| HTTPbis Working Group | J. Reschke |
| Internet-Draft | greenbytes |
| Updates: 2616 (if approved) | March 14, 2011 |
| Intended status: Standards Track | |
| Expires: September 15, 2011 |
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 Internet-Draft will expire on September 15, 2011.¶
Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶
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 (ietf-http-wg@w3.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.12.¶
RFC 2616 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.¶
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 (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 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
Defined in [RFC5987]:
ext-value = <ext-value, defined in [RFC5987], Section 3.2>
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; 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).¶
Unknown or unhandled disposition types SHOULD be handled by recipients 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 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: ¶
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 consider the actual filename (where 'path segment' 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 SHOULD 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.
To enable future extensions, recipients SHOULD ignore unrecognized parameters (see also [RFC2183], Section 2.8).¶
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).¶
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, Stewart Bryant, Bjoern Hoehrmann, Alfred Hoenes, Roar Lauritzsen, Alexey Melnikov, 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: ¶
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 March 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 |
|---|---|---|---|---|
| Chrome | yes | yes | yes | yes |
| Firefox | yes (*) | yes | no | yes |
| Internet Explorer | yes (**) | no | yes | no |
| Konqueror | yes | no | no | no |
| Opera | yes | no | no | no |
| Safari | no | no | no | yes |
(*) Does not implement the fallback behavior to "filename" described in Section 4.3; a fix is planned for Firefox 5.¶
(**) 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. 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.¶
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: ¶
Update Appendix C.4; Opera 10.63 RC implements the recommended fallback behavior.¶
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".¶
Closed issues: ¶
Rephrase the requirement about well-known file system locations, and also clarify that by "last path segment" we mean the actual filename. Added a forward reference from "invalid" to the section that defines a valid header field.¶