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Internet Engineering Task Force (IETF)                        J. ReschkeRequest for Comments: 6266                                    greenbytesUpdates:2616                                                  June 2011Category: Standards TrackISSN: 2070-1721Use of the Content-Disposition Header Field in theHypertext Transfer Protocol (HTTP)AbstractRFC 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.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6266.Copyright Notice   Copyright (c) 2011 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.Reschke                      Standards Track                    [Page 1]

RFC 6266               Content-Disposition in HTTP             June 2011Table of Contents1. Introduction ....................................................22. Notational Conventions ..........................................33. Conformance and Error Handling ..................................34. Header Field Definition .........................................34.1. Grammar ....................................................44.2. Disposition Type ...........................................54.3. Disposition Parameter: 'Filename' ..........................54.4. Disposition Parameter: Extensions ..........................64.5. Extensibility ..............................................75. Examples ........................................................76. Internationalization Considerations .............................87. Security Considerations .........................................88. IANA Considerations .............................................88.1. Registry for Disposition Values and Parameters .............88.2. Header Field Registration ..................................89. Acknowledgements ................................................910. References .....................................................910.1. Normative References ......................................910.2. Informative References ....................................9Appendix A. Changes from theRFC 2616 Definition ..................11Appendix B. Differences Compared toRFC 2183 ......................11Appendix C. Alternative Approaches to Internationalization ........11C.1.RFC 2047 Encoding ..........................................12C.2. Percent Encoding ...........................................12C.3. Encoding Sniffing ..........................................12Appendix D. Advice on Generating Content-Disposition Header               Fields ................................................131.  IntroductionRFC 2616 defines the Content-Disposition response header field   (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 (UAs), 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.Reschke                      Standards Track                    [Page 2]

RFC 6266               Content-Disposition in HTTP             June 2011      Note: This document does not apply to Content-Disposition header      fields appearing in payload bodies transmitted over HTTP, such as      when using the media type "multipart/form-data" ([RFC2388]).2.  Notational Conventions   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 (ABNF) notation defined inSection 2.1 of [RFC2616], including its rules for implied linear   whitespace (LWS).3.  Conformance and Error Handling   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 behavior (e.g., the   implementation is a validator).  As such, the default handling of   invalid fields is to ignore them.4.  Header Field Definition   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.Reschke                      Standards Track                    [Page 3]

RFC 6266               Content-Disposition in HTTP             June 20114.1.  Grammar     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>   Content-Disposition 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 (e.g., "en"); this is of limited use   for filenames and is likely to be ignored by recipients.Reschke                      Standards Track                    [Page 4]

RFC 6266               Content-Disposition in HTTP             June 20114.2.  Disposition Type   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).4.3.  Disposition Parameter: 'Filename'   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 (seeSection 5   for an example).Reschke                      Standards Track                    [Page 5]

RFC 6266               Content-Disposition in HTTP             June 2011   It is essential that recipients treat the specified filename as   advisory only, and thus be very careful in extracting the desired   information.  In particular:   o  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 considering the actual filename (where 'path      segments' are the components of the field value delimited by the      path separator characters "\" and "/").   o  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 that make use of      file extensions to determine the media type MUST ensure that a      file extension is used that is safe, optimally matching the media      type of the received payload.   o  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.   o  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.      Note: Many user agents do not properly handle the escape character      "\" when using the quoted-string form.  Furthermore, some user      agents erroneously try to perform unescaping of "percent" escapes      (seeAppendix C.2), and thus might misinterpret filenames      containing the percent character followed by two hex digits.4.4.  Disposition Parameter: Extensions   To enable future extensions, recipients SHOULD ignore unrecognized   parameters (see also[RFC2183], Section 2.8).Reschke                      Standards Track                    [Page 6]

RFC 6266               Content-Disposition in HTTP             June 20114.5.  Extensibility   Note thatSection 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.5.  Examples   Direct the UA to show "save as" dialog, with a filename of   "example.html":     Content-Disposition: Attachment; filename=example.html   Direct the 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 the 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.   This example is the same as the one above, but adding the "filename"   parameter for compatibility with user agents not implementingRFC 5987:     Content-Disposition: attachment;                          filename="EURO rates";                          filename*=utf-8''%e2%82%ac%20rates   Note: Those user agents that do not support theRFC 5987 encoding   ignore "filename*" when it occurs after "filename".Reschke                      Standards Track                    [Page 7]

RFC 6266               Content-Disposition in HTTP             June 20116.  Internationalization Considerations   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.7.  Security Considerations   Using server-supplied information for constructing local filenames   introduces many risks.  These are summarized inSection 4.3.   Furthermore, implementers ought to be aware of the security   considerations applying to HTTP (seeSection 15 of [RFC2616]), and   also the parameter encoding defined in [RFC5987] (seeSection 5).8.  IANA Considerations8.1.  Registry for Disposition Values and Parameters   This specification does not introduce any changes to the registration   procedures for disposition values and parameters that are defined inSection 9 of [RFC2183].8.2.  Header Field Registration   This document updates the definition of the Content-Disposition HTTP   header field in the permanent HTTP header field registry (see   [RFC3864]).   Header field name:  Content-Disposition   Applicable protocol:  http   Status:  standard   Author/Change controller:  IETF   Specification document:  this specification (Section 4)   Related information:  noneReschke                      Standards Track                    [Page 8]

RFC 6266               Content-Disposition in HTTP             June 20119.  Acknowledgements   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.10.  References10.1.  Normative References   [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 2010.10.2.  Informative 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.   [RFC2183]     Troost, R., Dorner, S., and K. Moore, Ed.,                 "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.Reschke                      Standards Track                    [Page 9]

RFC 6266               Content-Disposition in HTTP             June 2011   [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.   [US-ASCII]    American National Standards Institute, "Coded Character                 Set -- 7-bit American Standard Code for Information                 Interchange", ANSI X3.4, 1986.Reschke                      Standards Track                   [Page 10]

RFC 6266               Content-Disposition in HTTP             June 2011Appendix A.  Changes from theRFC 2616 Definition   Compared toSection 19.5.1 of [RFC2616], the following normative   changes reflecting actual implementations have been made:   o  According toRFC 2616, the disposition type "attachment" only      applies to content of type "application/octet-stream".  This      restriction has been removed, because recipients in practice do      not check the content type, and it also discourages properly      declaring the media type.   oRFC 2616 only allows "quoted-string" for the filename parameter.      This would be an exceptional parameter syntax, and also doesn't      reflect actual use.   o  The definition for the disposition type "inline" ([RFC2183],      Section 2.1) has been re-added with a suggestion for its      processing.   o  This specification requires support for the extended parameter      encoding defined in [RFC5987].Appendix B.  Differences Compared toRFC 2183Section 2 of [RFC2183] defines several additional disposition   parameters: "creation-date", "modification-date", "quoted-date-time",   and "size".  The majority of user agents do not implement these;   thus, they have been omitted from this specification.Appendix C.  Alternative Approaches to Internationalization   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 explain how they are inferior to theRFC 5987 encoding used in this specification.Reschke                      Standards Track                   [Page 11]

RFC 6266               Content-Disposition in HTTP             June 2011C.1.RFC 2047 EncodingRFC 2047 defines an encoding mechanism for header fields, but this   encoding is not supposed to be used for header field parameters --   seeSection 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.C.2.  Percent Encoding   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.C.3.  Encoding Sniffing   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.Reschke                      Standards Track                   [Page 12]

RFC 6266               Content-Disposition in HTTP             June 2011Appendix D.  Advice on Generating Content-Disposition Header Fields   To successfully interoperate with existing and future user agents,   senders of the Content-Disposition header field are advised to:   o  Include a "filename" parameter when US-ASCII ([US-ASCII]) is      sufficiently expressive.   o  Use the 'token' form of the filename parameter only when it does      not contain disallowed characters (e.g., spaces); in such cases,      the quoted-string form should be used.   o  Avoid including the percent character followed by two hexadecimal      characters (e.g., %A9) in the filename parameter, since some      existing implementations consider it to be an escape character,      while others will pass it through unchanged.   o  Avoid including the "\" character in the quoted-string form of the      filename parameter, as escaping is not implemented by some user      agents, and "\" can be considered an illegal path character.   o  Avoid using non-ASCII characters in the filename parameter.      Although most existing implementations will decode them as      ISO-8859-1, some will apply heuristics to detect UTF-8, and thus      might fail on certain names.   o  Include a "filename*" parameter where the desired filename cannot      be expressed faithfully using the "filename" form.  Note that      legacy user agents will not process this, and will fall back to      using the "filename" parameter's content.   o  When a "filename*" parameter is sent, to also generate a      "filename" parameter as a fallback for user agents that do not      support the "filename*" form, if possible.  This can be done by      substituting characters with US-ASCII sequences (e.g., Unicode      character point U+00E4 (LATIN SMALL LETTER A WITH DIARESIS) by      "ae").  Note that this may not be possible in some locales.   o  When a "filename" parameter is included as a fallback (as per      above), "filename" should occur first, due to parsing problems in      some existing implementations.   o  Use UTF-8 as the encoding of the "filename*" parameter, when      present, because at least one existing implementation only      implements that encoding.Reschke                      Standards Track                   [Page 13]

RFC 6266               Content-Disposition in HTTP             June 2011   Note that this advice is based upon UA behavior 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.Author's Address   Julian F. Reschke   greenbytes GmbH   Hafenweg 16   Muenster, NW  48155   Germany   EMail: julian.reschke@greenbytes.de   URI:http://greenbytes.de/tech/webdav/Reschke                      Standards Track                   [Page 14]