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.
            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
            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
            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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            RFC 1343       Multimedia Mail Configuration       June 1992
            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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            RFC 1343       Multimedia Mail Configuration       June 1992
                 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
            -- 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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            RFC 1343       Multimedia Mail Configuration       June 1992
            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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            RFC 1343       Multimedia Mail Configuration       June 1992
            -- 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; \
                 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 = * <any CHAR except
                            ";", "\", and CTLS>
                 qchar = "\" CHAR ; may quote any char
                 field = flag / namedfield
                 namedfield = fieldname "=" mtext
                 flag = "needsterminal"   ; All these literals are to
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            RFC 1343       Multimedia Mail Configuration       June 1992
                      / "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
            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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            RFC 1343       Multimedia Mail Configuration       June 1992
            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
                 /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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            RFC 1343       Multimedia Mail Configuration       June 1992
            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
                 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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            RFC 1343       Multimedia Mail Configuration       June 1992
            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
          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
                 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
                 image/*; xview %s; test=/usr/local/bin/RunningX
                 # The ATOMICMAIL interpreter uses curses, so needs a
                 application/atomicmail; /usr/local/bin/atomicmail %s; \
                 # 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; \;
                 # The next silly example demonstrates the use of
                 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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            RFC 1343       Multimedia Mail Configuration       June 1992
            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.
            [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.
            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
            Phone: +1 201 829 4270
            Fax:  +1 201 829 7019
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