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

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.¶

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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 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.¶

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 implied linear whitespace (LWS).¶

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, 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 behaviour (e.g., the implementation is a validator). As such, the default handling of invalid fields is to ignore them.¶

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.¶

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 "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 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: those user agents that do not support the RFC 5987 encoding ignore "filename*" when it occurs after "filename".

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.¶

Using server-supplied information for constructing local filenames introduces many risks. These are summarized in Section 4.3.¶

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 Adam Barth, Rolf Eike Beer, Bjoern Hoehrmann, Alfred Hoenes, Roar Lauritzsen, Henrik Nordstrom, and Mark Nottingham 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 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].

Section 2 of [RFC2183] defines several additional disposition parameters: "creation-date", "modification-date", "quoted-date-time", and "size". The majority of user agents does not implement these, thus they 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.¶

• Include a "filename" parameter when 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 as 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. [fallbackbug: Firefox is known to pick the wrong parameter; a bug fix is scheduled for Firefox 5. --jre]
• 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 behaviour at the time of writing, and might be superseded. <http://purl.org/NET/http/content-disposition-tests> provides an overview of current levels of support in various implementations.¶

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>.¶