HTTP Header Lookup
Look up an HTTP header, or paste response headers to have each one explained.
Explained
Examples
Headers
Request
Response
Either side
Questions
How do I see a page's response headers?
In the browser, open the developer tools, go to Network, click a request and look at Response Headers. From a terminal, run curl -I followed by the address. Paste either here to have each header explained.
Are header names case-sensitive?
No. Content-Type and content-type are the same header. HTTP/2 and HTTP/3 send them all in lower case, which is why devtools often shows them that way.
What are X- headers?
Custom headers, usually set by one server, CDN or framework, like X-Served-By or X-Request-Id. The X- prefix is no longer recommended for new headers, but many remain.
Where are these headers defined?
Most in RFC 9110 (HTTP semantics) and RFC 9111 (caching), with CORS in the WHATWG Fetch standard and cookies in RFC 6265. Each page names its source.
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