Use of the Content-Disposition Header Field in the Hypertext Transfer Protocol (HTTP)
draft-ietf-httpbis-content-disp-02

Abstract

HTTP/1.1 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.

Editorial Note (To be removed by RFC Editor before publication)

This specification is expected to replace the definition of Content-Disposition in the HTTP/1.1 specification, as currently revised by the IETF HTTPbis working group. See also http://trac.tools.ietf.org/wg/httpbis/trac/ticket/123.

Discussion of this draft should take place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org). The current issues list is at http://trac.tools.ietf.org/wg/httpbis/trac/query?component=content-disp and related documents (including fancy diffs) can be found at http://tools.ietf.org/wg/httpbis/.

The changes in this draft are summarized in Appendix D.6 (Since draft-ietf-httpbis-content-disp-01).

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress.”

This Internet-Draft will expire on March 26, 2011.

Copyright Notice

Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

1.  Introduction
2.  Notational Conventions
3.  Header Field Definition
    3.1.  Grammar
    3.2.  Disposition Type
    3.3.  Disposition Parameter: 'Filename'
    3.4.  Disposition Parameter: Extensions
    3.5.  Extensibility
4.  Examples
5.  Internationalization Considerations
6.  Security Considerations
7.  IANA Considerations
    7.1.  Registry for Disposition Values and Parameter
    7.2.  Header Field Registration
8.  Acknowledgements
9.  References
    9.1.  Normative References
    9.2.  Informative References
Appendix A.  Changes from the RFC 2616 Definition
Appendix B.  Differences compared to RFC 2183
Appendix C.  Alternative Approaches to Internationalization
    C.1.  RFC 2047 Encoding
    C.2.  Percent Encoding
    C.3.  Encoding Sniffing
    C.4.  Implementations (to be removed by RFC Editor before publication)
Appendix D.  Change Log (to be removed by RFC Editor before publication)
    D.1.  Since draft-reschke-rfc2183-in-http-00
    D.2.  Since draft-reschke-rfc2183-in-http-01
    D.3.  Since draft-reschke-rfc2183-in-http-02
    D.4.  Since draft-reschke-rfc2183-in-http-03
    D.5.  Since draft-ietf-httpbis-content-disp-00
    D.6.  Since draft-ietf-httpbis-content-disp-01
§  Index

1.  Introduction

HTTP/1.1 defines the Content-Disposition response header field in Section 19.5.1 of [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.), 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, it fully defines a profile of the features defined in the Multipurpose Internet Mail Extensions (MIME) variant ([RFC2183] (Troost, R., Dorner, S., and K. Moore, “Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field,” August 1997.)) of the header field, and also clarifies internationalization aspects.

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

This specification uses the augmented BNF notation defined in Section 2.1 of [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.), including its rules for linear whitespace (LWS).

3.  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.

3.1.  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] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.):

  token       = <token, defined in [RFC2616], Section 2.2>
  value       = <value, defined in [RFC2616], Section 3.6>

Defined in [RFC5987] (Reschke, J., “Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters,” August 2010.):

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

3.2.  Disposition Type

If the disposition type matches "attachment" (case-insensitively), this indicates that the user agent 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.

Unknown or unhandled disposition types SHOULD be handled the same way as "attachment" (see also [RFC2183] (Troost, R., Dorner, S., and K. Moore, “Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field,” August 1997.), Section 2.8).

3.3.  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] (Reschke, J., “Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters,” August 2010.), allowing the use of characters not present in the ISO-8859-1 character set ([ISO‑8859‑1] (International Organization for Standardization, “Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1,” 1998.)).

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 4 (Examples) for an example).

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

• When the value contains path separator characters, all but the last segment SHOULD be ignored. This prevents unintentional overwriting of well-known file system location (such as "/etc/passwd").
• Many platforms do not use Internet Media Types ([RFC2046] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types,” November 1996.)) 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 need to ensure that a file extension is used that is safe, optimally matching the media type of the received payload.
• 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.

3.4.  Disposition Parameter: Extensions

To enable future extensions, unknown parameters SHOULD be ignored (see also [RFC2183] (Troost, R., Dorner, S., and K. Moore, “Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field,” August 1997.), Section 2.8).

3.5.  Extensibility

Note that Section 9 of [RFC2183] (Troost, R., Dorner, S., and K. Moore, “Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field,” August 1997.) 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.

4.  Examples

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

Content-Disposition: Attachment; filename=example.html

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

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

Direct UA to show "save as" dialog, with a filename of "an example":

Content-Disposition: Attachment; Filename*=UTF-8'en'an%20example

Note that this example uses the extended encoding defined in [RFC5987] (Reschke, J., “Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters,” August 2010.) to specify that the natural language of the filename is English, and also to encode the space character which is not allowed in the token production.

Direct 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] (Reschke, J., “Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters,” August 2010.) is also used to encode the non-ISO-8859-1 character.

Same as 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: as of September 2010, those user agents that do not support the RFC 5987 encoding ignore "filename*" when it occurs after "filename". Unfortunately, some user agents that do support RFC 5987 do pick the "filename" rather than the "filename*" parameter when it occurs first; it is expected that this situation is going to improve soon.

5.  Internationalization Considerations

The "filename*" parameter (Section 3.3 (Disposition Parameter: 'Filename')), using the encoding defined in [RFC5987] (Reschke, J., “Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters,” August 2010.), 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.

6.  Security Considerations

Using server-supplied information for constructing local filenames introduces many risks. These are summarized in Section 3.3 (Disposition Parameter: 'Filename').

Furthermore, implementers also ought to be aware of the Security Considerations applying to HTTP (see Section 15 of [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.)), and also the parameter encoding defined in [RFC5987] (Reschke, J., “Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters,” August 2010.) (see Section 5).

7.  IANA Considerations

7.1.  Registry for Disposition Values and Parameter

This specification does not introduce any changes to the registration procedures for disposition values and parameters that are defined in Section 9 of [RFC2183] (Troost, R., Dorner, S., and K. Moore, “Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field,” August 1997.).

7.2.  Header Field Registration

This document updates the definition of the Content-Disposition HTTP header field in the permanent HTTP header field registry (see [RFC3864] (Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields,” September 2004.)).

Header field name:

Content-Disposition

Applicable protocol:

http

Status:

standard

Author/Change controller:

IETF

Specification document:

this specification (Section 3 (Header Field Definition))

8.  Acknowledgements

Thanks to Rolf Eike Beer, Bjoern Hoehrmann, Alfred Hoenes, Roar Lauritzsen, Henrik Nordstrom, and Mark Nottingham for their valuable feedback.

9.  References

9.1. Normative References

9.2. Informative References

Appendix A.  Changes from the RFC 2616 Definition

Compared to Section 19.5.1 of [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.), the following normative changes reflecting actual implementations have been made:

• According to RFC 2616, the disposition type "attachment" only applies to content of type "application/octet-stream". This restriction has been removed, because user agents in practice do not check the content type, and it also discourages properly declaring the media type.
• RFC 2616 only allows "quoted-string" for the filename parameter. This would be an exceptional parameter syntax, and also doesn't reflect actual use.
• The definition for the disposition type "inline" ([RFC2183] (Troost, R., Dorner, S., and K. Moore, “Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field,” August 1997.), Section 2.1) has been re-added with a suggestion for its processing.
• This specification requires support for the extended parameter encoding defined in [RFC5987] (Reschke, J., “Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters,” August 2010.).

Appendix B.  Differences compared to RFC 2183

Section 2 of [RFC2183] (Troost, R., Dorner, S., and K. Moore, “Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field,” August 1997.) defines several additional disposition parameters: "creation-date", "modification-date", "quoted-date-time", and "size". These do not appear to be implemented by any user agent, thus 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] (International Organization for Standardization, “Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1,” 1998.)) character encoding (see [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.), 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] (Freed, N. and K. Moore, “MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations,” November 1997.), clarified and profiled for HTTP in [RFC5987] (Reschke, J., “Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters,” August 2010.)).

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

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] (Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text,” November 1996.):

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] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.), Section 2.1) sequences of characters encoded using the UTF-8 ([RFC3629] (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.)) character encoding.

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.

Furthermore, the first user agent to implement this did choose the encoding based on local settings; thus making it very hard to use in multi-lingual environments.

C.3.  Encoding Sniffing

Some user agents inspect the value (which defaults to ISO-8859-1) 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.

C.4.  Implementations (to be removed by RFC Editor before publication)

Unfortunately, as of September 2010, neither the encoding defined in RFCs 2231 and 5987, nor any of the alternate approaches discussed above was implemented interoperably. Thus, this specification recommends the approach defined in RFC 5987, which at least has the advantage of actually being specified properly.

The table below shows the implementation support for the various approaches:

(*) Does not implement the fallback behavior to "filename" described in Section 3.3 (Disposition Parameter: 'Filename').

Appendix D.  Change Log (to be removed by RFC Editor before publication)

D.1.  Since draft-reschke-rfc2183-in-http-00

Adjust terminology ("header" -> "header field"). Update rfc2231-in-http reference.

D.2.  Since draft-reschke-rfc2183-in-http-01

Update rfc2231-in-http reference. Actually define the "filename" parameter. Add internationalization considerations. Add examples using the RFC 5987 encoding. Add overview over other approaches, plus a table reporting implementation status. Add and resolve issue "nodep2183". Add issues "asciivsiso", "deplboth", "quoted", and "registry".

D.3.  Since draft-reschke-rfc2183-in-http-02

Add and close issue "docfallback". Close issues "asciivsiso", "deplboth", "quoted", and "registry".

D.4.  Since draft-reschke-rfc2183-in-http-03

Updated to be a Working Draft of the IETF HTTPbis Working Group.

D.5.  Since draft-ietf-httpbis-content-disp-00

Closed issues:

• http://tools.ietf.org/wg/httpbis/trac/ticket/242: "handling of unknown disposition types"

Slightly updated the notes about the proposed fallback behavior.

D.6.  Since draft-ietf-httpbis-content-disp-01

None yet.

Index

Author's Address