Internet DRAFT - draft-hoehrmann-date-parsing
draft-hoehrmann-date-parsing
Network Working Group B. Hoehrmann
Internet-Draft September 20, 2010
Intended status: Informational
Expires: March 24, 2011
Parsing malformed HTTP dates
draft-hoehrmann-date-parsing-01
Abstract
The HTTP/1.1 specification encourages recipients of date values, as
found in HTTP headers like Date and Last-Modified, to be robust in
accepting date values that may have been sent by non-HTTP software.
This memo defines an error-tolerant parsing algorithm based on the
date formats permitted by the HTTP specification for this purpose.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on March 24, 2011.
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1. Introduction
HTTP/1.1 defines three different date formats for use in HTTP (see
[RFC2616], section 3.3). Some deployed software generates values
that do not strictly match any of the three formats, and the HTTP
specification encourages implementations to be robust in accepting
them. Differences include for instance whether numeric values are
padded with spaces or with leading zeros, and which delimiters are
used.
This specification defines a grammar for HTTP date values that
tolerates these minor differences to accomodate malformed formats
that are known to occur relatively frequently in malformed values and
are supported by widely deployed implementations. No effort is made
to mirror a particular set of existing implementations or entirely
different date formats.
This specification does not update the HTTP specification; values
that match the grammar in this document but not the requirements of
the HTTP standard continue to be non-compliant. Implementations
continue to conform (or not) to the HTTP specification whether or not
they follow the requirements defined in this specification.
2. Terminology and Conformance
A software module that interprets dates expressed as sequences of
octets conforms to this specification if and only if it interprets
dates that match the "recoverable" rule defined in this document
according to this specification.
3. Tolerant HTTP date syntax
The following ABNF [RFC5234] grammar corresponds to the HTTP-Date
grammar with some minor modifications: the leading weekday is a free-
form string separated from the remainder by a comma or white space,
hyphen and space are valid separators between month, day, and year
regardless of the exact format, an optional trailing time zone (a
free form string) is allowed for all variants, the three-letter month
names may be succeeded by other letters to accomodate spelled-out
names, and sequences of white space and digits do not have to have
the exact length specified in the HTTP specification.
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recoverable = [ wkday wsc ] (dmyt / mdty) [ ws tz ]
dmyt = day sep month sep year ws time ; rfc1123-ish
mdty = month sep day ws time ws year ; asctime-ish
time-of-day = hour ":" minute ":" second
hour = 1*2digit
minute = 1*2digit
second = 1*2digit
day = 1*2digit
year = 2digit / 4digit
ws = 1*sp
wsc = ws / "," *sp
wkday = %x00-1f / %x21-2b / %2d-ff
tz = *octet
sep = 1*sp / "-"
month = "Jan" / "Feb" / "Mar" / "Apr" / "May" / "Jun"
/ "Jul" / "Aug" / "Sep" / "Oct" / "Nov" / "Dec"
month-ish = month *alpha
sp = <as defined in [RFC5234], appendix B.1>
digit = <as defined in [RFC5234], appendix B.1>
octet = <as defined in [RFC5234], appendix B.1>
alpha = <as defined in [RFC5234], appendix B.1>
The interpretation of time-of-day, day, and month is the same as
defined in the HTTP specification, with the clarification that where
the HTTP specification requires more digits than are matched, the
missing digits are assumed to be leading zeros; for year, the rules
are the same as defined for the rfc850-date format. This document
does not define an interpretation for the free-form time zone string,
though it is common to simply ignore it.
4. IANA Considerations
None.
5. Security Considerations
The HTTP specification already encourages implementers to be robust
in accepting malformed date values and there are no new security
concerns with the particular error recovery rules defined in this
document.
6. References
[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.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
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Specifications: ABNF", STD 68, RFC 5234, January 2008.
Author's Address
Bjoern Hoehrmann
Mittelstrasse 50
39114 Magdeburg
Germany
EMail: mailto:bjoern@hoehrmann.de
URI: http://bjoern.hoehrmann.de
Note: Please write "Bjoern Hoehrmann" with o-umlaut (U+00F6) wherever
possible, e.g., as "Björn Höhrmann" in HTML and XML.
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