If-Modified-Since Caching — Conditional Request and 304 Not Modified Errors

Fix If-Modified-Since and If-None-Match conditional caching errors causing stale content, cache validation failures, and unnecessary full downloads. This guide covers everything you need to know about this topic, including common causes, step-by-step solutions, and answers to frequently asked questions.

Here are the key things to understand: Conditional requests use If-Modified-Since or If-None-Match headers to validate cached content. Server responds with 304 Not Modified if the content has not changed, saving bandwidth. If-Modified-Since uses a date; If-None-Match uses an ETag (entity tag) for more precise validation. Incorrect date formats or clock skew between client and server cause validation failures. Some servers do not support conditional requests, always returning full 200 responses. Understanding these fundamentals will help you diagnose and resolve this issue more effectively.

The most common reasons this occurs include: Server not sending Last-Modified or ETag headers on initial responses. Date format in If-Modified-Since not matching the required HTTP date format (RFC 7231). Clock skew between client and server causing premature or delayed cache invalidation. Load-balanced servers generating different ETags for the same content. Identifying the root cause is the first step toward finding the right solution.

To resolve this, follow these recommended steps: Ensure your server sends Last-Modified and ETag headers on cacheable responses. Implement conditional request handling: compare If-Modified-Since/If-None-Match and return 304 when content is unchanged. Use strong ETags based on content hash (not timestamp) for consistent validation across load-balanced servers. Test conditional requests: curl -H 'If-None-Match: "etag-value"' -v https://your-server.com/resource. Configure your CDN to support and forward conditional request headers to the origin. If these steps do not resolve the issue, consider consulting additional resources or a qualified professional.

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