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rfc:rfc1524

Network Working Group N. Borenstein Request for Comments: 1524 Bellcore Category: Informational September 1993

                A User Agent Configuration Mechanism
               For Multimedia Mail Format Information

Status of This Memo

 This memo provides information for the Internet community.  It 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 (STD 10), 822 (STD 11), 934, 1049 (STD 11), 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-1521].
 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

Borenstein [Page 1] RFC 1524 Multimedia Mail Configuration September 1993

 example, that if the content-type 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.

Borenstein [Page 2] RFC 1524 Multimedia Mail Configuration September 1993

 Thus the overall format of a mailcap file is given, in the modified
 BNF of RFC 822, as:
       Mailcap-File = *Mailcap-Line
       Mailcap-Line = Comment / Mailcap-Entry
       Comment = NEWLINE  /  "#" *CHAR NEWLINE
       NEWLINE = <newline as defined by OS convention>
 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:
  1. - 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.
  1. - The "composetyped" field is similar to the "compose" field, but is

to be used when the composing program needs to specify the

Borenstein [Page 3] RFC 1524 Multimedia Mail Configuration September 1993

    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 include auxilliary parameters, and the
    composition program must then know enough about mail formats to
    produce output that includes the mail type information.
  1. - 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.
  1. - 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.
  1. - 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.
  1. - 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.
  1. - 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.

Borenstein [Page 4] RFC 1524 Multimedia Mail Configuration September 1993

  1. - 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.
  1. - The "textualnewlines" field, if set to any non-zero value,

indicates that this type of data is line-oriented and that, if

    encoded in base64, all newlines should be converted to canonical
    form (CRLF) before encoding, and will be in that form after
    decoding.  In general, this field is needed only if there is
    line-oriented data of some type other than text/* or non-line-
    oriented data that is a subtype of text.
  1. - 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.
  1. - The "nametemplate" field gives a file name format, in which %s

will be replaced by a short unique string to give the name of the

    temporary file to be passed to the viewing command.  This is only
    expected to be relevant in environments where filename extensions
    are meaningful, e.g., one coulld specify that a GIF file being
    passed to a gif viewer should have a name eding in ".gif" by using
    "nametemplate=%s.gif".
 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:

Borenstein [Page 5] RFC 1524 Multimedia Mail Configuration September 1993

       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 = * <any CHAR except ";","\", and CTLS>
       qchar = "\" CHAR ; may quote any char
       field = flag / namedfield
       namedfield = fieldname "=" mtext
       flag = "needsterminal"   ; All these literals are to
            / "copiousoutput"   ; be interpreted as
            / x-token           ; case-insensitive
       fieldname =    / "compose"      ;Also all of these
                      / "composetyped" ;are case-insensitive.
                      / "print"
                      / "edit"
                      / "test"
                      / "x11-bitmap"
                      / "textualnewlines"
                      / "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.

Borenstein [Page 6] RFC 1524 Multimedia Mail Configuration September 1993

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

Borenstein [Page 7] RFC 1524 Multimedia Mail Configuration September 1993

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 fie ld, 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.)
 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

Borenstein [Page 8] RFC 1524 Multimedia Mail Configuration September 1993

 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
 If the content-type is "multipart" (any subtype), then the two
 characters "%n" will be replaced by an integer giving the number of
 sub-parts within the multipart entity.  Also, the two characters "%F"
 will be replaced by a set of arguments, twice as many arguments as
 the number of sub-parts, consisting of alternating content-types and
 file names for each part in turn.  Thus if multipart entity has three
 parts, "%F" will be replaced by the equivalent of "content-type1
 file-name1 content-type2 file-name2 content-type3 file-name3".

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
 test program should return an exit code of zero if the condition is
 true, and a non-zero code otherwise.

Borenstein [Page 9] RFC 1524 Multimedia Mail Configuration September 1993

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 in the standard manner defined by MIME.  Because such

Borenstein [Page 10] RFC 1524 Multimedia Mail Configuration September 1993

 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

Borenstein [Page 11] RFC 1524 Multimedia Mail Configuration September 1993

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 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", STD 11, RFC 822, UDEL, August 1982.
   [RFC-1521] Borenstein, N., and N.  Freed, "MIME (Multipurpose
   Internet Mail Extensions) Part One: Mechanisms for Specifying and
   Describing the Format of Internet Message Bodies", RFC 1521,
   Bellcore, Innosoft, September 1993.

Borenstein [Page 12]

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