Apache Module mod_log_config

BufferedLogs Directive

The BufferedLogs directive causes mod_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.

CustomLog Directive

The CustomLog 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 the ServerRoot.

pipe

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

Security:

If a program is used, then it will be run as the user who started httpd. 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 a nickname defined by a previous LogFormat directive, or it can be an explicit format string as described in the log formats section.

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

# CustomLog with format nickname
LogFormat "%h %l %u %t \"%r\" %>s %b" common
CustomLog logs/access_log common

# CustomLog with explicit format string
CustomLog 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 server environment. Alternatively, the condition can be expressed as arbitrary boolean expression. 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 the mod_setenvif and/or mod_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-image
CustomLog gif-requests.log common env=gif-image
CustomLog 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 localreferer
CustomLog referer.log referer env=!localreferer

LogFormat Directive

This directive specifies the format of the access log file.

The LogFormat 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 subsequent TransferLog directives. The single argument can specify an explicit format as discussed in the custom log formats section above. Alternatively, it can use a nickname to refer to a log format defined in a previous LogFormat directive as described below.

The second form of the LogFormat directive associates an explicit format with a nickname. This nickname can then be used in subsequent LogFormat or CustomLog directives rather than repeating the entire format string. A LogFormat directive that defines a nickname does nothing else -- that is, it only defines the nickname, it doesn't actually apply the format and make it the default. Therefore, it will not affect subsequent TransferLog 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

TransferLog Directive

This directive has exactly the same arguments and effect as the CustomLog 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 specified LogFormat 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