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rfc:rfc6266

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.

Reschke Standards Track [Page 1] 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.

Reschke Standards Track [Page 2] RFC 6266 Content-Disposition in HTTP June 2011

    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.

Reschke Standards Track [Page 3] RFC 6266 Content-Disposition in HTTP June 2011

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.

Reschke Standards Track [Page 4] RFC 6266 Content-Disposition in HTTP June 2011

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).

Reschke Standards Track [Page 5] RFC 6266 Content-Disposition in HTTP June 2011

 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).

Reschke Standards Track [Page 6] RFC 6266 Content-Disposition in HTTP June 2011

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".

Reschke Standards Track [Page 7] RFC 6266 Content-Disposition in HTTP June 2011

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

Reschke Standards Track [Page 8] RFC 6266 Content-Disposition in HTTP June 2011

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.

Reschke Standards Track [Page 9] RFC 6266 Content-Disposition in HTTP June 2011

 [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.

Reschke Standards Track [Page 10] RFC 6266 Content-Disposition in HTTP June 2011

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.

Reschke Standards Track [Page 11] RFC 6266 Content-Disposition in HTTP June 2011

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.

Reschke Standards Track [Page 12] RFC 6266 Content-Disposition in HTTP June 2011

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.

Reschke Standards Track [Page 13] RFC 6266 Content-Disposition in HTTP June 2011

 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/

Reschke Standards Track [Page 14]

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