Network Working Group Arnt Gulbrandsen
Internet-Draft Oryx Mail Systems GmbH
Intended Status: Proposed Standard October 21, 2008
IMAP Response Codes
draft-gulbrandsen-imap-response-codes-03.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].
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 for the best that can EASILY be done, not for the best that can
POSSIBLY be done.
3. Response Codes
This section defines all the new response codes. Each definition is
followed by one or more examples.
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
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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]
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.
C: f select "/archive/projects/experiment-iv"
S: f NO [ACL] Access denied
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.
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C: g remove "/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
is.
C: k1 select "/archive/projects/experiment-iv"
[...]
S: k1 OK [READ-ONLY] Done
C: k2 status "/archive/projects/experiment-iv"
[...]
S: k2 OK [CLIENTBUG] Done
CANNOT The operation violates some invariant of the server and
can never succeed.
C: l create "///////"
S: l NO [CANNOT] Unsupported name
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
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OVERQUOTA The user is or would be over quota after the operation.
(The user may or may not be over quota already.)
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. Implementations MUST
accept these strings in a case-insensitive fashion.
resp-text-code =/ "UNAVAILABLE" / "AUTHENTICATIONFAILED" /
"AUTHORIZATIONFAILED" / "EXPIRED" /
"CONTACTADMIN" / "ACL" / "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.
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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.
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
BADURL RFC 4469
HIGHESTMODSEQ RFC 4551
NOMODSEQ RFC 4551
MODIFIED RFC 4551
COMPRESSIONACTIVE RFC 4978
CLOSED RFC 5162
BADCOMPARATOR RFC 5255
ANNOTATE TOOBIG RFC 5257
ANNOTATE TOOMANY RFC 5257
ANNOTATIONS RFC 5257
UNAVAILABLE RFC (this)
AUTHENTICATIONFAILED RFC (this)
AUTHORIZATIONFAILED RFC (this)
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EXPIRED RFC (this)
CONTACTADMIN RFC (this)
ACL 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 registrly's URL instead.
7. Acknowledgements
Peter Coates, Mark Crispin, Philip Guenther, Philip Van Hoof, Alexey
Melnikov, Ken Murchison, Chris Newman and Dale Wiggins 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
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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
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.
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.
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
implements.
- CONTACTADMIN vs. ALERT, an open issue.
- Added an IANA considerations section registering every (?) response
code defined so far.
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- 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.
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