Network Working Group N. Borenstein, Bellcore
Request for Comments: 1343 June 1992
A User Agent Configuration Mechanism
For Multimedia Mail Format Information
Status of This Memo
This is an informational memo for the Internet community,
and requests discussion and suggestions for improvements.
This memo does not specify an Internet standard.
Distribution of this memo is unlimited.
Abstract
This memo suggests a file format to be used to inform
multiple mail reading user agent programs about the
locally-installed facilities for handling mail in various
formats. The mechanism is explicitly designed to work with
mail systems based Internet mail as defined by RFC's 821,
822, 934, 1049, 1113, and the Multipurpose Internet Mail
Extensions, known as MIME. However, with some extensions it
could probably be made to work for X.400-based mail systems
as well. The format and mechanism are proposed in a manner
that is generally operating-system independent. However,
certain implementation details will inevitably reflect
operating system differences, some of which will have to be
handled in a uniform manner for each operating system. This
memo makes such situations explicit, and, in an appendix,
suggests a standard behavior under the UNIX operating
system.
Introduction
The electronic mail world is in the midst of a transition
from single-part text-only mail to multi-part, multi-media
mail. In support of this transition, various extensions to
RFC 821 and RFC 822 have been proposed and/or adopted,
notably including MIME [RFC-1341]. Various parties have
demonstrated extremely high-functionality multimedia mail,
but the problem of mail interchange between different user
agents has been severe. In general, only text messages have
been shared between user agents that were not explicitly
designed to work together. This limitation is not
compatible with a smooth transition to a multi-media mail
world.
One approach to this transition is to modify diverse sets of
mail reading user agents so that, when they need to display
mail of an unfamiliar (non-text) type, they consult an
external file for information on how to display that file.
That file might say, for example, that if the content-type
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of a message is "foo" it can be displayed to the user via
the "displayfoo" program.
This approach means that, with a one-time modification, a
wide variety of mail reading programs can be given the
ability to display a wide variety of types of message.
Moreover, extending the set of media types supported at a
site becomes a simple matter of installing a binary and
adding a single line to a configuration file. Crucial to
this scheme, however, is that all of the user agents agree
on a common representation and source for the configuration
file. This memo proposes such a common representation.
Location of Configuration Information
Each user agent must clearly obtain the configuration
information from a common location, if the same information
is to be used to configure all user agents. However,
individual users should be able to override or augment a
site's configuration. The configuration information should
therefore be obtained from a designated set of locations.
The overall configuration will be obtained through the
virtual concatenation of several individual configuration
files known as mailcap files. The configuration information
will be obtained from the FIRST matching entry in a mailcap
file, where "matching" depends on both a matching content-
type specification, an entry containing sufficient
information for the purposes of the application doing the
searching, and the success of any test in the "test=" field,
if present.
The precise location of the mailcap files is operating-
system dependent. A standard location for UNIX is specified
in Appendix A.
Overall Format of a Mailcap File
Each mailcap file consists of a set of entries that describe
the proper handling of one media type at the local site.
For example, one line might tell how to display a message in
Group III fax format. A mailcap file consists of a sequence
of such individual entries, separated by newlines (according
to the operating system's newline conventions). Blank lines
and lines that start with the "#" character (ASCII 35) are
considered comments, and are ignored. Long entries may be
continued on multiple lines if each non-terminal line ends
with a backslash character ('\', ASCII 92), in which case
the multiple lines are to be treated as a single mailcap
entry. Note that for such "continued" lines, the backslash
must be the last character on the line to be continued.
Thus the overall format of a mailcap file is given, in the
modified BNF of RFC 822, as:
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Mailcap-File = *Mailcap-Line
Mailcap-Line = Comment / Mailcap-Entry
Comment = NEWLINE / "#" *CHAR NEWLINE
NEWLINE =
Note that the above specification implies that comments must
appear on lines all to themselves, with a "#" character as
the first character on each comment line.
Format of a Mailcap Entry
Each mailcap entry consists of a number of fields, separated
by semi-colons. The first two fields are required, and must
occur in the specified order. The remaining fields are
optional, and may appear in any order.
The first field is the content-type, which indicates the
type of data this mailcap entry describes how to handle. It
is to be matched against the type/subtype specification in
the "Content-Type" header field of an Internet mail message.
If the subtype is specified as "*", it is intended to match
all subtypes of the named content-type.
The second field, view-command, is a specification of how
the message or body part can be viewed at the local site.
Although the syntax of this field is fully specified, the
semantics of program execution are necessarily somewhat
operating system dependent. UNIX semantics are given in
Appendix A.
The optional fields, which may be given in any order, are as
follows:
-- The "compose" field may be used to specify a program that
can be used to compose a new body or body part in the given
format. Its intended use is to support mail composing
agents that support the composition of multiple types of
mail using external composing agents. As with the view-
command, the semantics of program execution are operating
system dependent, with UNIX semantics specified in Appendix
A. The result of the composing program may be data that is
not yet suitable for mail transport -- that is, a Content-
Transfer-Encoding may need to be applied to the data.
-- The "composetyped" field is similar to the "compose"
field, but is to be used when the composing program needs to
specify the Content-type header field to be applied to the
composed data. The "compose" field is simpler, and is
preferred for use with existing (non-mail-oriented) programs
for composing data in a given format. The "composetyped"
field is necessary when the Content-type information must
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include auxilliary parameters, and the composition program
must then know enough about mail formats to produce output
that includes the mail type information.
-- The "edit" field may be used to specify a program that
can be used to edit a body or body part in the given format.
In many cases, it may be identical in content to the
"compose" field, and shares the operating-system dependent
semantics for program execution.
-- The "print" field may be used to specify a program that
can be used to print a message or body part in the given
format. As with the view-command, the semantics of program
execution are operating system dependent, with UNIX
semantics specified in Appendix A.
-- The "test" field may be used to test some external
condition (e.g. the machine architecture, or the window
system in use) to determine whether or not the mailcap line
applies. It specifies a program to be run to test some
condition. The semantics of execution and of the value
returned by the test program are operating system dependent,
with UNIX semantics specified in Appendix A. If the test
fails, a subsequent mailcap entry should be sought.
Multiple test fields are not permitted -- since a test can
call a program, it can already be arbitrarily complex.
-- The "needsterminal" field indicates that the view-command
must be run on an interactive terminal. This is needed to
inform window-oriented user agents that an interactive
terminal is needed. (The decision is not left exclusively
to the view-command because in some circumstances it may not
be possible for such programs to tell whether or not they
are on interactive terminals.) The needsterminal command
should be assumed to apply to the compose and edit commands,
too, if they exist. Note that this is NOT a test -- it is a
requirement for the environment in which the program will be
executed, and should typically cause the creation of a
terminal window when not executed on either a real terminal
or a terminal window.
-- The "copiousoutput" field indicates that the output from
the view-command will be an extended stream of output, and
is to be interpreted as advice to the UA (User Agent mail-
reading program) that the output should be either paged or
made scrollable. Note that it is probably a mistake if
needsterminal and copiousoutput are both specified.
-- The "description" field simply provides a textual
description, optionally quoted, that describes the type of
data, to be used optionally by mail readers that wish to
describe the data before offering to display it.
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-- The "x11-bitmap" field names a file, in X11 bitmap (xbm)
format, which points to an appropriate icon to be used to
visually denote the presence of this kind of data.
-- Any other fields beginning with "x-" may be included for
local or mailer-specific extensions of this format.
Implementations should simply ignore all such unrecognized
fields to permit such extensions, some of which might be
standardized in a future version of this document.
Some of the fields above, such as "needsterminal", apply to
the actions of the view-command, edit-command, and compose-
command, alike. In some unusual cases, this may not be
desirable, but differentiation can be accomplished via
separate mailcap entries, taking advantage of the fact that
subsequent mailcap entries are searched if an earlier
mailcap entry does not provide enough information:
application/postscript; ps-to-terminal %s; \
needsterminal
application/postscript; ps-to-terminal %s; \
compose=idraw %s
In RFC 822 modified BNF, the following grammar describes a
mailcap entry:
Mailcap-Entry = typefield ; view-command
[";" 1#field]
typefield = propertype / implicit-wild
propertype = type "/" wildsubtype
implicitwild = type
wildsubtype = subtype / "*"
view-command = mtext
mtext = *mchar
mchar = schar / qchar
schar = *
qchar = "\" CHAR ; may quote any char
field = flag / namedfield
namedfield = fieldname "=" mtext
flag = "needsterminal" ; All these literals are to
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/ "copiousoutput" ; be interpreted as
/ x-token ; case-insensitive
fieldname = / "compose" ;Also all of these
/ "composetyped" ;are case-insensitive.
/ "print"
/ "edit"
/ "test"
/ "x11-bitmap"
/ "description"
/ x-token
Note that "type", "subtype", and "x-token" are defined in
MIME. Note also that while the definition of "schar"
includes the percent sign, "%", this character has a special
meaning in at least the UNIX semantics, and will therefore
need to be quoted as a qchar to be used literally.
Appendix A: Implementation Details for UNIX
Although this memo fully specifies a syntax for "mailcap"
files, the semantics of the mailcap file are of necessity
operating-system dependent in four respects. In order to
clarify the intent, and to promote a standard usage, this
appendix proposes a UNIX semantics for these four cases. If
a mailcap mechanism is implemented on non-UNIX systems,
similar semantic decisions should be made and published.
Location of the Mailcap File(s)
For UNIX, a path search of mailcap files is specified. The
default path search is specified as including at least the
following:
$HOME/.mailcap:/etc/mailcap:/usr/etc/mailcap:/usr/local/etc/mailcap
However, this path may itself be overridden by a path
specified by the MAILCAPS environment variable.
Semantics of executable commands
Several portions of a mailcap entry specify commands to be
executed. In particular, the mandatory second field, the
view-command, takes a command to be executed, as do the
optional print, edit, test, and compose fields.
On a UNIX system, such commands will each be a full shell
command line, including the path name for a program and its
arguments. (Because of differences in shells and the
implementation and behavior of the same shell from one
system to another, it is specified that the command line be
intended as input to the Bourne shell, i.e. that it is
implicitly preceded by "/bin/sh -c " on the command line.)
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The two characters "%s", if used, will be replaced by the
name of a file for the actual mail body data. In the case
of the edit adn view-command, the body part will be passed
to this command as standard input unless one or more
instances of "%s" appear in the view-command, in which case
%s will be replaced by the name of a file containing the
body part, a file which may have to be created before the
view-command program is executed. (Such files cannot be
presumed to continue to exist after the view-command program
exits. Thus a view-command that wishes to exit and continue
processing in the background should take care to save the
data first.) In the case of the compose and composetyped
commands, %s should be replaced by the name of a file to
which the composed data should be written by the programs
named in the compose or composedtyped commands. Thus, the
calling program will look in that file later in order to
retrieve the composed data. If %s does not appear in the
compose or composetyped commands, then the composed data
will be assumed to be written by the composing programs to
standard output.
Furthermore, any occurrence of "%t" will be replaced by the
content-type and subtype specification. (That is, if the
content-type is "text/plain", then %t will be replaced by
"text/plain".) A literal % character may be quoted as \%.
Finally, named parameters from the Content-type field may be
placed in the command execution line using "%{" followed by
the parameter name and a closing "}" character. The entire
parameter should appear as a single command line argument,
regardless of embedded spaces. Thus, if the message has a
Content-type line of:
Content-type: multipart/mixed; boundary=42
and the mailcap file has a line of:
multipart/*; /usr/local/bin/showmulti \
%t %{boundary}
then the equivalent of the following command should be
executed:
/usr/local/bin/showmulti multipart/mixed 42
Semantics of the "test" field
The "test" field specifies a program to be used to test
whether or not the current mailcap line applies. This can
be used, for example, to have a mailcap line that only
applies if the X window system is running, or if the user is
running on a SPARCstation with a /dev/audio. The value of
the "test" field is a program to run to test such a
condition. The precise program to run and arguments to give
it are determined as specified in the previous section. The
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test program should return an exit code of zero if the
condition is true, and a non-zero code otherwise.
Semantics of the "compose" field
On UNIX, the composing program is expected to produce a data
stream for such a body part as its standard output. The
program will be executed with the command line arguments
determined as specified above. The data returned via its
standard output will be given a Content-Type field that has
no supplementary parameters. For example, the following
mailcap entry:
audio/basic; /usr/local/bin/showaudio %t
compose = /usr/local/bin/recordaudio
would result in tagging the data composed by the
"recordaudio" program as:
Content-Type: audio/basic
If this is unacceptable -- for example, in the case of
multipart mail a "boundary" parameter is required -- then
the "compose" field cannot be used. Instead, the
"composetyped" field should be used in the mailcap file.
Semantics of the "composetyped" field
The "composetyped" filed is much like the "compose" field,
except that it names a composition program that produces,
not raw data, but data that includes a MIME-conformant type
specification. The program will be executed with the
command line arguments determined as specified above. The
data returned via its standard output must begin with a
Content-Type header, followed optionally by other Content-*
headers, and then by a blank line and the data. For
example, the following mailcap entry:
multipart/mixed; /usr/local/bin/showmulti %t \
%{boundary}; \
composetyped = /usr/local/bin/makemulti
would result in executing the "makemulti" program, which
would be expected to begin its output with a line of the
form:
Content-Type: multipart/mixed; boundary=foobar
Note that a composition program need not encode binary data
in base64 or quoted-printable. It remains the responsibility
of the software calling the composition program to encode
such data as necessary. However, if a composing program
does encode data, which is not encouraged, it should
announce that fact using a Content-Transfer-Encoding header
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in the standard manner defined by MIME. Because such
encodings must be announced by such a header, they are an
option only for composetyped programs, not for compose
programs.
Appendix B: Sample Mailcap File
The following is an example of a mailcap file for UNIX that
demonstrates most of the syntax above. It contains
explanatory comments where necessary.
# Mailcap file for Bellcore lab 214.
#
# The next line sends "richtext" to the richtext
program
text/richtext; richtext %s; copiousoutput
#
# Next, basic u-law audio
audio/*; showaudio; test=/usr/local/bin/hasaudio
#
# Next, use the xview program to handle several image
formats
image/*; xview %s; test=/usr/local/bin/RunningX
#
# The ATOMICMAIL interpreter uses curses, so needs a
terminal
application/atomicmail; /usr/local/bin/atomicmail %s; \
needsterminal
#
# The next line handles Andrew format,
# if ez and ezview are installed
x-be2; /usr/andrew/bin/ezview %s; \
print=/usr/andrew/bin/ezprint %s ; \
compose=/usr/andrew/bin/ez -d %s \;
edit=/usr/andrew/bin/ez -d %s; \;
copiousoutput
#
# The next silly example demonstrates the use of
quoting
application/*; echo "This is \\"%t\\" but \
is 50 \% Greek to me" \; cat %s; copiousoutput
Appendix C: A Note on Format Translation
It has been suggested that another function of a mailcap-
like mechanism might be to specify the locally available
tools for document format translation. For example, the
file could designate a program for translating from format A
to format B, another for translating from format B to format
C, and finally a mechanism for displaying format C.
Although this mechanism would be somewhat richer than the
current mailcap file, and might conceivably also have
utility at the message transport layer, it significantly
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complicates the processing effort necessary for a user agent
that simply wants to display a message in format A. Using
the current, simpler, mailcap scheme, a single line could
tell such a user agent to display A-format mail using a
pipeline of translators and the C-format viewer. This memo
resists the temptation to complicate the necessary
processing for a user agent to accomplish this task. Using
the mailcap format defined here, it is only necessary to
find the correct single line in a mailcap file, and to
execute the command given in that line.
References
[RFC 822] Crocker, D., "Standard for the format of ARPA
Internet text messages", RFC 822, UDEL, August, 1982.
[RFC 1341] Borenstein, N., and N. Freed, "MIME
(Multipurpose Internet Mail Extensions): Mechanisms for
Specifying and Describing the Format of Internet Message
Bodies", RFC 1341, Bellcore, June, 1992.
Acknowledgements
The author wishes to thank Malcolm Bjorn Gillies, Dan
Heller, Olle Jaernefors, Keith Moore, Luc Rooijakkers, and
the other members of the IETF task force on mail extensions
for their comments on earlier versions of this draft. If
other acknowledgements were neglected, please let me know,
as it was surely accidental.
Security Considerations
Security issues are not discussed in this memo. However,
the use of the mechanisms described in this memo can make
it easier for implementations to slip into the kind of
security problems discussed in the MIME document.
Implementors and mailcap administrators should be aware of
these security considerations, and in particular should
exercise caution in the choice of programs to be listed in a
mailcap file for automatic execution.
Author's Address
Nathaniel S. Borenstein
MRE 2D-296, Bellcore
445 South St.
Morristown, NJ 07962-1910
Email: nsb@bellcore.com
Phone: +1 201 829 4270
Fax: +1 201 829 7019
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