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Apache Module mod_log_config

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Description:Logging of the requests made to the server
Module Identifier:log_config_module
Source File:mod_log_config.c


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, and CustomLog to define a log file and format in one step. The TransferLog and CustomLog directives can be used multiple times in each server to cause each request to be logged to multiple files.

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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:

Format String Description
%% The percent sign.
%a Client IP address of the request (see the mod_remoteip module).
%{c}a Underlying peer IP address of the connection (see the mod_remoteip module).
%A Local IP-address.
%B Size of response in bytes, excluding HTTP headers.
%b Size of response in bytes, excluding HTTP headers. In CLF format, i.e. a '-' rather than a 0 when no bytes are sent.
%{VARNAME}C The contents of cookie VARNAME in the request sent to the server. Only version 0 cookies are fully supported.
%D The time taken to serve the request, in microseconds.
%{VARNAME}e The contents of the environment variable VARNAME.
%f Filename.
%h Remote hostname. Will log the IP address if HostnameLookups is set to Off, which is the default. If it logs the hostname for only a few hosts, you probably have access control directives mentioning them by name. See the Require host documentation.
%{c}h Like %h, but always reports on the hostname of the underlying TCP connection and not any modifications to the remote hostname by modules like mod_remoteip.
%H The request protocol.
%{VARNAME}i The contents of VARNAME: header line(s) in the request sent to the server. Changes made by other modules (e.g. mod_headers) affect this. If you're interested in what the request header was prior to when most modules would have modified it, use mod_setenvif to copy the header into an internal environment variable and log that value with the %{VARNAME}e described above.
%k Number of keepalive requests handled on this connection. Interesting if KeepAlive is being used, so that, for example, a '1' means the first keepalive request after the initial one, '2' the second, etc...; otherwise this is always 0 (indicating the initial request).
%l Remote logname (from identd, if supplied). This will return a dash unless mod_ident is present and IdentityCheck is set On.
%L The request log ID from the error log (or '-' if nothing has been logged to the error log for this request). Look for the matching error log line to see what request caused what error.
%m The request method.
%{VARNAME}n The contents of note VARNAME from another module.
%{VARNAME}o The contents of VARNAME: header line(s) in the reply.
%p The canonical port of the server serving the request.
%{format}p The canonical port of the server serving the request, or the server's actual port, or the client's actual port. Valid formats are canonical, local, or remote.
%P The process ID of the child that serviced the request.
%{format}P The process ID or thread ID of the child that serviced the request. Valid formats are pid, tid, and hextid. hextid requires APR 1.2.0 or higher.
%q The query string (prepended with a ? if a query string exists, otherwise an empty string).
%r First line of request.
%R The handler generating the response (if any).
%s Status. For requests that have been internally redirected, this is the status of the original request. Use %>s for the final status.
%t Time the request was received, in the format [18/Sep/2011:19:18:28 -0400]. The last number indicates the timezone offset from GMT
%{format}t The time, in the form given by format, which should be in an extended strftime(3) format (potentially localized). If the format starts with begin: (default) the time is taken at the beginning of the request processing. If it starts with end: it is the time when the log entry gets written, close to the end of the request processing. In addition to the formats supported by strftime(3), the following format tokens are supported:
secnumber of seconds since the Epoch
msecnumber of milliseconds since the Epoch
usecnumber of microseconds since the Epoch
msec_fracmillisecond fraction
usec_fracmicrosecond fraction
These tokens can not be combined with each other or strftime(3) formatting in the same format string. You can use multiple %{format}t tokens instead.
%T The time taken to serve the request, in seconds.
%{UNIT}T The time taken to serve the request, in a time unit given by UNIT. Valid units are ms for milliseconds, us for microseconds, and s for seconds. Using s gives the same result as %T without any format; using us gives the same result as %D. Combining %T with a unit is available in 2.4.13 and later.
%u Remote user if the request was authenticated. May be bogus if return status (%s) is 401 (unauthorized).
%U The URL path requested, not including any query string.
%v The canonical ServerName of the server serving the request.
%V The server name according to the UseCanonicalName setting.
%X Connection status when response is completed:
X = Connection aborted before the response completed.
+ = Connection may be kept alive after the response is sent.
- = Connection will be closed after the response is sent.
%I Bytes received, including request and headers. Cannot be zero. You need to enable mod_logio to use this.
%O Bytes sent, including headers. May be zero in rare cases such as when a request is aborted before a response is sent. You need to enable mod_logio to use this.
%S Bytes transferred (received and sent), including request and headers, cannot be zero. This is the combination of %I and %O. You need to enable mod_logio to use this.
%{VARNAME}^ti The contents of VARNAME: trailer line(s) in the request sent to the server.
%{VARNAME}^to The contents of VARNAME: trailer line(s) in the response sent from the server.


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.

Format String Meaning
%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.
%!200,304,302{Referer}i Logs Referer on all requests that do not return one of the three specified codes, "-" otherwise.

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, 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.

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 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.


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

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

Description:Buffer log entries in memory before writing to disk
Syntax:BufferedLogs On|Off
Default:BufferedLogs Off
Context:server config

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.

This directive should be used with caution as a crash might cause loss of logging data.

CustomLog Directive

Description:Sets filename and format of log file
Syntax:CustomLog file|pipe format|nickname [env=[!]environment-variable| expr=expression]
Context:server config, virtual host

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:

A filename, relative to the ServerRoot.
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.


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.


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

GlobalLog Directive

Description:Sets filename and format of log file
Syntax:GlobalLogfile|pipe format|nickname [env=[!]environment-variable| expr=expression]
Context:server config
Compatibility:Available in Apache HTTP Server 2.4.19 and later

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

The GlobalLog directive is identical to the CustomLog directive, apart from the following differences:


LogFormat Directive

Description:Describes a format for use in a log file
Syntax:LogFormat format|nickname [nickname]
Default:LogFormat "%h %l %u %t \"%r\" %>s %b"
Context:server config, virtual host

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

Description:Specify location of a log file
Syntax:TransferLog file|pipe
Context:server config, virtual host

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

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