Work out what an HTTP header does

The job

Debugging an API call or a caching surprise usually means reading headers. The quick questions are always the same: what does this header do, and who sends it?

Look it up

Search the header reference by name or keyword. Each row shows whether the header travels on requests, responses, or both, plus a one-line meaning and notes on common mistakes — like Content-Type going on both sides but meaning different things.

• request — sent by the client (Authorization, If-None-Match).
• response — sent by the server (ETag, Content-Security-Policy).
• shared — valid in either direction (Cache-Control, Date).

Worked example

Your CDN seems to ignore cache settings. Searching 'cache' shows Cache-Control (shared), ETag and Last-Modified (response validators), and If-Modified-Since / If-None-Match (the request-side conditions). Checking the trace against those rows tells you which side forgot its half of the conversation.

Limits

Curated coverage of headers you will actually meet — the full registry is far larger. Descriptions are summaries; the RFC text is authoritative for edge cases like header merging and list syntax.