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

Note: a later version of this document has been published as RFC6266.

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

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This Internet-Draft will expire in December 2011.¶

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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.¶

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.¶

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.¶

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).¶

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

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 "/").
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.
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.
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.

4.4. Disposition Parameter: Extensions

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

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".

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].¶

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.
[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.

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: ¶

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.
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], 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].

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.¶

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.¶

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:¶

Include a "filename" parameter when US-ASCII ([US-ASCII]) is sufficiently expressive.
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.
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.
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.
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.
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.
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.
When a "filename" parameter is included as a fallback (as per above), "filename" should occur first, due to parsing problems in some existing implementations.
Use UTF-8 as the encoding of the "filename*" parameter, when present, because at least one existing implementation only implements that encoding.

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/