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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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 (firstname.lastname@example.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 E.10.
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.¶
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.¶
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
Header field values with multiple instances of the same parameter name are invalid.¶
Furthermore note that the format used for ext-value allows specifying a natural language; this is of limited use for filenames and is likely to be ignored by recipients.¶
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).¶
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, thus be very careful in extracting the desired information. In particular: ¶
When the value contains path separator characters ("\" or "/"), recipients SHOULD ignore all but the last path segment. This prevents unintentional overwriting of well-known file system locations (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.
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.¶
Thanks to Adam Barth, Rolf Eike Beer, Bjoern Hoehrmann, Alfred Hoenes, Roar Lauritzsen, Henrik Nordstrom, and Mark Nottingham for their valuable feedback.¶
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.¶
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.¶
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.¶
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.¶
Unfortunately, as of February 2011, 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 Agent||RFC 2231/5987||RFC 2047||Percent Encoding||Encoding Sniffing|
|Internet Explorer||yes (**)||no||yes||no|
(**) Starting with IE9RC, but only implements UTF-8.¶
To successfully interoperate with existing and future user agents, senders of the Content-Disposition header field are advised to:¶
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>.¶
Adjust terminology ("header" -> "header field"). Update rfc2231-in-http reference.¶
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".¶
Add and close issue "docfallback". Close issues "asciivsiso", "deplboth", "quoted", and "registry".¶
Updated to be a Working Draft of the IETF HTTPbis Working Group.¶
Closed issues: ¶
Slightly updated the notes about the proposed fallback behavior.¶
Various editorial improvements.¶
Closed issues: ¶
Closed issues: ¶
Updated implementation information (Chrome 9 implements RFC 5987, IE 9 RC implements it for UTF-8 only).¶
Clarify who requirements are on, add a section discussing conformance and handling of invalid field values in general.¶
Closed issues: ¶
Editorial changes: Fixed two typos where the new Conformance section said "Content-Location" instead of "Content-Disposition". Cleaned up terminology ("user agent", "recipient", "sender", "message body", ...). Stated what the escape character for quoted-string is. Explained a use case for "inline" disposition type. Updated implementation notes with respect to the fallback behavior.¶
Added appendix "Advice on Generating Content-Disposition Header Fields".¶