed ALREADYEXISTS to avoid confusion with the EXISTS
     response. Mustn't overload developer brains. (Do unto others.)

   - Added a security note about how response codes makes some
     information leaks worse.

   - A couple of open issues.

Changes since -01

   - Two people independently argued that merging ALREADYEXISTS and
     NONEXISTENT was bad because of RENAME. Open issue closed.

   - An example for each response code.

   - EXPUNGED renamed, see EXISTS above.

   - EXPUNGEISSUED semantics changed to be 2180-neutral. It should now
     be equally useful no matter which part of 2180 the server

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   - CONTACTADMIN vs. ALERT, an open issue.

   - Added an IANA considerations section registering every (?) response
     code defined so far.

   - Added contact details to CONTACTADMIN, by request.

   - Resolved the CA/SB/C issue: The three responses may be handled
     similarly by some clients, but they may equally well be handled
     differently, so they should not be folded.

Changes since -02

   - Removed the contact details for CONTACTADMIN. I think that was
     creeping featuritis, not likely to be implemented.

   - Removed NOBODYPART, noone suggested use for it.

   - Edited CORRUPTION to suggest that detailed information belongs in
     the server logs. The client/user can bug the admin to look in the
     log, but expecting users to transmit information is stupid.

   - Updated the IANA list for 5255 and 5257.

Changes since -03

   - Explained the criteria for inclusion/exclusion better.

   - Fixed remove/delete typo, fix status type

   - Better text in the CANNOT example

   - Instruct the IANA to extend the registry only when an RFC is

Changes since -04

   - ACL renamed NOPERM on request of Timo.

   - Added METADATA, which I had overlooked.

   - Turned ANNOTATE <VARIOUS> into just ANNOTATE, added METADATA
     <VARIOUS> in the same way.

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Changes since -05

   - Fix typo (by rewriting the sentence)

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