Apache module mod_log_config

### Apache HTTP Server Version 1.3

# Module mod_log_config

This module is contained in the `mod_log_config.c` file, and is compiled in by default in Apache 1.2. mod_log_config replaces mod_log_common in Apache 1.2. Prior to version 1.2, mod_log_config was an optional module. It provides for logging of the requests made to the server, using the Common Log Format or a user-specified format.

## Summary

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 go. The `TransferLog` and `CustomLog` directives can be used multiple times in each server to cause each request to be logged to multiple files.

### 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 re-configure Apache to use 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 settable 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.

## Log File Formats

Unless told otherwise with `LogFormat` the log files created by `TransferLog` will be in standard "Common Log Format" (CLF). The contents of each line in a CLF file are explained below. Alternatively, the log file can be customized (and if multiple log files are used, each can have a different format). Custom formats are set with `LogFormat` and `CustomLog`.

### Common Log Format

The Common Log Format (CLF) file contains a separate line for each request. A line is composed of several tokens separated by spaces:

> host ident authuser date request status bytes

If a token does not have a value then it is represented by a hyphen (-). The meanings and values of these tokens are as follows:

host
  The fully-qualified domain name of the client, or its IP number if the name is not available.
ident
  If IdentityCheck is enabled and the client machine runs identd, then this is the identity information reported by the client.
authuser
  If the request was for an password protected document, then this is the userid used in the request.
date
  The date and time of the request, in the following format:

    > `date = [day/month/year:hour:minute:second zone]
    > day = 2*digit
    > month = 3*letter
    > year = 4*digit
    > hour = 2*digit
    > minute = 2*digit
    > second = 2*digit
    > zone = (`+' | `-') 4*digit`

request
  The request line from the client, enclosed in double quotes (`"`).
status
  The three digit status code returned to the client.
bytes
  The number of bytes in the object returned to the client, not including any headers.

### Custom Log Formats

The format argument to the `LogFormat` and `CustomLog` is a string. This string is logged to the log file for each request. It can contain literal characters copied into the log files, and `%' directives which are replaced in the log file by the values as follows:

    %...b:          Bytes sent, excluding HTTP headers.
    %...f:          Filename
    %...{FOOBAR}e:  The contents of the environment variable FOOBAR
    %...h:          Remote host
    %...a:          Remote IP-address
    %...{Foobar}i:  The contents of Foobar: header line(s) in the request
                    sent to the server.
    %...l:          Remote logname (from identd, if supplied)
    %...{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.
    %...r:          First line of request
    %...s:          Status.  For requests that got internally redirected, this
                    is status of the *original* request --- %...>s for the last.
    %...t:          Time, in common log format time format
    %...{format}t:  The time, in the form given by format, which should
                    be in strftime(3) format.
    %...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.
    %...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). Note that there is no escaping performed on the strings from %r, %...i and %...o; some with long memories may remember that I thought this was a bad idea, once upon a time, and I'm still not comfortable with it, but it is difficult to see how to `do the right thing' with all of `%..i', unless we URL-escape everything and break with CLF.

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 the common log format is defined by the string `"%h %l %u %t \"%r\" %s %b"`, which can be used as the basis for extending for format if desired (e.g., to add extra fields at the end). NCSA's extended/combined log format would be `"%h %l %u %t \"%r\" %s %b \"%{Referer}i\" \"%{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.

## Using Multiple Log Files

The `TransferLog` and `CustomLog` directives can be given more than once to log requests to multiple log files. Each request will be logged to all the log files defined by either of these directives.

### Use with Virtual Hosts

If a <VirtualHost> section does not contain any `TransferLog` or `CustomLog` directives, the logs defined for the main server will be used. If it does contain one or more of these directives, requests serviced by this virtual host will only be logged in the log files defined within its definition, not in any of the main server's log files. See the examples below.

## 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.

## Directives

- CookieLog
- CustomLog
- CustomLog (conditional)
- LogFormat
- TransferLog

## CookieLog

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

Syntax: CustomLog file-pipe format-or-nickname
Context: server config, virtual host
Status: Base
Compatibility: Nickname only available in Apache 1.3 or later
Module: mod_log_config

The first argument is the filename to which log records should be written. This is used exactly like the argument to `TransferLog`; that is, it is either a full path or relative to the current server root.

The format argument specifies a format for each line of the log file. The options available for the format are exactly the same as for the argument of the `LogFormat` directive. If the format includes any spaces (which it will do in almost all cases) it should be enclosed in double quotes.

Instead of an actual format string, you can use a format nickname defined with the `LogFormat` directive.

## CustomLog (conditional)

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

The behaviour of this form of the `CustomLog` directive is almost identical to the standard `CustomLog` directive. The difference is that the '`env=`' clause controls whether a particular request will be logged in the specified file or not. 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 don't want to record requests for all GIF images on your server in a separate logfile but not 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

Syntax: LogFormat format [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 sets the format of the default logfile named by the `TransferLog` directive . See the section on Custom Log Formats for details on the format arguments.

If you include a nickname for the format on the directive line, you can use it in other `LogFormat` and `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.

## TransferLog

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

The TransferLog directive adds a log file in the format defined by the most recent `LogFormat` directive, or Common Log Format if no other default format has been specified. File-pipe is one of

A filename
  A filename relative to the ServerRoot.
`|' followed by a command
  A program to receive the agent log information on its standard input. Note the a new program will not be started for a VirtualHost if it inherits the TransferLog from the main server.

Security: if a program is used, then it will be run under the user who started httpd. This will be root if the server was started by root; be sure that the program is secure.

### Apache HTTP Server Version 1.3