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Network Working Group                                       B. Hoehrmann
Internet-Draft                                        September 20, 2010
Intended status: Informational
Expires: March 24, 2011

                      Parsing malformed HTTP dates


   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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   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 March 24, 2011.

Copyright Notice

   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
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   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.

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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

   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


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

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

   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 "Bj&#246;rn H&#246;hrmann" in HTML and XML.

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