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PROPOSED STANDARD
Errata Exist
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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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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