The Apache server saves diagnostic information and error messages that it encounters while processing requests. The default path of this file is usr/local/apache/logs/error.log in Linux. Identify the Apache error log from the following logs.

The Apache error log format is distinct from the access log. A typical error log entry includes a timestamp, the module and severity level (e.g., [error]), process and thread IDs, client information (e.g., [client 127.0.0.1]), and a descriptive message explaining the error. Option B is the only entry that precisely matches this standard structure, indicating a server-side issue—in this case, access denial due to server configuration. The other options represent different types of data or malformed entries.

A. This is a standard Apache access log entry in the Common Log Format, recording a successful (status 200) client request, not an error.
C. This is a URL containing a directory traversal attack payload. It is not a server log entry itself, but rather the content of a request that might be logged.
D. This is a syntactically incorrect and malformed log entry that improperly mixes elements of an access log (request, status code) with an error log tag.

1. Apache HTTP Server Project. (2023). Log Files - Apache HTTP Server Version 2.4. Apache Software Foundation. Retrieved from https://httpd.apache.org/docs/2.4/logs.html#errorlog.

Reference Detail: The section "Error Log" describes the format of error log entries. A provided example is: [Wed Oct 11 14:32:52 2000] [error] [client 127.0.0.1] client denied by server configuration: /export/home/live/ap/htdocs/test, which directly matches option B.

2. Bauer, L., & Cranor, L. F. (2018). Course 15-330/630: Introduction to Computer Security, Lecture 18: Web Security. Carnegie Mellon University, School of Computer Science.

Reference Detail: Lecture slides on web security and server administration frequently cover the analysis of log files. Slide decks for such courses illustrate the distinct formats of access logs (showing requests and status codes) versus error logs (showing server-side issues with timestamps and severity levels), confirming that option A is an access log and B is an error log.

3. Zwicky, E. D., Cooper, S., & Chapman, D. B. (2000). Building Internet Firewalls, 2nd Edition. O'Reilly & Associates. (Often used as a textbook in university network security courses).

Reference Detail: Chapter 13, "Logging and Auditing," details the formats of various server logs, including Apache. It distinguishes between the access log's Common Log Format and the more descriptive, free-form nature of the error log, which includes severity levels like [error].