Apache Module mod_log_forensic

Summary

This module provides for forensic logging of client requests. Logging is done before and after processing a request, so the forensic log contains two log lines for each request. The forensic logger is very strict, which means:

• The format is fixed. You cannot modify the logging format at runtime.
• If it cannot write its data, the child process exits immediately and may dump core (depending on yourCoreDumpDirectory configuration).

Thecheck_forensic script, which can be found in the distribution's support directory, may be helpful in evaluating the forensic log output.

Topics

• Forensic Log Format
• Security Considerations

Directives

• ForensicLog

Bugfix checklist

• httpd changelog
• Known issues
• Report a bug

See also

• Apache Log Files
• mod_log_config
• Comments

Forensic Log Format

Each request is logged two times. The first time isbefore it's processed further (that is, after receiving the headers). The second log entry is writtenafter the request processing at the same time where normal logging occurs.

In order to identify each request, a unique request ID is assigned. This forensic ID can be cross logged in the normal transfer log using the%{forensic-id}n format string. If you're usingmod_unique_id, its generated ID will be used.

The first line logs the forensic ID, the request line and all received headers, separated by pipe characters (|). A sample line looks like the following (all on one line):

The plus character at the beginning indicates that this is the first log line of this request. The second line just contains a minus character and the ID again:

Thecheck_forensic script takes as its argument the name of the logfile. It looks for those+/- ID pairs and complains if a request was not completed.

Security Considerations

See thesecurity 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.

The log files may contain sensitive data such as the contents ofAuthorization: headers (which can contain passwords), so they should not be readable by anyone except the user that starts the server.

ForensicLogDirective

TheForensicLog directive is used to log requests to the server for forensic analysis. Each log entry is assigned a unique ID which can be associated with the request using the normalCustomLog directive.mod_log_forensic creates a token calledforensic-id, which can be added to the transfer log using the%{forensic-id}n format string.

The argument, which specifies the location to which the logs will be written, can take one of the following two types of values:

filename

A filename, relative to theServerRoot.

pipe

The pipe character "|", followed by the path to a program to receive the log information on its standard input. The program name can be specified relative to theServerRoot directive.

Security:

If a program is used, then it will be run as the user who startedhttpd. This will be root if the server was started by root; be sure that the program is secure or switches to a less privileged user.

Note

When entering a file path on non-Unix platforms, care should be taken to make sure that only forward slashes 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.