---
title: "Common Log Format"
description: "The Common Log Format (CLF) is a standardized text format for web server access logs that records the client IP, timestamp, request, status code, and response size."
category: "Server Log Analysis"
date: "2026-03-05"
url: "https://getbeast.io/glossary/common-log-format/"
type: "glossary"
---

# Common Log Format

**Category:** Server Log Analysis | **Updated:** 2026-03-05

The Common Log Format (CLF) is a standardized text format for web server access logs that records the client IP, timestamp, request, status code, and response size.

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## What Is the Common Log Format?

The Common Log Format (CLF) is one of the oldest standardized formats for web server access logs. Each line contains seven fields: remote host, RFC 1413 identity, authenticated user, timestamp, request line, HTTP status code, and response size in bytes. It was defined by the NCSA and is supported by virtually every web server.

## Why It Matters

While the Common Log Format is simpler than the Combined format, it lacks the user-agent and referrer fields that are critical for SEO analysis. For this reason, most SEO professionals prefer the Combined Log Format. However, understanding CLF is important because some legacy systems and CDNs still output logs in this format.

## How to Upgrade to Combined Format

If your server is using CLF, switch to the Combined Log Format by updating your server configuration to include the `Referer` and `User-Agent` fields. This gives you the bot identification and traffic source data needed for comprehensive SEO log analysis.

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## Related Terms

- [Combined Log Format](/glossary/combined-log-format/)
- [W3C Log Format](/glossary/w3c-log-format/)
- [Access Log](/glossary/access-log/)
- [Log Parsing](/glossary/log-parsing/)

## Further Reading

- [Understanding Server Log Formats](/blog/log-formats/)

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*Part of the [GetBeast SEO Glossary](/glossary/). Visit [GetBeast.io](https://getbeast.io) for professional SEO and log analysis tools.*