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

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.¶

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Content-Disposition is not part of the HTTP standard, but since it is widely implemented, we are documenting its use and risks for implementors.¶

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 MIMEMultipurpose Internet Mail Extensions (MIME) variant ([RFC2183]) of the header field, and also clarifies internationalization aspects.¶

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).¶

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.¶

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

Content-Disposition: Attachment; filename=foo.html

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

Content-Disposition: INLINE; FILENAME= "foo.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-↑ I ↓ASCIIISO-8859-1 character.

The "filename*" parameter (Section 3.3), using the encoding defined in [RFC5987], allows the server to transmit characters outside the ↑ I ↓ASCIIISO-8859-1 character set↑ I ↓, and also to optionally specify the language in use.¶

Future parameters might also require internationalization, in which case the same encoding can be used.¶

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).¶

Thanks to Rolf Eike Beer, Alfred Hoenes, and Roar Lauritzsen for their valuable feedback.¶

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 user agents in practice do not check the content type, and it also discourages properly declaring the media type.
•  I 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].

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.¶

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.¶