    Internet Engineering Task Force (IETF)                        J. Reschke
    Request for Comments: 6266                                    greenbytes
    Updates: 2616                                                  June 2011
    Category: Standards Track
    ISSN: 2070-1721

               Use of the Content-Disposition Header Field in the
                       Hypertext Transfer Protocol (HTTP)

    Abstract

       RFC 2616 defines the Content-Disposition response header field, but
       points out that it is not part of the HTTP/1.1 Standard.  This
       specification takes over the definition and registration of Content-
       Disposition, as used in HTTP, and clarifies internationalization
       aspects.

    Status of This Memo

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 5741.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       http://www.rfc-editor.org/info/rfc6266.

    Copyright Notice

       Copyright (c) 2011 IETF Trust and the persons identified as the
       document authors.  All rights reserved.

       This document is subject to BCP 78 and the IETF Trust's Legal
       Provisions Relating to IETF Documents
       (http://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

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    RFC 6266               Content-Disposition in HTTP             June 2011

    Table of Contents

       1. Introduction ....................................................2
       2. Notational Conventions ..........................................3
       3. Conformance and Error Handling ..................................3
       4. Header Field Definition .........................................3
          4.1. Grammar ....................................................4
          4.2. Disposition Type ...........................................5
          4.3. Disposition Parameter: 'Filename' ..........................5
          4.4. Disposition Parameter: Extensions ..........................6
          4.5. Extensibility ..............................................7
       5. Examples ........................................................7
       6. Internationalization Considerations .............................8
       7. Security Considerations .........................................8
       8. IANA Considerations .............................................8
          8.1. Registry for Disposition Values and Parameters .............8
          8.2. Header Field Registration ..................................8
       9. Acknowledgements ................................................9
       10. References .....................................................9
          10.1. Normative References ......................................9
          10.2. Informative References ....................................9
       Appendix A. Changes from the RFC 2616 Definition ..................11
       Appendix B. Differences Compared to RFC 2183 ......................11
       Appendix C. Alternative Approaches to Internationalization ........11
         C.1. RFC 2047 Encoding ..........................................12
         C.2. Percent Encoding ...........................................12
         C.3. Encoding Sniffing ..........................................12
       Appendix D. Advice on Generating Content-Disposition Header
                   Fields ................................................13

    1.  Introduction

       RFC 2616 defines the Content-Disposition response header field
       (Section 19.5.1 of [RFC2616]) but points out that it is not part of
       the HTTP/1.1 Standard (Section 15.5):

          Content-Disposition is not part of the HTTP standard, but since it
          is widely implemented, we are documenting its use and risks for
          implementers.

       This specification takes over the definition and registration of
       Content-Disposition, as used in HTTP.  Based on interoperability
       testing with existing user agents (UAs), it fully defines a profile
       of the features defined in the Multipurpose Internet Mail Extensions
       (MIME) variant ([RFC2183]) of the header field, and also clarifies
       internationalization aspects.

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          Note: This document does not apply to Content-Disposition header
          fields appearing in payload bodies transmitted over HTTP, such as
          when using the media type "multipart/form-data" ([RFC2388]).

    2.  Notational Conventions

       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 [RFC2119].

       This specification uses the augmented BNF (ABNF) notation defined in
       Section 2.1 of [RFC2616], including its rules for implied linear
       whitespace (LWS).

    3.  Conformance and Error Handling

       This specification defines conformance criteria for both senders
       (usually, HTTP origin servers) and recipients (usually, HTTP user
       agents) of the Content-Disposition header field.  An implementation
       is considered conformant if it complies with all of the requirements
       associated with its role.

       This specification also defines certain forms of the header field
       value to be invalid, using both ABNF and prose requirements
       (Section 4), but it does not define special handling of these invalid
       field values.

       Senders MUST NOT generate Content-Disposition header fields that are
       invalid.

       Recipients MAY take steps to recover a usable field value from an
       invalid header field, but SHOULD NOT reject the message outright,
       unless this is explicitly desirable behavior (e.g., the
       implementation is a validator).  As such, the default handling of
       invalid fields is to ignore them.

    4.  Header Field Definition

       The Content-Disposition response header field is used to convey
       additional information about how to process the response payload, and
       also can be used to attach additional metadata, such as the filename
       to use when saving the response payload locally.

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    4.1.  Grammar

         content-disposition = "Content-Disposition" ":"
                                disposition-type *( ";" disposition-parm )

         disposition-type    = "inline" | "attachment" | disp-ext-type
                             ; case-insensitive
         disp-ext-type       = token

         disposition-parm    = filename-parm | disp-ext-parm

         filename-parm       = "filename" "=" value
                             | "filename*" "=" ext-value

         disp-ext-parm       = token "=" value
                             | ext-token "=" ext-value
         ext-token           = <the characters in token, followed by "*">

       Defined in [RFC2616]:

         token         = <token, defined in [RFC2616], Section 2.2>
         quoted-string = <quoted-string, defined in [RFC2616], Section 2.2>
         value         = <value, defined in [RFC2616], Section 3.6>
                       ; token | quoted-string

       Defined in [RFC5987]:

         ext-value   = <ext-value, defined in [RFC5987], Section 3.2>

       Content-Disposition header field values with multiple instances of
       the same parameter name are invalid.

       Note that due to the rules for implied linear whitespace (Section 2.1
       of [RFC2616]), OPTIONAL whitespace can appear between words (token or
       quoted-string) and separator characters.

       Furthermore, note that the format used for ext-value allows
       specifying a natural language (e.g., "en"); this is of limited use
       for filenames and is likely to be ignored by recipients.

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    4.2.  Disposition Type

       If the disposition type matches "attachment" (case-insensitively),
       this indicates that the recipient should prompt the user to save the
       response locally, rather than process it normally (as per its media
       type).

       On the other hand, if it matches "inline" (case-insensitively), this
       implies default processing.  Therefore, the disposition type "inline"
       is only useful when it is augmented with additional parameters, such
       as the filename (see below).

       Unknown or unhandled disposition types SHOULD be handled by
       recipients the same way as "attachment" (see also [RFC2183],
       Section 2.8).

    4.3.  Disposition Parameter: 'Filename'

       The parameters "filename" and "filename*", to be matched case-
       insensitively, provide information on how to construct a filename for
       storing the message payload.

       Depending on the disposition type, this information might be used
       right away (in the "save as..." interaction caused for the
       "attachment" disposition type), or later on (for instance, when the
       user decides to save the contents of the current page being
       displayed).

       The parameters "filename" and "filename*" differ only in that
       "filename*" uses the encoding defined in [RFC5987], allowing the use
       of characters not present in the ISO-8859-1 character set
       ([ISO-8859-1]).

       Many user agent implementations predating this specification do not
       understand the "filename*" parameter.  Therefore, when both
       "filename" and "filename*" are present in a single header field
       value, recipients SHOULD pick "filename*" and ignore "filename".
       This way, senders can avoid special-casing specific user agents by
       sending both the more expressive "filename*" parameter, and the
       "filename" parameter as fallback for legacy recipients (see Section 5
       for an example).

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       It is essential that recipients treat the specified filename as
       advisory only, and thus be very careful in extracting the desired
       information.  In particular:

       o  Recipients MUST NOT be able to write into any location other than
          one to which they are specifically entitled.  To illustrate the
          problem, consider the consequences of being able to overwrite
          well-known system locations (such as "/etc/passwd").  One strategy
          to achieve this is to never trust folder name information in the
          filename parameter, for instance by stripping all but the last
          path segment and only considering the actual filename (where 'path
          segments' are the components of the field value delimited by the
          path separator characters "\" and "/").

       o  Many platforms do not use Internet Media Types ([RFC2046]) to hold
          type information in the file system, but rely on filename
          extensions instead.  Trusting the server-provided file extension
          could introduce a privilege escalation when the saved file is
          later opened (consider ".exe").  Thus, recipients that make use of
          file extensions to determine the media type MUST ensure that a
          file extension is used that is safe, optimally matching the media
          type of the received payload.

       o  Recipients SHOULD strip or replace character sequences that are
          known to cause confusion both in user interfaces and in filenames,
          such as control characters and leading and trailing whitespace.

       o  Other aspects recipients need to be aware of are names that have a
          special meaning in the file system or in shell commands, such as
          "." and "..", "~", "|", and also device names.  Recipients SHOULD
          ignore or substitute names like these.

          Note: Many user agents do not properly handle the escape character
          "\" when using the quoted-string form.  Furthermore, some user
          agents erroneously try to perform unescaping of "percent" escapes
          (see Appendix C.2), and thus might misinterpret filenames
          containing the percent character followed by two hex digits.

    4.4.  Disposition Parameter: Extensions

       To enable future extensions, recipients SHOULD ignore unrecognized
       parameters (see also [RFC2183], Section 2.8).

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    4.5.  Extensibility

       Note that Section 9 of [RFC2183] defines IANA registries both for
       disposition types and disposition parameters.  This registry is
       shared by different protocols using Content-Disposition, such as MIME
       and HTTP.  Therefore, not all registered values may make sense in the
       context of HTTP.

    5.  Examples

       Direct the UA to show "save as" dialog, with a filename of
       "example.html":

         Content-Disposition: Attachment; filename=example.html

       Direct the UA to behave as if the Content-Disposition header field
       wasn't present, but to remember the filename "an example.html" for a
       subsequent save operation:

         Content-Disposition: INLINE; FILENAME= "an example.html"

       Note: This uses the quoted-string form so that the space character
       can be included.

       Direct the UA to show "save as" dialog, with a filename containing
       the Unicode character U+20AC (EURO SIGN):

         Content-Disposition: attachment;
                              filename*= UTF-8''%e2%82%ac%20rates

       Here, the encoding defined in [RFC5987] is also used to encode the
       non-ISO-8859-1 character.

       This example is the same as the one above, but adding the "filename"
       parameter for compatibility with user agents not implementing
       RFC 5987:

         Content-Disposition: attachment;
                              filename="EURO rates";
                              filename*=utf-8''%e2%82%ac%20rates

       Note: Those user agents that do not support the RFC 5987 encoding
       ignore "filename*" when it occurs after "filename".

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    6.  Internationalization Considerations

       The "filename*" parameter (Section 4.3), using the encoding defined
       in [RFC5987], allows the server to transmit characters outside the
       ISO-8859-1 character set, and also to optionally specify the language
       in use.

       Future parameters might also require internationalization, in which
       case the same encoding can be used.

    7.  Security Considerations

       Using server-supplied information for constructing local filenames
       introduces many risks.  These are summarized in Section 4.3.

       Furthermore, implementers ought to be aware of the security
       considerations applying to HTTP (see Section 15 of [RFC2616]), and
       also the parameter encoding defined in [RFC5987] (see Section 5).

    8.  IANA Considerations

    8.1.  Registry for Disposition Values and Parameters

       This specification does not introduce any changes to the registration
       procedures for disposition values and parameters that are defined in
       Section 9 of [RFC2183].

    8.2.  Header Field Registration

       This document updates the definition of the Content-Disposition HTTP
       header field in the permanent HTTP header field registry (see
       [RFC3864]).

       Header field name:  Content-Disposition

       Applicable protocol:  http

       Status:  standard

       Author/Change controller:  IETF

       Specification document:  this specification (Section 4)

       Related information:  none

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    9.  Acknowledgements

       Thanks to Adam Barth, Rolf Eike Beer, Stewart Bryant, Bjoern
       Hoehrmann, Alfred Hoenes, Roar Lauritzsen, Alexey Melnikov, Henrik
       Nordstrom, and Mark Nottingham for their valuable feedback.

    10.  References

    10.1.  Normative References

       [ISO-8859-1]  International Organization for Standardization,
                     "Information technology -- 8-bit single-byte coded
                     graphic character sets -- Part 1: Latin alphabet
                     No. 1", ISO/IEC 8859-1:1998, 1998.

       [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                     Requirement Levels", BCP 14, RFC 2119, March 1997.

       [RFC2616]     Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
                     Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
                     Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

       [RFC5987]     Reschke, J., "Character Set and Language Encoding for
                     Hypertext Transfer Protocol (HTTP) Header Field
                     Parameters", RFC 5987, August 2010.

    10.2.  Informative References

       [RFC2046]     Freed, N. and N. Borenstein, "Multipurpose Internet
                     Mail Extensions (MIME) Part Two: Media Types",
                     RFC 2046, November 1996.

       [RFC2047]     Moore, K., "MIME (Multipurpose Internet Mail
                     Extensions) Part Three: Message Header Extensions for
                     Non-ASCII Text", RFC 2047, November 1996.

       [RFC2183]     Troost, R., Dorner, S., and K. Moore, Ed.,
                     "Communicating Presentation Information in Internet
                     Messages: The Content-Disposition Header Field",
                     RFC 2183, August 1997.

       [RFC2231]     Freed, N. and K. Moore, "MIME Parameter Value and
                     Encoded Word Extensions: Character Sets, Languages, and
                     Continuations", RFC 2231, November 1997.

       [RFC2388]     Masinter, L., "Returning Values from Forms: multipart/
                     form-data", RFC 2388, August 1998.

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       [RFC3864]     Klyne, G., Nottingham, M., and J. Mogul, "Registration
                     Procedures for Message Header Fields", BCP 90,
                     RFC 3864, September 2004.

       [RFC3986]     Berners-Lee, T., Fielding, R., and L. Masinter,
                     "Uniform Resource Identifier (URI): Generic Syntax",
                     STD 66, RFC 3986, January 2005.

       [US-ASCII]    American National Standards Institute, "Coded Character
                     Set -- 7-bit American Standard Code for Information
                     Interchange", ANSI X3.4, 1986.

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    Appendix A.  Changes from the RFC 2616 Definition

       Compared to Section 19.5.1 of [RFC2616], the following normative
       changes reflecting actual implementations have been made:

       o  According to RFC 2616, the disposition type "attachment" only
          applies to content of type "application/octet-stream".  This
          restriction has been removed, because recipients in practice do
          not check the content type, and it also discourages properly
          declaring the media type.

       o  RFC 2616 only allows "quoted-string" for the filename parameter.
          This would be an exceptional parameter syntax, and also doesn't
          reflect actual use.

       o  The definition for the disposition type "inline" ([RFC2183],
          Section 2.1) has been re-added with a suggestion for its
          processing.

       o  This specification requires support for the extended parameter
          encoding defined in [RFC5987].

    Appendix B.  Differences Compared to RFC 2183

       Section 2 of [RFC2183] defines several additional disposition
       parameters: "creation-date", "modification-date", "quoted-date-time",
       and "size".  The majority of user agents do not implement these;
       thus, they have been omitted from this specification.

    Appendix C.  Alternative Approaches to Internationalization

       By default, HTTP header field parameters cannot carry characters
       outside the ISO-8859-1 ([ISO-8859-1]) character encoding (see
       [RFC2616], Section 2.2).  For the "filename" parameter, this of
       course is an unacceptable restriction.

       Unfortunately, user agent implementers have not managed to come up
       with an interoperable approach, although the IETF Standards Track
       specifies exactly one solution ([RFC2231], clarified and profiled for
       HTTP in [RFC5987]).

       For completeness, the sections below describe the various approaches
       that have been tried, and explain how they are inferior to the
       RFC 5987 encoding used in this specification.

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    C.1.  RFC 2047 Encoding

       RFC 2047 defines an encoding mechanism for header fields, but this
       encoding is not supposed to be used for header field parameters --
       see Section 5 of [RFC2047]:

          An 'encoded-word' MUST NOT appear within a 'quoted-string'.

          ...

          An 'encoded-word' MUST NOT be used in parameter of a MIME Content-
          Type or Content-Disposition field, or in any structured field body
          except within a 'comment' or 'phrase'.

       In practice, some user agents implement the encoding, some do not
       (exposing the encoded string to the user), and some get confused by
       it.

    C.2.  Percent Encoding

       Some user agents accept percent-encoded ([RFC3986], Section 2.1)
       sequences of characters.  The character encoding being used for
       decoding depends on various factors, including the encoding of the
       referring page, the user agent's locale, its configuration, and also
       the actual value of the parameter.

       In practice, this is hard to use because those user agents that do
       not support it will display the escaped character sequence to the
       user.  For those user agents that do implement this, it is difficult
       to predict what character encoding they actually expect.

    C.3.  Encoding Sniffing

       Some user agents inspect the value (which defaults to ISO-8859-1 for
       the quoted-string form) and switch to UTF-8 when it seems to be more
       likely to be the correct interpretation.

       As with the approaches above, this is not interoperable and,
       furthermore, risks misinterpreting the actual value.

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    Appendix D.  Advice on Generating Content-Disposition Header Fields

       To successfully interoperate with existing and future user agents,
       senders of the Content-Disposition header field are advised to:

       o  Include a "filename" parameter when US-ASCII ([US-ASCII]) is
          sufficiently expressive.

       o  Use the 'token' form of the filename parameter only when it does
          not contain disallowed characters (e.g., spaces); in such cases,
          the quoted-string form should be used.

       o  Avoid including the percent character followed by two hexadecimal
          characters (e.g., %A9) in the filename parameter, since some
          existing implementations consider it to be an escape character,
          while others will pass it through unchanged.

       o  Avoid including the "\" character in the quoted-string form of the
          filename parameter, as escaping is not implemented by some user
          agents, and "\" can be considered an illegal path character.

       o  Avoid using non-ASCII characters in the filename parameter.
          Although most existing implementations will decode them as
          ISO-8859-1, some will apply heuristics to detect UTF-8, and thus
          might fail on certain names.

       o  Include a "filename*" parameter where the desired filename cannot
          be expressed faithfully using the "filename" form.  Note that
          legacy user agents will not process this, and will fall back to
          using the "filename" parameter's content.

       o  When a "filename*" parameter is sent, to also generate a
          "filename" parameter as a fallback for user agents that do not
          support the "filename*" form, if possible.  This can be done by
          substituting characters with US-ASCII sequences (e.g., Unicode
          character point U+00E4 (LATIN SMALL LETTER A WITH DIARESIS) by
          "ae").  Note that this may not be possible in some locales.

       o  When a "filename" parameter is included as a fallback (as per
          above), "filename" should occur first, due to parsing problems in
          some existing implementations.

       o  Use UTF-8 as the encoding of the "filename*" parameter, when
          present, because at least one existing implementation only
          implements that encoding.

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       Note that this advice is based upon UA behavior at the time of
       writing, and might be superseded.  At the time of publication of this
       document, <http://purl.org/NET/http/content-disposition-tests>
       provides an overview of current levels of support in various
       implementations.

    Author's Address

       Julian F. Reschke
       greenbytes GmbH
       Hafenweg 16
       Muenster, NW  48155
       Germany

       EMail: julian.reschke@greenbytes.de
       URI:   http://greenbytes.de/tech/webdav/

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