CacheStaleOnError and CacheIgnoreHeaders interaction for stale error responses

Direct Answer

Yes, CacheIgnoreHeaders strips headers from stale responses served under CacheStaleOnError. The directive applies to cached objects at storage time, and the same filtering logic runs when retrieving stale content for 5xx errors.

Confirmed Behavior

The interaction works as follows:

1. Cache storage: When a response is cached, CacheIgnoreHeaders removes listed headers before storing the object.
2. Stale delivery: When CacheStaleOnError serves a stale copy for a 5xx response, the headers have already been stripped during caching.
3. No additional configuration: No extra directive is needed; the combination achieves the desired behavior.

Version History

This behavior has been consistent since Apache 2.4's mod_cache refactor and remains unchanged in 2.4.x and 2.5.x releases.

Verification Steps

To confirm this in your environment:

1. Configure a test virtual host with:

CacheEnable disk /
CacheStaleOnError on
CacheIgnoreHeaders X-Powered-By Custom-Header

1. Prime the cache with a successful request containing headers to be stripped
2. Force the origin to return a 5xx error (via rewrite rule or temporary endpoint)
3. Use curl -I to request the resource while origin returns 5xx
4. Verify stripped headers are absent from the response

Caution: Header Selection Matters

Avoid stripping headers like:

• Set-Cookie - may break authentication flows
• Authorization - security implications
• Content-Length - client parsing issues

Potential Side Effects

CacheIgnoreHeaders can cause issues if:

• The origin varies responses based on stripped headers (e.g., Vary: Authorization)
• Clients expect stripped headers for functionality

Performance Impact

Minimal - header removal occurs during cache storage/retrieval, not per-request processing.