Understanding HTTP
A comprehensive guide to HTTP — what it is, why it was designed the way it was, and how HTTP/1.1 works at the wire level. Start here before diving into the individual test categories.
What is HTTP?
Application-layer request/response protocol, client-server model, stateless design, and core design goals.
Message Syntax
Request and response structure, methods (GET, POST, PUT...), status codes (1xx–5xx), and the request-line grammar.
Headers
Header structure, common request and response headers, the Host header, and why it's the only required header.
Connections
Persistent connections, keep-alive, pipelining, head-of-line blocking, Upgrade, and 100 Continue.
Body and Framing
Content-Length, chunked transfer encoding, trailers, and why CL+TE conflicts cause request smuggling.
Caching and Negotiation
Content negotiation with Accept headers, Cache-Control, ETags, conditional requests, and Vary.
History and Future
HTTP/0.9 to HTTP/3, the current IETF work, alternatives to HTTP, and learning resources.
Http11Probe — HTTP/1.1 compliance & smuggling testerSource on GitHub