Custom Log Formats

The format argument to the LogFormat and CustomLog 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 "%". For example, "%400,501{User-agent}i" logs User-agent on 400 errors and 501 errors only. For other status codes, the literal string "-" will be logged. The status code list may be preceded by a "!" to indicate negation: "%!200,304,302{Referer}i" logs Referer on all requests that do not return one of the three specified codes.

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.

Some 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, where hh 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.

In 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 by mod_logio will log the actual number of bytes sent over the network.

Note: mod_cache is implemented as a quick-handler and not as a standard handler. Therefore, the %R format string will not return any handler information when content caching is involved.

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 like msec_frac:

Timestamp including milliseconds

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

Security Considerations

See the security 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.

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.

CookieLog Directive

The CookieLog directive sets the filename for logging of cookies. The filename is relative to the ServerRoot. This directive is included only for compatibility with mod_cookies, and is deprecated.

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:

The third argument is optional and controls whether or not to log a particular request based on the presence or absence of a particular variable in the server environment. If the specified environment variable is set for the request (or is not set, in the case of a 'env=!name' clause), then the request will be logged.

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:

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

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 (%).

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.