HTTPbis Working GroupJ. Reschke
Internet-Draftgreenbytes
Updates: 2616 (if approved)October 25, 2010
Intended status: Standards Track
Expires: April 28, 2011

Use of the Content-Disposition Header Field in the Hypertext Transfer Protocol (HTTP)
draft-ietf-httpbis-content-disp-03

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.7.

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 April 28, 2011.

Copyright Notice

Copyright © 2010 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.


Table of Contents

1. Introduction

HTTP/1.1 defines the Content-Disposition response header field in 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, 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.

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 notation defined in Section 2.1 of [RFC2616], 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]:

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

Defined in [RFC5987]:

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

Parameter names MUST NOT be repeated; a header field value with multiple instances of the same parameter SHOULD be treated as invalid.

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], 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], 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 4 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]) 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.

  • Recipients are advised to 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.

  • 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.

Note: Many user agents do not properly handle escape characters 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.

3.4 Disposition Parameter: Extensions

To enable future extensions, unknown parameters SHOULD be ignored (see also [RFC2183], Section 2.8).

3.5 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.

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] 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] 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 October 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), 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.

6. Security Considerations

Using server-supplied information for constructing local filenames introduces many risks. These are summarized in Section 3.3.

Furthermore, implementers also 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).

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].

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]).

Header field name:
Content-Disposition
Applicable protocol:
http
Status:
standard
Author/Change controller:
IETF
Specification document:
this specification (Section 3)

8. Acknowledgements

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

9. References

9.1 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.

9.2 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, “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.
[RFC3629]Yergeau, F., “UTF-8, a transformation format of ISO 10646”, STD 63, RFC 3629, November 2003.
[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.

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:

B. Differences compared to RFC 2183

Section 2 of [RFC2183] 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.

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 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]:

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 encoded using the UTF-8 ([RFC3629]) 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 October 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:

User AgentRFC 2231/5987RFC 2047Percent EncodingEncoding Sniffing
Chromenoyesyesyes
Firefoxyes (*)yesnoyes
Internet Explorernonoyesno
Konqueroryesnonono
Operayesnonono
Safarinononoyes

(*) Does not implement the fallback behavior to "filename" described in Section 3.3.

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

Note: the issues names in the change log entries for draft-reschke-rfc2183-in-http refer to <http://greenbytes.de/tech/webdav/draft-reschke-rfc2183-in-http-issues.html>.

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:

Slightly updated the notes about the proposed fallback behavior.

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

Various editorial improvements.

D.7 Since draft-ietf-httpbis-content-disp-02

Closed issues:

Update Appendix C.4; Opera 10.63 RC implements the recommended fallback behavior.

Index

C H I R

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/