Network Working Group E. Hall
Request for Comments: 4155 September 2005
Category: Informational
The application/mbox Media Type
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This memo requests that the application/mbox media type be authorized
for allocation by the IESG, according to the terms specified in RFC
2048. This memo also defines a default format for the mbox database,
which must be supported by all conformant implementations.
1. Background and Overview
UNIX-like operating systems have historically made widespread use of
"mbox" database files for a variety of local email purposes. In the
common case, mbox files store linear sequences of one or more
electronic mail messages, with local email clients treating the
database as a logical folder of email messages. mbox databases are
also used by a variety of other messaging tools, such as mailing list
management programs, archiving and filtering utilities, messaging
servers, and other related applications. In recent years, mbox
databases have also become common on a large number of non-UNIX
computing platforms, for similar kinds of purposes.
The increased pervasiveness of these files has led to an increased
demand for a standardized, network-wide interchange of these files as
discrete database objects. In turn, this dictates a need for a
general media type definition for mbox files, which is the subject
and purpose of this memo.
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2. About the mbox Database
The mbox database format is not documented in an authoritative
specification, but instead exists as a well-known output format that
is anecdotally documented, or which is only authoritatively
documented for a specific platform or tool.
mbox databases typically contain a linear sequence of electronic mail
messages. Each message begins with a separator line that identifies
the message sender, and also identifies the date and time at which
the message was received by the final recipient (either the last-hop
system in the transfer path, or the system which serves as the
recipient's mailstore). Each message is typically terminated by an
empty line. The end of the database is usually recognized by either
the absence of any additional data, or by the presence of an explicit
end-of-file marker.
The structure of the separator lines vary across implementations, but
usually contain the exact character sequence of "From", followed by a
single Space character (0x20), an email address of some kind, another
Space character, a timestamp sequence of some kind, and an end-of-
line marker. However, due to the lack of any authoritative
specification, each of these attributes are known to vary widely
across implementations. For example, the email address can reflect
any addressing syntax that has ever been used on any messaging system
in all of history (specifically including address forms that are not
compatible with Internet messages, as defined by RFC 2822 [RFC2822]).
Similarly, the timestamp sequences can also vary according to system
output, while the end-of-line sequences will often reflect platform-
specific requirements. Different data formats can even appear within
a single database as a result of multiple mbox files being
concatenated together, or because a single file was accessed by
multiple messaging clients, each of which has used its own syntax for
the separator line.
Message data within mbox databases often reflects site-specific
peculiarities. For example, it is entirely possible for the message
body or headers in an mbox database to contain untagged eight-bit
character data that implicitly reflects a site-specific default
language or locale, or that reflects local defaults for timestamps
and email addresses; none of this data is widely portable beyond the
local scope. Similarly, message data can also contain unencoded
eight-bit binary data, or can use encoding formats that represent a
specific platform (e.g., BINHEX or UUENCODE sequences).
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Many implementations are also known to escape message body lines that
begin with the character sequence of "From ", so as to prevent
confusion with overly-liberal parsers that do not search for full
separator lines. In the common case, a leading Greater-Than symbol
(0x3E) is used for this purpose (with "From " becoming ">From ").
However, other implementations are known not to escape such lines
unless they are immediately preceded by a blank line or if they also
appear to contain an email address and a timestamp. Other
implementations are also known to perform secondary escapes against
these lines if they are already escaped or quoted, while others
ignore these mechanisms altogether.
A comprehensive description of mbox database files on UNIX-like
systems can be found at http://qmail.org./man/man5/mbox.html, which
should be treated as mostly authoritative for those variations that
are otherwise only documented in anecdotal form. However, readers
are advised that many other platforms and tools make use of mbox
databases, and that there are many more potential variations that can
be encountered in the wild.
In order to mitigate errors that may arise from such vagaries, this
specification defines a "format" parameter to the application/mbox
media type declaration, which can be used to identify the specific
kind of mbox database that is being transferred. Furthermore, this
specification defines a "default" database format which MUST be
supported by implementations that claim to be compliant with this
specification, and which is to be used as the implicit format for
undeclared application/mbox data objects. Additional format types
are to be defined in subsequent specifications. Messaging systems
that receive an mbox database with an unknown format parameter value
SHOULD treat the data as an opaque binary object, as if the data had
been declared as application/octet-stream
Refer to Appendix A for a description of the default mbox format.
Note that RFC 2046 [RFC2046] defines the multipart/digest media type
for transferring platform-independent message files. Because that
specification defines a set of neutral and strict formatting rules,
the multipart/digest media type already facilitates highly-
predictable transfer and conversion operations; as such, implementers
are strongly encouraged to support and use that media type where
possible.
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3. Prerequisites and Terminology
Readers of this document are expected to be familiar with the
specification for MIME [RFC2045] and MIME-type registrations
[RFC2048].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
4. The application/mbox Media Type Registration
This section provides the media type registration application (as per
[RFC2048]).
MIME media type name: application
MIME subtype name: mbox
Required parameters: none
Optional parameters: The "format" parameter identifies the format of
the mbox database and the messages contained therein. The default
value for the "format" parameter is "default", and refers to the
formatting rules defined in Appendix A of this memo. mbox databases
that do not have a "format" parameter SHOULD be interpreted as having
the implicit "format" value of "default". mbox databases that have
an unknown value for the "format" parameter SHOULD be treated as
opaque data objects, as if the media type had been specified as
application/octet-stream. Additional values for the format parameter
are to be defined in subsequent specifications, and registered with
IANA.
Encoding considerations: If an email client receives an mbox database
as a message attachment, and then stores that attachment within a
local mbox database, the contents of the two database files may
become irreversibly intermingled, such that both databases are
rendered unrecognizable. In order to avoid these collisions,
messaging systems that support this specification MUST encode an mbox
database (or at a minimum, the separator lines) with non-transparent
transfer encoding (such as BASE64 or Quoted-Printable) whenever an
application/mbox object is transferred via messaging protocols.
Other transfer services are generally encouraged to adopt similar
encoding strategies in order to allow for any subsequent
retransmission that might occur, but this is not a requirement.
Implementers should also be prepared to encode mbox data locally if
non-compliant data is received.
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Security considerations: mbox data is passive, and does not generally
represent a unique or new security threat. However, there is risk in
sharing any kind of data, because unintentional information may be
exposed, and this risk certainly applies to mbox data as well.
Interoperability considerations: Due to the lack of a single
authoritative specification for mbox databases, there are a large
number of variations between database formats (refer to the
introduction text for common examples), and it is expected that non-
conformant data will be erroneously tagged or exchanged. Although
the "default" format specified in this memo does not allow for these
kinds of vagaries, prior negotiation or agreement between humans may
sometimes be needed.
Published specification: see Appendix A.
Applications that use this media type: hundreds of messaging products
make use of the mbox database format, in one form or another.
Magic number(s): mbox database files can be recognized by having a
leading character sequence of "From", followed by a single Space
character (0x20), followed by additional printable character data
(refer to the description in Appendix A for details). However,
implementers are cautioned that all such files will not be compliant
with all of the formatting rules, therefore implementers should treat
these files with an appropriate amount of circumspection.
File extension(s): mbox database files sometimes have an ".mbox"
extension, but this is not required nor expected. As with magic
numbers, implementers should avoid reflexive assumptions about the
contents of such files.
Macintosh File Type Code(s): None are known to be common.
Person & email address to contact for further information: Eric A.
Hall (ehall@ntrg.com)
Intended usage: COMMON
5. Security Considerations
See the discussion in section 4.
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6. IANA Considerations
The IANA has registered the application/mbox media type in the MIME
registry, using the application provided in section 4 above.
Furthermore, IANA has established and will maintain a registry of
values for the "format" parameter as described in this memo. The
first registration is the "default" value, using the description
provided in Appendix A. Subsequent values for the "format" parameter
MUST be accompanied by some form of recognizable, complete, and
legitimate specification, such as an IESG-approved specification, or
some kind of authoritative vendor documentation.
7. Normative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[RFC2048] Freed, N., Klensin, J., and J. Postel, "Multipurpose
Internet Mail Extensions (MIME) Part Four: Registration
Procedures", BCP 13, RFC 2048, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April
2001.
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Appendix A. The "default" mbox Database Format
In order to improve interoperability among messaging systems, this
memo defines a "default" mbox database format, which MUST be
supported by all implementations that claim to be compliant with this
specification.
The "default" mbox database format uses a linear sequence of Internet
messages, with each message being immediately prefaced by a separator
line, and being terminated by an empty line. More specifically:
o Each message within the database MUST follow the syntax and
formatting rules defined in RFC 2822 [RFC2822] and its related
specifications, with the exception that the canonical mbox
database MUST use a single Line-Feed character (0x0A) as the
end-of-line sequence, and MUST NOT use a Carriage-Return/Line-
Feed pair (NB: this requirement only applies to the canonical
mbox database as transferred, and does not override any other
specifications). This usage represents the most common
historical representation of the mbox database format, and
allows for the least amount of conversion.
o Messages within the default mbox database MUST consist of
seven-bit characters within an eight-bit stream. Eight-bit data
within the stream MUST be converted to a seven-bit form (using
appropriate, standardized encoding) and appropriately tagged
(with the correct header fields) before the database is
transferred.
o Message headers and data in the default mbox database MUST be
fully-qualified, as per the relevant specification(s). For
example, email addresses in the various header fields MUST have
legitimate domain names (as per RFC 2822), while extended
characters and encodings MUST be specified in the appropriate
location (as per the appropriate MIME specifications), and so
forth.
o Each message in the mbox database MUST be immediately preceded
by a single separator line, which MUST conform to the following
syntax:
The exact character sequence of "From";
a single Space character (0x20);
the email address of the message sender (as obtained from the
message envelope or other authoritative source), conformant
with the "addr-spec" syntax from RFC 2822;
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a single Space character;
a timestamp indicating the UTC date and time when the message
was originally received, conformant with the syntax of the
traditional UNIX 'ctime' output sans timezone (note that the
use of UTC precludes the need for a timezone indicator);
an end-of-line marker.
o Each message in the database MUST be terminated by an empty
line, containing a single end-of-line marker.
Note that the first message in an mbox database will only be prefaced
by a separator line, while every other message will begin with two
end-of-line sequences (one at the end of the message itself, and
another to mark the end of the message within the mbox database file
stream) and a separator line (marking the new message). The end of
the database is implicitly reached when no more message data or
separator lines are found.
Also note that this specification does not prescribe any escape
syntax for message body lines that begin with the character sequence
of "From ". Recipient systems are expected to parse full separator
lines as they are documented above.
Author's Address
Eric A. Hall
EMail: ehall@ntrg.com
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