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Network Working Group                                   Arnt Gulbrandsen
Internet-Draft                                    Oryx Mail Systems GmbH
Intended Status: Proposed Standard                     February 15, 2008


                          IMAP Response Codes
              draft-gulbrandsen-imap-response-codes-01.txt


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   This Internet-Draft expires in August 2008.


Copyright Notice

   Copyright (C) The IETF Trust (2008).


Abstract

   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.





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1.  Conventions Used in This Document

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


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 servers
   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.


3. Response Codes

   This section defines all the new response codes.

   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.

   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.

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




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   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.

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

   ACL         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.

   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
               later.

   EXPUNGED    One or more messages related to a client command have
               been expunged. The client might want to send a NOOP
               command. [RFC2180] discusses this subject in depth.

   CORRUPTION  The server discovered that some relevant data (e.g. the
               mailbox) are corrupt.

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

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

   NOBODYPART  The specified bodypart does not exist.

               (This seems unjustified. It will be removed before IETF
               LC unless I learn a sensible use case before then.)

   CANNOT      The operation violates some invariant of the server and
               can never succeed. For example, creating a mailbox whose
               name is legal according to IMAP but impossible for the
               server ("///").

   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.



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   OVERQUOTA   The operation failed because the user would be over quota
               afterwards. (The user may or may not be over quota
               already.)

   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.

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


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.  Implementations MUST
   accept these strings in a case-insensitive fashion.

        resp-text-code =/ "UNAVAILABLE" / "AUTHENTICATIONFAILED" /
                         "AUTHORIZATIONFAILED" / "EXPIRED" /
                         "CONTACTADMIN" / "ACL" / "INUSE" / "EXPUNGED" /
                         "CORRUPTION" / "SERVERBUG" / "CLIENTBUG" /
                         "NOBODYPART" / "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 ACL and authentication-related
   responses.



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6. IANA considerations

   None. (Or should this document create a registry and populate it with
   all the entries from 3501 and various extensions?)


7. Acknowledgements

   Peter Coates, Mark Crispin, Philip Van Hoof, Alexey Melnikov and Ken
   Murchison 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.


9. Informative References

   [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
   Germany

   Fax: +49 89 4502 9758

   Email: arnt@oryx.com









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Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.







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          (RFC Editor: Please delete everything after this point)


Open Issues

   Is it worth adding an IANA registry?

   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."

   CONTACTADMIN, SERVERBUG and CORRUPTED are all different ways to say
   "tell the admin that ...". Should they be merged because they're so
   similar?

   ALREADYEXISTS and DOESNOTEXIST could be merged to a single EXISTENCE,
   if that isn't troublesome for clients. Could be. Should be?


Changes since -00

   - CHILDMAILBOXEXISTS merged into INUSE.

   - 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.
















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