Apache Module mod_log_config

Summary

This module provides for flexible logging of client requests. Logs are written in a customizable format, and may be written directly to a file, or to an external program. Conditional logging is provided so that individual requests may be included or excluded from the logs based on characteristics of the request.

Three directives are provided by this module:TransferLog to create a log file,LogFormat to set a custom format, andCustomLog to define a log file and format in one step. TheTransferLog andCustomLog directives can be used multiple times in each server to cause each request to be logged to multiple files.

Topics

• Custom Log Formats
• Security Considerations

Directives

• BufferedLogs
• CustomLog
• GlobalLog
• LogFormat
• TransferLog

Bugfix checklist

• httpd changelog
• Known issues
• Report a bug

See also

• Apache Log Files
• Comments

Custom Log Formats

The format argument to theLogFormat andCustomLog directives is a string. This string is used to log each request to the log file. It can contain literal characters copied into the log files and the C-style control characters "\n" and "\t" to represent new-lines and tabs. Literal quotes and backslashes should be escaped with backslashes.

The characteristics of the request itself are logged by placing "%" directives in the format string, which are replaced in the log file by the values as follows:

Modifiers

Particular items can be restricted to print only for responses with specific HTTP status codes by placing a comma-separated list of status codes immediately following the "%". The status code list may be preceded by a "!" to indicate negation.

The modifiers "<" and ">" can be used for requests that have been internally redirected to choose whether the original or final (respectively) request should be consulted. By default, the% directives%s, %U, %T, %D, and%r look at the original request while all others look at the final request. So for example,%>s can be used to record the final status of the request and%<u can be used to record the original authenticated user on a request that is internally redirected to an unauthenticated resource.

Format Notes

For security reasons, starting with version 2.0.46, non-printable and other special characters in%r,%i and%o are escaped using\xhh sequences, wherehh stands for the hexadecimal representation of the raw byte. Exceptions from this rule are" and\, which are escaped by prepending a backslash, and all whitespace characters, which are written in their C-style notation (\n,\t, etc). In versions prior to 2.0.46, no escaping was performed on these strings so you had to be quite careful when dealing with raw log files.

Since httpd 2.0, unlike 1.3, the%b and%B format strings do not represent the number of bytes sent to the client, but simply the size in bytes of the HTTP response (which will differ, for instance, if the connection is aborted, or if SSL is used). The%O format provided bymod_logio will log the actual number of bytes sent over the network.

Examples

Some commonly used log format strings are:

Common Log Format (CLF)

"%h %l %u %t \"%r\" %>s %b"

Common Log Format with Virtual Host

"%v %h %l %u %t \"%r\" %>s %b"

NCSA extended/combined log format

"%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-agent}i\""

Referer log format

"%{Referer}i -> %U"

Agent (Browser) log format

"%{User-agent}i"

You can use the%{format}t directive multiple times to build up a time format using the extended format tokens likemsec_frac:

Timestamp including milliseconds

"%{%d/%b/%Y %T}t.%{msec_frac}t %{%z}t"

Security Considerations

See thesecurity tips document for details on why your security could be compromised if the directory where logfiles are stored is writable by anyone other than the user that starts the server.

BufferedLogsDirective

TheBufferedLogs directive causesmod_log_config to store several log entries in memory and write them together to disk, rather than writing them after each request. On some systems, this may result in more efficient disk access and hence higher performance. It may be set only once for the entire server; it cannot be configured per virtual-host.

CustomLogDirective

TheCustomLog directive is used to log requests to the server. A log format is specified, and the logging can optionally be made conditional on request characteristics using environment variables.

The first argument, which specifies the location to which the logs will be written, can take one of the following two types of values:

file

A filename, relative to theServerRoot.

pipe

The pipe character "|", followed by the path to a program to receive the log information on its standard input. See the notes onpiped logs for more information.

Security:

If a program is used, then it will be run as the user who startedhttpd. This will be root if the server was started by root; be sure that the program is secure.

Note

When entering a file path on non-Unix platforms, care should be taken to make sure that only forward slashed are used even though the platform may allow the use of back slashes. In general it is a good idea to always use forward slashes throughout the configuration files.

The second argument specifies what will be written to the log file. It can specify either anickname defined by a previousLogFormat directive, or it can be an explicitformat string as described in thelog formats section.

For example, the following two sets of directives have exactly the same effect:

# CustomLog with format nicknameLogFormat "%h %l %u %t \"%r\" %>s %b" commonCustomLog "logs/access_log" common# CustomLog with explicit format stringCustomLog "logs/access_log" "%h %l %u %t \"%r\" %>s %b"

The third argument is optional and controls whether or not to log a particular request. The condition can be the presence or absence (in the case of a 'env=!name' clause) of a particular variable in the serverenvironment. Alternatively, the condition can be expressed as arbitrary booleanexpression. If the condition is not satisfied, the request will not be logged. References to HTTP headers in the expression will not cause the header names to be added to the Vary header.

Environment variables can be set on a per-request basis using themod_setenvif and/ormod_rewrite modules. For example, if you want to record requests for all GIF images on your server in a separate logfile but not in your main log, you can use:

SetEnvIf Request_URI \.gif$ gif-imageCustomLog "gif-requests.log" common env=gif-imageCustomLog "nongif-requests.log" common env=!gif-image

Or, to reproduce the behavior of the old RefererIgnore directive, you might use the following:

SetEnvIf Referer example\.com localrefererCustomLog "referer.log" referer env=!localreferer

GlobalLogDirective

TheGlobalLog directive defines a log shared by the main server configuration and all defined virtual hosts.

TheGlobalLog directive is identical to theCustomLog directive, apart from the following differences:

• GlobalLog is not valid in virtual host context.
• GlobalLog is used by virtual hosts that define their ownCustomLog, unlike a globally specifiedCustomLog.

LogFormatDirective

This directive specifies the format of the access log file.

TheLogFormat directive can take one of two forms. In the first form, where only one argument is specified, this directive sets the log format which will be used by logs specified in subsequentTransferLog directives. The single argument can specify an explicitformat as discussed in thecustom log formats section above. Alternatively, it can use anickname to refer to a log format defined in a previousLogFormat directive as described below.

The second form of theLogFormat directive associates an explicitformat with anickname. Thisnickname can then be used in subsequentLogFormat orCustomLog directives rather than repeating the entire format string. ALogFormat directive that defines a nicknamedoes nothing else -- that is, itonly defines the nickname, it doesn't actually apply the format and make it the default. Therefore, it will not affect subsequentTransferLog directives. In addition,LogFormat cannot use one nickname to define another nickname. Note that the nickname should not contain percent signs (%).

LogFormat "%v %h %l %u %t \"%r\" %>s %b" vhost_common

TransferLogDirective

This directive has exactly the same arguments and effect as theCustomLog directive, with the exception that it does not allow the log format to be specified explicitly or for conditional logging of requests. Instead, the log format is determined by the most recently specifiedLogFormat directive which does not define a nickname. Common Log Format is used if no other format has been specified.

LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-agent}i\""TransferLog logs/access_log