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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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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(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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