Extended Log File Format

W3C Working Draft WD-logfile-960323

This version:
Latest version:
Phillip M. Hallam-Baker <hallam@w3.org>
Brian Behlendorf <brian@organic.com>

Status of this document

This is a W3C Working Draft for review by W3C members and other interested parties. It is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to use W3C Working Drafts as reference material or to cite them as other than "work in progress". A list of current W3C working drafts can be found at: http://www.w3.org/pub/WWW/TR

Note: since working drafts are subject to frequent change, you are advised to reference the above URL, rather than the URLs for working drafts themselves.


An improved format for Web server log files is presented. The format is extensible, permitting a wider range of data to be captured. This proposal is motivated by the need to capture a wider range of data for demographic analysis and also the needs of proxy caches.


Most Web servers offer the option to store logfiles in either the common log format or a proprietary format. The common log file format is supported by the majority of analysis tools but the information about each server transaction is fixed. In many cases it is desirable to record more information. Sites sensitive to personal data issues may wish to omit the recording of certain data. In addition ambiguities arise in analyzing the common log file format since field separator characters may in some cases occur within fields. The extended log file format is designed to meet the following needs:

The log file format described permits customized logfiles to be recorded in a format readable by generic analysis tools. A header specifying the data types recorded is written out at the start of each log.

This work is in part motivated by the need to support collection of demographic data. This work is discussed at greater length in companion drafts describing session identifier URIs [Hallam96a] and more consistent proxy behaviour [Hallam96b].


An extended log file contains a sequence of lines containing ASCII characters terminated by either the sequence LF or CRLF. Log file generators should follow the line termination convention for the platform on which they are executed. Analyzers should accept either form. Each line may contain either a directive or an entry.

Entries consist of a sequence of fields relating to a single HTTP transaction. Fields are separated by whitespace, the use of tab characters for this purpose is encouraged. If a field is unused in a particular entry dash "-" marks the omitted field. Directives record information about the logging process itself.

Lines beginning with the # character contain directives. The following directives are defined:

Version: <integer>.<integer>
The version of the extended log file format used. This draft defines version 1.0.
Fields: [<specifier>...]
Specifies the fields recorded in the log.
Software: string
Identifies the software which generated the log.
Start-Date: <date> <time>
The date and time at which the log was started.
End-Date:<date> <time>
The date and time at which the log was finished.
Date:<date> <time>
The date and time at which the entry was added.
Remark: <text>
Comment information. Data recorded in this field should be ignored by analysis tools.

The directives Version and Fields are required and should precede all entries in the log. The Fields directive specifies the data recorded in the fields of each entry.


The following is an example file in the extended log format:

#Version: 1.0
#Date: 12-Jan-1996 00:00:00
#Fields: time cs-method cs-uri
00:34:23 GET /foo/bar.html
12:21:16 GET /foo/bar.html
12:45:52 GET /foo/bar.html
12:57:34 GET /foo/bar.html


The #Fields directive lists a sequence of field identifiers specifying the information recorded in each entry. Field identifiers may have one of the following forms:

Identifier relates to the transaction as a whole.
Identifier relates to information transfer between parties defined by the value prefix.
Identifies the value of the HTTP header field header for transfer between parties defined by the value prefix. Fields specified in this manner always have the value <string>.

The following prefixes are defined:

Client to Server.
Server to Client.
Server to Remote Server, this prefix is used by proxies.
Remote Server to Server, this prefix is used by proxies.
Application specific identifier.

The identifier cs-method thus refers to the method in the request sent by the client to the server while sc(Referer) refers to the referer: field of the reply. The identifier c-ip refers to the client's ip address.


The following identifiers do not require a prefix

Date at which transaction completed, field has type <date>
Time at which transaction completed, field has type <time>
Time taken for transaction to complete in seconds, field has type <fixed>
bytes transferred, field has type <integer>
Records whether a cache hit occurred, field has type <integer> 0 indicates a cache miss.

The following identifiers require a prefix

IP address and port, field has type <address>
DNS name, field has type <name>
Status code, field has type <integer>
Comment returned with status code, field has type <text>
Method, field has type <name>
URI, field has type <uri>
Stem portion alone of URI (omitting query), field has type <uri>
Query portion alone of URI, field has type <uri>

Special fields for log summaries.

Analysis tools may generate log summaries. A log summary entry begins with a count specifying the number of times a particular even occurred. For example a site may be interested in a count of the number of requests for a particular URI with a given referer: field but not be interested in recording information about individual requests such as the IP address.

The following field is mandatory and must precede all others:

The number of entries for which the listed data, field has type <integer>

The following fields may be used in place of time to allow aggregation of log file entries over intervals of time.

Time at which sampling began, field has type <time>
Time at which sampling ended, field has type <time>
Time over which sampling occurred in seconds, field has type <integer>


This section describes the data formats for log file field entries. These formats are chosen so as to avoid ambiguity, minimize the difficulty of generation and parsing and provide for human readability.

Each logfile entry consists of a sequence of fields separated by whitespace and terminated by a CR or CRLF sequence. The meanings of the fields are defined by a preceding #Fields directive. If a field is omitted for a particular entry a single dash "-" is substituted.

Log file parsers should be tolerant of errors. If an entry contains corrupt data or is terminated unexpectedly the parser should resynchronize using the end of line marker and continue to parse the following entries. Entries must not contain any ASCII control characters.

<entry> = *<field> <end-of-line>

<field> = <integer> | <fixed> | <uri> | <date> | <time> | <string>

<digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"


<integer>       = 1*<digit>

Integers are represented as a sequence of digits.

Fixed Format Float

<fixed> = 1*<digit> [. *<digit>]


A URI as specified by RFC1738, relative URIs are specified by RFC1808. URIs cannot by definition include whitespace or ASCII control characters. Consequently no ambiguity arises from their use.


<date>  = 4<digit> "-" 2<digit> "-" 2<digit>

Dates are recorded in the format YYYY-MM-DD where YYYY, MM and DD stand for the numeric year, month and day respectively. All dates are specified in GMT. This format is chosen to assist collation using sort.


<time>  = 2<digit> ":" 2<digit> [":" 2<digit> ["." *<digit>]

Times are recorded in the form HH:MM, HH:MM:SS or HH:MM:SS.S where HH is the hour in 24 hour format, MM is minutes and SS is seconds. All times are specified in GMT.


<string>        = '"' *<schar> '"'

<schar> = xchar | '"' '"'

Strings are output in quoted form. If a string contains a quotation character the character is repeated. This format is unambiguous since fields are by definition separated by whitespace. The dash character must not be used as an abbreviation for an empty string.

No mechanism for incorporating control characters is defined.


<text>  =  <char>* 

The text field is used only by directives.


<name>  =  <alpha> [ "." *<alpha> ] 

DNS name.


<name>  =  <integer> [ "." *<integer> ] [ ":" <integer> ]

Numeric IP address and optional port specifier.


Robert Thau provided useful advice and some code. John Mallery and Roger Hurwitz helped develop many of the ideas.


R. Fielding Relative Uniform Resource Locators, June 1995
T. Berners-Lee, L. Masinter, Uniform Resource Locators (URL), December 1994
A. Luotonen, The Common Logfile Format, 1995, http://www.w3.org/pub/WWW/Daemon/User/Config/Logging.html
P. M. Hallam-Baker Session Identification URI World Wide Web Consortium Working Draft, WD-session-id.
P. M. Hallam-Baker Notification for Proxy Caches World Wide Web Consortium Working Draft, WD-proxy.