Internet DRAFT - draft-ietf-appsawg-multipart-form-data
draft-ietf-appsawg-multipart-form-data
APPSAWG L. Masinter
Internet-Draft Adobe
Obsoletes: 2388 (if approved) April 10, 2015
Intended status: Standards Track
Expires: October 12, 2015
Returning Values from Forms: multipart/form-data
draft-ietf-appsawg-multipart-form-data-11
Abstract
This specification defines the multipart/form-data Internet Media
Type, which can be used by a wide variety of applications and
transported by a wide variety of protocols as a way of returning a
set of values as the result of a user filling out a form. It
obsoletes RFC 2388.
Status of This Memo
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/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 12, 2015.
Copyright Notice
Copyright (c) 2015 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. percent-encoding option . . . . . . . . . . . . . . . . . . . 3
3. Advice for Forms and Form Processing . . . . . . . . . . . . 3
4. Definition of multipart/form-data . . . . . . . . . . . . . . 4
4.1. Boundary parameter of multipart/form-data . . . . . . . . 4
4.2. Content-Disposition header for each part . . . . . . . . 4
4.3. filename attribute of content-distribution part header . 4
4.4. Multiple files for one form field . . . . . . . . . . . . 5
4.5. Content-Type header for each part . . . . . . . . . . . . 5
4.6. The charset parameter for text/plain form data . . . . . 5
4.7. The _charset_ field for default charset . . . . . . . . . 6
4.8. Content-Transfer-Encoding deprecated . . . . . . . . . . 6
4.9. Other Content- headers . . . . . . . . . . . . . . . . . 7
5. Operability considerations . . . . . . . . . . . . . . . . . 7
5.1. Non-ASCII field names and values . . . . . . . . . . . . 7
5.1.1. Avoid non-ASCII field names . . . . . . . . . . . . . 7
5.1.2. Interpreting forms and creating form-data . . . . . . 7
5.1.3. Parsing and interpreting form data . . . . . . . . . 8
5.2. Ordered fields and duplicated field names . . . . . . . . 8
5.3. Interoperability with web applications . . . . . . . . . 8
5.4. Correlating form data with the original form . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Media type registration for multipart/form-data . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Changes from RFC 2388 . . . . . . . . . . . . . . . 12
Appendix B. Alternatives . . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
In many applications, it is possible for a user to be presented with
a form. The user will fill out the form, including information that
is typed, generated by user input, or included from files that the
user has selected. When the form is filled out, the data from the
form is sent from the user to the receiving application.
The definition of "multipart/form-data" is derived from one of those
applications, originally set out in [RFC1867] and subsequently
incorporated into HTML 3.2 [W3C.REC-html32-19970114], where forms are
expressed in HTML, and in which the form data is sent via HTTP or
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electronic mail. This representation is widely implemented in
numerous web browsers and web servers.
However, "multipart/form-data" is also used for forms that are
presented using representations other than HTML (spreadsheets, PDF,
etc.), and for transport using means other than electronic mail or
HTTP; it is used in distributed applications which do not involve
forms at all, or do not have users filling out the form. For this
reason, this document defines a general syntax and semantics
independent of the application for which it is used, with specific
rules for web applications noted in context.
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 BCP 14, RFC 2119
[RFC2119].
2. percent-encoding option
Within this specification, "percent-encoding" (as defined in
[RFC3986]) is offered as a possible way of encoding characters in
file names that are otherwise disallowed, including non-ASCII
characters, spaces, control characters and so forth. The encoding is
created replacing each non-ASCII or disallowed character with a
sequence, where each byte of the UTF-8 encoding of the character is
represented by a percent-sign (%) followed by the (case-insensitive)
hexadecimal of that byte.
3. Advice for Forms and Form Processing
The representation and interpretation of forms and the nature of form
processing is not specified by this document. However, for forms and
form-processing that result in generation of multipart/form-data,
some suggestions are included.
In a form, there is generally a sequence of fields, where each field
is expected to be supplied with a value, e.g. by a user who fills out
the form. Each field has a name. After a form has been filled out,
and the form's data is "submitted": the form processing results in a
set of values for each field-- the "form data".
In forms that work with multipart/form-data, field names could be
arbitrary Unicode strings; however, restricting field names to ASCII
will help avoid some interoperability issues (see Section 5.1).
Within a given form, ensuring field names are unique is also helpful.
Some fields may have default values or presupplied values in the form
itself. Fields with presupplied values might be hidden or invisible;
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this allows using generic processing for form data from a variety of
actual forms.
4. Definition of multipart/form-data
The media-type "multipart/form-data" follows the model of multipart
MIME data streams as specified in [RFC2046] Section 5.1; changes are
noted in this document.
A "multipart/form-data" body contains a series of parts, separated by
a boundary.
4.1. Boundary parameter of multipart/form-data
As with other multipart types, the parts are delimited with a
boundary delimiter, constructed using CRLF, "--", the value of the
boundary parameter. The boundary is supplied as a "boundary"
parameter to the "multipart/form-data" type. As noted in [RFC2046]
Section 5.1, the boundary delimiter MUST NOT appear inside any of the
encapsulated parts, and it is often necessary to enclose the boundary
parameter values in quotes on the Content-type line.
4.2. Content-Disposition header for each part
Each part MUST contain a "content-disposition" header [RFC2183] and
where the disposition type is "form-data". The "content-disposition"
header MUST also contain an additional parameter of "name"; the value
of the "name" parameter is the original field name from the form
(possibly encoded; see Section 5.1). For example, a part might
contain a header:
Content-Disposition: form-data; name="user"
with the body of the part containing the form data of the "user"
field.
4.3. filename attribute of content-distribution part header
For form data that represents the content of a file, a name for the
file SHOULD be supplied as well, by using a "filename" parameter of
the "content-disposition" header. The file name isn't mandatory for
cases where the file name isn't available or is meaningless or
private; this might result, for example, from selection or drag-and-
drop or where the form data content is streamed directly from a
device.
If a filename parameter is supplied, the requirements of [RFC2183]
Section 2.3 for "receiving MUA" apply to recievers of "multipart/
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form-data" as well: Do not use the file name blindly, check and
possibly change to match local filesystem conventions if applicable,
do not use directory path information that may be present.
In most multipart types, the MIME headers in each part are restricted
to US-ASCII; for compatibility with those systems, file names
normally visible to users MAY be encoded using the percent-encoding
method in Section 2, following how a "file:" URI
[I-D.ietf-appsawg-file-scheme] might be encoded.
NOTE: The encoding method described in [RFC5987], which would add a
"filename*" paramter to the "Content-Disposition" header, MUST NOT be
used.
Some commonly deployed systems use multipart/form-data with file
names directly encoded including octets outside the US-ASCII range.
The encoding used for the file names is typically UTF-8, although
HTML forms will use the charset associated with the form.
4.4. Multiple files for one form field
The form data for a form field might include multiple files.
[RFC2388] suggested that multiple files for a single form field be
transmitted using a nested multipart/mixed part. This usage is
deprecated.
To match widely deployed implementations, multiple files MUST be sent
by supplying each file in a separate part, but all with the same
"name" parameter.
Receiving applications intended for wide applicability (e.g.
multipart/form-data parsing libraries) SHOULD also support the older
method of supplying multiple files.
4.5. Content-Type header for each part
Each part MAY have an (optional) "content-type", which defaults to
"text/plain". If the contents of a file are to be sent, the file
data SHOULD be labeled with an appropriate media type, if known, or
"application/octet-stream".
4.6. The charset parameter for text/plain form data
In the case where the form data is text, the charset parameter for
the "text/plain" Content-Type MAY be used to indicate the character
encoding used in that part. For example, a form with a text field in
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which a user typed "Joe owes <eu>100" where <eu> is the Euro symbol
might have form data returned as:
--AaB03x
content-disposition: form-data; name="field1"
content-type: text/plain;charset=UTF-8
content-transfer-encoding: quoted-printable
Joe owes =E2=82=AC100.
--AaB03x
In practice, many widely deployed implementations do not supply a
charset parameter in each part, but, rather, they rely on the notion
of a "default charset" for a multipart/form-data instance.
Subsequent sections will explain how the default charset is
established.
4.7. The _charset_ field for default charset
Some form processing applications (including HTML) have the
convention that the value of a form entry with entry name "_charset_"
and type "hidden" is automatically set when the form is opened; the
value is used as the default charset of text field values (see form-
charset in Section 5.1.2). In such cases, the value of the default
charset for each text/plain part without a charset parameter is the
supplied value. For example:
--AaB03x
content-disposition: form-data; name="_charset_"
iso-8859-1
--AaB03x--
content-disposition: form-data; name="field1"
...text encoded in iso-8859-1 ...
AaB03x--
4.8. Content-Transfer-Encoding deprecated
Previously, it was recommended that senders use a "Content-Transfer-
Encoding" encoding (such as "quoted-printable") for each non-ASCII
part of a multipart/form-data body, because that would allow use in
transports that only support a "7BIT" encoding. This use is
deprecated for use in contexts that support binary data such as HTTP.
Senders SHOULD NOT generate any parts with a "Content-Transfer-
Encoding" header.
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Currently, no deployed implementations that send such bodies have
been discovered.
4.9. Other Content- headers
The "multipart/form-data" media type does not support any MIME
headers in the parts other than Content-Type, Content-Disposition,
and (in limited circumstances) Content-Transfer-Encoding. Other
headers MUST NOT be included and MUST be ignored.
5. Operability considerations
5.1. Non-ASCII field names and values
Normally, MIME headers in multipart bodies are required to consist
only of 7-bit data in the US-ASCII character set. While [RFC2388]
suggested that non-ASCII field names be encoded according to the
method in [RFC2047], this practice doesn't seem to have been followed
widely.
This specification makes three sets of recommendations for three
different states of workflow.
5.1.1. Avoid non-ASCII field names
For broadest interoperability with existing deployed software, those
creating forms SHOULD avoid non-ASCII field names. This should not
be a burden, because in general the field names are not visible to
users. The field names in the underlying need not match what the
user sees on the screen.
If non-ASCII field names are unavoidable, form or application
creators SHOULD use UTF-8 uniformly. This will minimize
interoperability problems.
5.1.2. Interpreting forms and creating form-data
Some applications of this specification will supply a character
encoding to be used for interpretation of the multipart/form-data
body. In particular, HTML 5 [W3C.REC-html5-20141028] uses:
o The content of a '_charset_' field, if there is one.
o the value of an accept-charset attribute of the <form> element, if
there is one,
o the character encoding of the document containing the form, if it
is US-ASCII compatible,
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o otherwise UTF-8.
Call this value the form-charset. Any text, whether field name,
field value, or (text/plain) form data which is uses characters
outside the ASCII range MAY be represented directly encoded in the
form-charset.
5.1.3. Parsing and interpreting form data
While this specification provides guidance for creation of multipart/
form-data, parsers and interpreters should be aware of the variety of
implementations. File systems differ as to whether and how they
normalize Unicode names, for example. The matching of form elements
to form-data parts may rely on a fuzzier match. In particular, some
multipart/form-data generators might have followed the previous
advice of [RFC2388] and used the [RFC2047] "encoded-word" method of
encoding non-ASCII values:
encoded-word = "=?" charset "?" encoding "?" encoded-text "?="
Others have been known to follow [RFC2231], to send unencoded UTF-8,
or even strings encoded in the form-charset.
For this reason, interpreting "multipart/form-data" (even from
conforming generators) may require knowing the charset used in form
encoding, in cases where the _charset_ field value or a charset
parameter of a text/plain Content-Type header is not supplied.
5.2. Ordered fields and duplicated field names
Form processors given forms with a well-defined ordering SHOULD send
back results in order (note that there are some forms which do not
define a natural order.) Intermediaries MUST NOT reorder the
results. Form parts with identical field names MUST NOT be
coalesced.
5.3. Interoperability with web applications
Many web applications use the "application/x-url-encoded" method for
returning data from forms. This format is quite compact, e.g.:
name=Xavier+Xantico&verdict=Yes&colour=Blue&happy=sad&Utf%F6r=Send
However, there is no opportunity to label the enclosed data with
content type, apply a charset, or use other encoding mechanisms.
Many form-interpreting programs (primarily web browsers) now
implement and generate multipart/form-data, but an existing
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application might need to optionally support both the application/x-
url-encoded format as well.
5.4. Correlating form data with the original form
This specification provides no specific mechanism by which multipart/
form-data can be associated with the form that caused it to be
transmitted. This separation is intentional; many different forms
might be used for transmitting the same data. In practice,
applications may supply a specific form processing resource (in HTML,
the ACTION attribute in a FORM tag) for each different form.
Alternatively, data about the form might be encoded in a "hidden
field" (a field which is part of the form but which has a fixed value
to be transmitted back to the form-data processor.)
6. IANA Considerations
Please update the Internet Media Type registration of multipart/form-
data to point to this document, using the template in Section 8. In
addition, please update the registrations of the "name" parameter and
the "form-data" value in the "Content Disposition Values and
Parameters" registry to both point to this document.
7. Security Considerations
All form processing software should treat user supplied form-data
with sensitivity, as it often contains confidential or personally
identifying information. There is widespread use of form "auto-fill"
features in web browsers; these might be used to trick users to
unknowingly send confidential information when completing otherwise
innoccuous tasks. Multipart/form-data does not supply any features
for checking integrity, ensuring confidentiality, avoiding user
confusion, or other security features; those concerns must be
addressed by the form-filling and form-data-interpreting
applications.
Applications which receive forms and process them must be careful not
to supply data back to the requesting form processing site that was
not intended to be sent.
It is important when interpreting the filename of the Content-
Disposition header to not overwrite files in the recipient's file
space inadvertently.
User applications that request form information from users must be
careful not to cause a user to send information to the requestor or a
third party unwillingly or unwittingly. For example, a form might
request 'spam' information to be sent to an unintended third party,
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or private information to be sent to someone that the user might not
actually intend. While this is primarily an issue for the
representation and interpretation of forms themselves (rather than
the data representation of the form data), the transportation of
private information must be done in a way that does not expose it to
unwanted prying.
With the introduction of form-data that can reasonably send back the
content of files from a user's file space, the possibility arises
that a user might be sent an automated script that fills out a form
and then sends one of the user's local files to another address.
Thus, additional caution is required when executing automated
scripting where form-data might include a user's files.
Files sent via multipart/form-data may contain arbitrary executable
content, and precautions against malicious content are necessary.
The considerations of [RFC2183] Sections 2.3 and 5 with respect to
the filename parameter of the Content-Disposition header also apply
to its usage here.
8. Media type registration for multipart/form-data
This section is the [RFC6838] media type registration.
Type name: multipart
Subtype name: form-data
Required parameters: boundary
Optional parameters: none
Encoding considerations: Common use is BINARY.
In limited use (or transports that restrict the encoding to 7BIT
or 8BIT each part is encoded separately using Content-Transfer-
Encoding Section 4.8.
Security considerations: See Section 7 of this document.
Interoperability considerations: This document makes several
recommendations for interoperability with deployed
implementations, including Section 4.8.
Published specification: This document.
Applications that use this media type: Numerous web browsers,
servers, and web applications.
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Fragment identifier considerations: None: Fragment identifiers are
not defined for this type.
Additional information: None: no deprecated alias names, magic
numbers, file extensions or Macintosh ssssfile type codes.
Person & email address to contact for further information
Author of this document.
Intended Usage: COMMON
Restrictions on usage: none
Author: Author of this document.
Change controller: IETF
Provisional registration: N/A
9. References
9.1. Normative References
[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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
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9.2. Informative References
[I-D.ietf-appsawg-file-scheme]
Kerwin, M., "The file URI Scheme", draft-ietf-appsawg-
file-scheme-00 (work in progress), January 2015.
[RFC1867] Nebel, E. and L. Masinter, "Form-based File Upload in
HTML", RFC 1867, November 1995.
[RFC2388] Masinter, L., "Returning Values from Forms: multipart/
form-data", RFC 2388, August 1998.
[RFC5987] Reschke, J., "Character Set and Language Encoding for
Hypertext Transfer Protocol (HTTP) Header Field
Parameters", RFC 5987, August 2010.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13, RFC
6838, January 2013.
[W3C.REC-html32-19970114]
Raggett, D., "HTML 3.2 Reference Specification", World
Wide Web Consortium Recommendation REC-html32-19970114,
January 1997, <http://www.w3.org/TR/REC-html32-19970114>.
[W3C.REC-html5-20141028]
Hickson, I., Berjon, R., Faulkner, S., Leithead, T.,
Navara, E., O'Connor, E., and S. Pfeiffer, "HTML5",
World Wide Web Consortium Recommendation REC-
html5-20141028, October 2014,
<http://www.w3.org/TR/2014/REC-html5-20141028>.
Appendix A. Changes from RFC 2388
The handling of non-ASCII field names changed-- no longer
recommending the RFC 2047 method, instead suggesting senders send
UTF-8 field names directly, and file names directly in the form-
charset.
The handling of multiple files submitted as the result of a single
form field (e.g. HTML's <input type=file multiple> element) results
in each file having its own top level part with the same name
parameter; the method of using a nested "multipart/mixed" from
[RFC2388] is no longer recommended for creators, and not required for
receivers as there are no known implementations of senders.
The _charset_ convention and use of an explicit form-data charset is
documented.
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'boundary' is a required parameter in Content-Type.
The relationship of the ordering of fields within a form and the
ordering of returned values within multipart/form-data was not
defined before, nor was the handling of the case where a form has
multiple fields with the same name.
Editorial: Removed obsolete discussion of alternatives in appendix.
Update references. Move outline of form processing into
Introduction.
Appendix B. Alternatives
There are numerous alternative ways in which form data can be
encoded; many are listed in [RFC2388] section 5.2. The multipart/
form-data encoding is verbose, especially if there are many fields
with short values. In most use cases, this overhead isn't
significant.
More problematic are the differences introduced when implementors
opted to not follow [RFC2388] when encoding non-ASCII field names
(perhaps because "may" should have been "MUST"). As a result,
parsers need to be more complex for matching against the possible
outputs of various encoding methods.
Author's Address
Larry Masinter
Adobe
Email: masinter@adobe.com
URI: http://larry.masinter.net
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