HTTP Headers Explained: Security & Performance (2025)

HTTP headers are the invisible metadata that travel with every request and response. They control security, caching, content negotiation, authentication, and performance. Getting them right is the difference between a fast, secure website and one that's vulnerable to common attacks. This guide explains the headers that matter most — organized by category, with practical examples you can deploy today. Inspecting Headers Before optimizing, you need to see what's there. Use the DevPlaybook HTTP Headers Inspector to check any URL instantly, or use the command line: # View all response headers curl -I https://example.com # Include headers + body curl -i https://example.com # Specific header (case-insensitive) curl -sI https://example.com | grep -i cache-control # Full verbose output (request + response) curl -v https://example.com 2>&1 | grep -E "^[<>]" In the browser: DevTools → Network tab → click any request → Headers tab. Security Headers Security headers protect your users