Unified User-Agent String
draft-karcz-uuas-01
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Mateusz Karcz
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2021-11-24
(latest revision 2014-11-10)
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Independent M. Karcz
Internet-Draft UKLO Tczew
Updates: 7231 (if approved) November 10, 2014
Intended status: Experimental
Expires: May 14, 2015
Unified User-Agent String
draft-karcz-uuas-01
Abstract
User-Agent is a HTTP request-header field. It contains information
about the user agent originating the request, which is often used by
servers to help identify the scope of reported interoperability
problems, to work around or tailor responses to avoid particular user
agent limitations, and for analytics regarding browser or operating
system use. Over the years contents of this field got complicated
and ambiguous. That was the reaction for sending altered version of
websites to web browsers other than popular ones. During the
development of the WWW, authors of the new web browsers used to
construct User-Agent strings similar to Netscape's one. Nowadays
contents of the User-Agent field are much longer than 15 years ago.
This Memo proposes the Uniform User-Agent String as a way to simplify
the User-Agent field contents, while maintaining the previous
possibility of their use.
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 May 14, 2015.
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Copyright Notice
Copyright (c) 2014 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may not be modified, and derivative works of it may not
be created, except to format it for publication as an RFC or to
translate it into languages other than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conformance . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 3
1.2.1. Whitespaces . . . . . . . . . . . . . . . . . . . . . 3
2. Use of the User-Agent strings . . . . . . . . . . . . . . . . 3
3. Definition of Proposed Format . . . . . . . . . . . . . . . . 3
3.1. Standard String . . . . . . . . . . . . . . . . . . . . . 4
3.2. Regular String . . . . . . . . . . . . . . . . . . . . . 4
3.3. Web Browser String . . . . . . . . . . . . . . . . . . . 5
4. ABNF Definition of UUAS . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Nowadays User-Agent strings are long, complicated and often
ambiguous. (e.g. "Mozilla/4.0 (compatible; MSIE 6.0; X11; Linux
i686; en) Opera 8.01" - it is Opera Browser, but it can be read as
Internet Explorer or Netscape Navigator.) This document specifies a
new, easy and clear format of Unified User-Agent String (UUAS), which
allows simple distinction between user agents, maintaining most of
the features of the existing solutions.
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1.1. Conformance
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].
1.2. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234]. Section 4 contains a full syntax definition
of the Unified User-Agent String.
1.2.1. Whitespaces
This specification uses two rules to denote the use of linear
whitespace: OWS (optional whitespace) and RWS (required whitespace).
They are defined in Section 3.2.3 of [RFC7230].
2. Use of the User-Agent strings
Generally, the User-Agent header field was intended for statistical
purposes. However, in mid-90. during the "browser wars" data
provided by this field became used to alter the content of the
resources before sending them to the user, or even to prevent users
of particular browser the access to resources. To avoid these
protections, software vendors started to change their identifiers in
a way resembling User-Agent strings of the most popular browsers.
During the years it has made these identifiers much more complicated,
ambiguous and difficult to parse.
Nowadays User-Agent strings are still used for statistical purposes,
but also for avoiding limitations of particular implementations.
However, in modern browsers these limitations greatly decreased and
"user agent spoofing" is now unnecessary. Unfortunately, there are a
lot of websites still discriminating particular web browsers.
Unified User-Agent String is intended to propose a way for
simplifying, clarifying and standarizing the content of User-Agent
HTTP header field. Furthermore, if it becomes widespread, it will be
able to reduce the practice of "user agent spoofing" and
discrimination of particular groups of the Internet users.
3. Definition of Proposed Format
This document proposes a formal definition of three types of User-
Agent string: standard string, regular string and web browser string.
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User-Agent = uuas
uuas = standard-string / regular-string / browser-string
Standard string is intended to maintain backward compatibility with
existing implementions and it is the same simple format as defined in
[RFC7230].
Regular string introduces a degree of standardization making every
theoretical UUAS parser able to obtain information from it.
Web browser string is designed for modern graphical web browsers and
proposes a set of signatures, which should form together a clear and
unequivocal application identifier.
3.1. Standard String
The standard User-Agent string MUST be generated in conformance with
Section 5.5.3 of [RFC7231]. The standard User-Agent string consists
of one or more product identifiers, each followed by zero or more
comments (Section 3.2 of [RFC7230]), which together identify the user
agent software.
Standard string syntax definition:
standard-string = product *( RWS ( product / comment ) )
The product identifiers and comments SHOULD be listed in decreasing
order of their significance. Each of them consists of a name and
OPTIONAL version number.
In the standard string a sender SHOULD limit generated product
identifiers to what is necessary to identify the product; a sender
MUST NOT generate advertising or other nonessential information
within the product identifier. A sender SHOULD NOT place non-
version-related information in version number part of product
identifier. In the standard string successive versions of the same
product SHOULD differ only in the version part of the identifier.
Example:
CERN-LineMode/2.15 libwww/2.17b3
3.2. Regular String
Regular Unified User-Agent String is intended for request senders
other than graphical web browsers and general web crawlers. It MUST
provide a signature of the operating system or platform (eg. in case
of runtime environments) used to generate the request at the first
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position in the comment after the first product identifier. After
this signature the regular string MAY contain any comments and next
product identifiers. Only this information MUST be provided, because
this format is designed for cases, when the server does not need to
know the exact parameters of the application originating the request.
In such cases this string can be applicable in statistical purposes
or in adapting the server's response to capabilities of particular
software platforms (eg. for indicating the need for adding carriage
returns before the newlines).
Regular string syntax definition:
regular-string = product RWS "(" os [ sc 1*ctext ] ")"
*( RWS ( product / comment ) )
Regular Unified User-Agent Strings are syntactically compliant with
the standard definition.
Example:
Wget/1.11.1 (Red Hat modified)
3.3. Web Browser String
Web Browser User-Agent String is a format of this field-value
intended for identifying modern graphical web browsers, which are
compatible with HTML5, CSS3 or other modern web technologies. Web
browser string MUST contain "Mozilla/5.0" tag at the beginning for
historical reasons. This helps avoid the recognition of browsers as
very old ones. Web Browser UUAS MUST also contain "Gecko" tag. This
can avoid delivering impaired versions of websites to modern but not
Gecko-based client applications. It is also in conformance with
Section 6.6.1.1 of [W3C.REC-html5-20141028].
Web browser string syntax definition:
browser-string = Mozilla-tag
RWS "(" *( signature sc ) os
*( sc signature ) [ sc language ]
*( sc signature ) [ sc rvtag ] ")"
RWS Gecko-string
*( RWS ( product / comment ) )
Like regular string, Web Browser Unified User-Agent String MUST
provide information about software platform. Fields contained
between brakets (comments) SHOULD be separated by semicolons with
optional space. Application MAY also include language tag in its
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User-Agent string. Then it MUST be a Language-Tag in accordance with
[RFC5646].
Due to the fact that the application originating the request cannot
provide its version info in the first product identifier, it SHOULD
place its version number in the separate revision tag.
Of course, a sender can add to the string any valid product
identifiers and comments, but this Memo is intended to simplify and
clarify this element of the protocol. In the web browser string
there MUST be at least one signature allowing to identify particular
client application product. Also the order of platform, language and
revision signatures MUST NOT be changed.
This type of UUAS SHOULD be also used by general web crawlers. It
can help avoid certain unfair practices relying on delivering other
resources to web browsers, other to web crawlers.
Example:
Mozilla/5.0 (Windows NT 6.3; Trident/7.0; rv:11.0) like Gecko
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4. ABNF Definition of UUAS
; Unified User-Agent String general definition
User-Agent = uuas
uuas = standard-string / regular-string / browser-string
; Standard string, as described in [RFC7231]
standard-string = product *( RWS ( product / comment ) )
; Regular string, recommended for non-browsers
regular-string = product RWS "(" os [ sc 1*ctext ] ")"
*( RWS ( product / comment ) )
; String recommended for web browsers and crawlers
browser-string = Mozilla-tag
RWS "(" *( signature sc ) os
*( sc signature ) [ sc language ]
*( sc signature ) [ sc rvtag ] ")"
RWS Gecko-string
*( RWS ( product / comment ) )
; Tags and signatures definitions
signature = product / 1*schar
os = 1*schar
language = <Language-Tag, see [RFC5646], Section 2.1>
rvtag = "rv:" OWS token
Mozilla-tag = "Mozilla/5.0"
Gecko-string = Gecko-tag
/ ( product RWS "(" *ctext
RWS Gecko-tag [ RWS 1*ctext ] ")" )
Gecko-tag = ["like "] "Gecko" ["/20100101"]
; Additional definitions
product = <product, see [RFC7231], Section 5.5.3>
comment = <comment, see [RFC7230], Section 3.2.6>
ctext = <ctext, see [RFC7230], Section 3.2.6>
schar = tchar / HTAB / SP / obs-text
token = <token, see [RFC7230], Section 3.2.6>
tchar = <tchar, see [RFC7230], Section 3.2.6>
obs-text = <obs-text, see [RFC7230], Section 3.2.6>
sc = ";" OWS
OWS = <OWS, see [RFC7230], Section 3.2.3>
RWS = <RWS, see [RFC7230], Section 3.2.3>
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5. Security Considerations
Implementations are encouraged not to use the product tokens of other
implementations in order to declare compatibility or identity with
them beyond the scope prescribed in this document, as this
circumvents the purpose of the User-Agent field.
A user agent SHOULD NOT generate a User-Agent field containing
needlessly fine-grained detail and SHOULD limit the addition of
subproducts by third parties. Overly detailed User-Agent strings
increase request latency and the risk of a user being identified
against their wishes. In theory, this can make it easier for an
attacker to exploit known security holes; in practice, attackers tend
to try all potential holes regardless of the software being used.
But when User-Agent string is combined with other characteristics of
the application, particularly if the client application sends
excessive details about the user's system or extensions, the risk of
successful attack gets higher.
As User-Agent strings are text data, they can be used to carry out
attacks by causing buffer overfows or changing formatting strings.
Implementers should secure their applications against such practices.
Data provided by User-Agent header field can be used to discriminate
the users of particular client applications by preventing them
accessing the requested resources or replacing them with false ones.
6. IANA Considerations
This document has no actions for IANA.
7. Acknowledgments
I would like to thank my English teacher, who devoted her time to
conduct a linguistic revision of this Memo.
8. References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP 14, March 1997.
[RFC5646] Phillips, A. and M. Davis, "Tags for Identifying
Languages", RFC 5646, September 2009.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 5234, STD 68, January 2008.
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[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Message Syntax and Routing", RFC 7230, June
2014.
[RFC7231] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Semantics and Content", RFC 7231, June 2014.
[W3C.REC-html5-20141028]
Hickson, I., Berjon, R., Faulkner, S., Leithead, T., Doyle
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/>.
Author's Address
Mateusz Karcz
Uniwersyteckie Katolickie Liceum Ogolnoksztalcace w Tczewie
6 Wodna Street
Tczew, PM 83-100
PL
Email: mateusz.karcz(at)interia.eu
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