| Network Working Group | J. Reschke |
| Internet-Draft | greenbytes |
| Updates: 2616 (if approved) | August 24, 2010 |
| Intended status: Standards Track | |
| Expires: February 25, 2011 |
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Copyright (c) 2010 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://www3.tools.ietf.org/wg/httpbis/trac/ticket/123>.¶
Distribution of this document is unlimited. Although this is not a work item of the HTTPbis Working Group, comments should be sent to the Hypertext Transfer Protocol (HTTP) mailing list at ietf-http-wg@w3.org, which may be joined by sending a message with subject "subscribe" to ietf-http-wg-request@w3.org.¶
Discussions of the HTTPbis Working Group are archived at <http://lists.w3.org/Archives/Public/ietf-http-wg/>.¶
XML versions, latest edits and the issues list for this document are available from <http://greenbytes.de/tech/webdav/#draft-reschke-rfc2183-in-http>. A collection of test cases is available at <http://greenbytes.de/tech/tc2231/>.¶
| I edit (type: edit, status: open) | ||
| julian.reschke@greenbytes.de | 2009-10-16 | Umbrella issue for editorial fixes/enhancements. |
| Associated changes in this document: <#rfc.change.edit.1>, 1, 3, 3, 3.1, 3.1, 3.2, 3.2, 3.2, 3.2, 3.2, 3.3, 3.4, 4, <#rfc.change.edit.15>, 6, 8, 9.1, 9.2, 9.2, 9.2, A, B, <#rfc.change.edit.24>, D. | ||
| I nodep2183 (type: change, status: closed) | ||
| julian.reschke@greenbytes.de | 2010-08-23 | Make sure we do not have a normative dependency on RFC 2183. |
| 2010-08-23 | Resolution: Done. | |
| Associated changes in this document: 1, 3.2, 3.4, 9.1, 9.2. | ||
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 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 ↓fullydefines a profile of the features defined in the MIME variant ([RFC2183]) of the header field, and also clarifies internationalization ↑↓considerationsaspects.¶
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.
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 "*">
| I quoted (type: change, status: open) | ||
| julian.reschke@greenbytes.de | 2010-08-23 | Can value be quoted-pair as well? It is "value" only in RFC 2183, but "quoted-string" only in 2616. UAs seem to handle quoted-strings, although some have trouble unescaping backslashes. |
Defined in [RFC2616]:
token = <token, defined in [RFC2616], Section 2.2> value = <value, defined in [RFC2616], Section 3.6>
Defined in ↑↓[draft-reschke-rfc2231-in-http][RFC5987]:
ext-value = <ext-value, defined in ↑↓[draft-reschke-rfc2231-in-http], Section 3.2[RFC5987], Section 3.2>
If the disposition type matches "attachment" (case-insensitively), ↑↓the implied suggestion isthis indicates that the user agent should not display the response, but directly enter a "save ↑↓responseas..." dialog.¶
On the other hand, if it matches "inline"↑↓(case-insensitively), this implies ↑↓regulardefault processing. ↑↓Note that this type may be used when it is desirable to transport filename information for the case of a subsequent, user-initiated, save operation. ¶
Other disposition types SHOULD be handled the same way as "attachment" (↑↓see also[RFC2183], Section 2.8).¶
[rfc.comment.1: Talk about expected behavior, mention security considerations.]
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).
"filename" and "filename*" behave the same, except that "filename*" uses the encoding defined in [RFC5987], allowing the use of non-ASCII characters ([USASCII]). When both "filename" and "filename*" are present, a recipient SHOULD pick "filename*" and ignore "filename" - this will make it possible to send the same header value to clients that do not support "filename".
| I asciivsiso (type: change, status: open) | ||
| julian.reschke@greenbytes.de | 2010-08-24 | We should be consistent about what RFC 2616 defaults to (ASCII vs ISO-8859-1). |
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, all but the last segment SHOULD be ignored. 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 later on the file is opened locally (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.
Other aspects recipients need to be aware of are names that have a special meaning in the filesystem or in shell commands, such as "." and "..", "~", "|", and also device names.
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-ASCII character.
| I deplboth (type: change, status: open) | ||
| julian.reschke@greenbytes.de | 2010-08-24 | Add an example that uses both "filename" and "filename*" and mention current UA behavior. |
The "filename*" parameter (Section 3.3), using the encoding defined in [RFC5987], allows the server to transmit characters outside the ASCII character set.
Future parameters might also require internationalization, in which case the same encoding can be used.
[csec: Both refer to 2183, and also mention: long filenames, dot and dotdot, absolute paths, mismatches between media type and extension]
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].¶
| I registry (type: change, status: open) | ||
| julian.reschke@greenbytes.de | 2010-08-23 | The registry technically is for the MIME header, but has been used for C-D in other protocols already. What's missing are instructions that new registrations should state which protocol they're for. Do we want to attempt to modify the registry? |
This document updates the definition of the Content-Disposition HTTP header field in the permanent HTTP header field registry (see [RFC3864]).¶
↑↓[rfc.comment.2: TBD.]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: ¶
[rfc.comment.3: Mention: RFC 2047, IE, Safari]
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 encoded using the UTF-8 ([RFC3629]) character encoding.
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.
Furthermore, the first user agent to implement this did choose the encoding based on local settings; thus making it very hard to use in multi-lingual environments.
Some user agents inspect the value (which defaults to ISO-8859-1) 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.
The table below shows the various encoding approaches and for them in released user agent versions as of August 2010, based on the test cases published at <http://greenbytes.de/tech/tc2231>.
| User Agent | RFC 2231/5987 | RFC 2047 | Percent Encoding | Encoding Sniffing |
|---|---|---|---|---|
| Chrome | no | yes | yes | yes |
| Firefox | yes | yes | no | yes |
| Internet Explorer | no | no | yes | no |
| Konqueror | yes | no | no | no |
| Opera | yes | no | no | no |
| Safari | no | no | no | yes |
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".