Apache module mod_log_config

### Apache HTTP Server Version 1.3

# Module mod_log_config

This module provides for logging of the requests made to the server, using the Common Log Format or a user-specified format.

Status: Base
Source File: mod_log_config.c
Module Identifier: config_log_module
Compatibility: Was an extension module prior to Apache 1.2.

## 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, 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.

See also: Apache Log Files.

## Directives

- CookieLog
- CustomLog
- LogFormat
- TransferLog

## 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 back-slashes should be escaped with back-slashes.

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

    %...a:          Remote IP-address
    %...A:          Local IP-address
    %...B:          Bytes sent, excluding HTTP headers.
    %...b:          Bytes sent, excluding HTTP headers. In CLF format
            i.e. a '-' rather than a 0 when no bytes are sent.
    %...c:          Connection status when response was 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.
    %...{FOOBAR}e:  The contents of the environment variable FOOBAR
    %...f:          Filename
    %...h:          Remote host
    %...H       The request protocol
    %...{Foobar}i:  The contents of Foobar: header line(s) in the request
                    sent to the server.
    %...l:          Remote logname (from identd, if supplied)
    %...m       The request method
    %...{Foobar}n:  The contents of note "Foobar" from another module.
    %...{Foobar}o:  The contents of Foobar: header line(s) in the reply.
    %...p:          The canonical Port of the server serving the request
    %...P:          The process ID of the child that serviced the request.
    %...q       The query string (prepended with a ? if a query string exists,
            otherwise an empty string)
    %...r:          First line of request
    %...s:          Status.  For requests that got internally redirected, this is
                    the status of the *original* request --- %...>s for the last.
    %...t:          Time, in common log format time format (standard english format)
    %...{format}t:  The time, in the form given by format, which should
                    be in strftime(3) format. (potentially localized)
    %...T:          The time taken to serve the request, in seconds.
    %...u:          Remote user (from auth; may be bogus if return status (%s) is 401)
    %...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.

The "..." can be nothing at all (e.g., `"%h %u %r %s %b"`), or it can indicate conditions for inclusion of the item (which will cause it to be replaced with "-" if the condition is not met). The forms of condition are a list of HTTP status codes, which may or may not be preceded by "!". Thus, "%400,501{User-agent}i" logs User-agent: on 400 errors and 501 errors (Bad Request, Not Implemented) only; "%!200,304,302{Referer}i" logs Referer: on all requests which did not return some sort of normal status.

Note that in versions previous to 1.3.25 no escaping was performed on the strings from `%...r`, `%...i` and `%...o`. This was mainly to comply with the requirements of the Common Log Format. This implied that clients could insert control characters into the log, so you had to be quite careful when dealing with raw log files.

For security reasons starting with 1.3.25 non-printable and other special characters are escaped mostly by 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 that are written in their C-style notation (`\n`, `\t`, etc).

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"`

Note that the canonical ServerName and Port of the server serving the request are used for `%v` and `%p` respectively. This happens regardless of the UseCanonicalName setting because otherwise log analysis programs would have to duplicate the entire vhost matching algorithm in order to decide what host really served the request.

## 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.

## Compatibility notes

- This module is based on mod_log_config distributed with previous Apache releases, now updated to handle multiple logs. There is now no need to rebuild Apache to change configuration log formats.
- The module also implements the `CookieLog` directive, used to log user-tracking information created by mod_usertrack. The use of `CookieLog` is deprecated, and a `CustomLog` should be defined to log user-tracking information instead.
- As of Apache 1.3.5, this module allows conditional logging based upon the setting of environment variables. That is, you can control whether a request should be logged or not based upon whether an arbitrary environment variable is defined or not. This is configurable on a per-logfile basis.
- Beginning with Apache 1.3.5, the mod_log_config module has also subsumed the `RefererIgnore` functionality from mod_log_referer. The effect of `RefererIgnore` can be achieved by combinations of `SetEnvIf` directives and conditional `CustomLog` definitions.

## CookieLog directive

Syntax: CookieLog filename
Context: server config, virtual host
Module: mod_cookies
Compatibility: Only available in Apache 1.2 and above

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

Syntax: CustomLog file|pipe format|nickname [env=[!]environment-variable]
Context: server config, virtual host
Status: Base
Compatibility: Nickname only available in Apache 1.3 or later. Conditional logging available in 1.3.5 or later.
Module: mod_log_config

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. 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.

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 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:

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

## LogFormat directive

Syntax: LogFormat format|nickname [nickname]
Default: `LogFormat "%h %l %u %t \"%r\" %>s %b"`
Context: server config, virtual host
Status: Base
Compatibility: Nickname only available in Apache 1.3 or later
Module: mod_log_config

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 which 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.

For example:

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

## TransferLog directive

Syntax: TransferLog file|pipe
Default: none
Context: server config, virtual host
Status: Base
Module: mod_log_config

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 that does not define a nickname. Common Log Format is used if no other format has been specified.

Example:

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

### Apache HTTP Server Version 1.3