|HTTPbis Working Group||J. Reschke|
|Updates: 2616 (if approved)||January 1, 2011|
|Intended status: Standards Track|
|Expires: July 5, 2011|
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.
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 D.9.
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/.
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This Internet-Draft will expire on July 5, 2011.
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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.
Note: this document does not apply to Content-Disposition header fields appearing in message payloads transmitted over HTTP, such as when using the media type "multipart/form-data" ([RFC2388]).
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).
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
Defined in [RFC5987]:
ext-value = <ext-value, defined in [RFC5987], Section 3.2>
Senders MUST NOT generate header field values with multiple instances of the same parameter name. Recipients SHOULD treat these values as 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; 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 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).
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, recipients SHOULD ignore all but the last path segment. 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.
To enable future extensions, recipients SHOULD ignore unrecognized parameters (see also [RFC2183], Section 2.8).
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.
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: as of January 2011, 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.
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.
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).
This specification does not introduce any changes to the registration procedures for disposition values and parameters that are defined in Section 9 of [RFC2183].
This document updates the definition of the Content-Disposition HTTP header field in the permanent HTTP header field registry (see [RFC3864]).
Thanks to Adam Barth, Rolf Eike Beer, Bjoern Hoehrmann, Alfred Hoenes, Roar Lauritzsen, Henrik Nordstrom, and Mark Nottingham for their valuable feedback.
|[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.|
|[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.|
|[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.|
Compared to Section 19.5.1 of [RFC2616], the following normative changes reflecting actual implementations have been made:
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.
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.
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 January 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|
(*) But currently being implemented.
(**) Does not implement the fallback behavior to "filename" described in Section 3.3.
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.
Slightly updated the notes about the proposed fallback behavior.
Various editorial improvements.
Update Appendix C.4; Opera 10.63 RC implements the recommended fallback behavior.
No non-editorial changes yet.