Topic
Proxies
Understand how forward proxies, reverse proxies, and CDNs communicate request context, caching, and upstream behavior.
- HTTP Status Codes
203 Non-Authoritative Information
203 means a proxy modified the origin response before passing it on. Learn when it is sent, how it differs from 200, caching rules and what clients should do.
- HTTP Status Codes
407 Proxy Authentication Required: Fix It
407 means your proxy wants credentials before forwarding the request. Fix it in curl, npm, pip, git and browsers, and learn how Proxy-Authenticate works.
- Comparisons
502 vs 503 vs 504: Which Gateway Error Is It
502, 503 and 504 side by side: who generated each, what the upstream did, the nginx error-log line that proves it, and Cloudflare 52x and ALB equivalents.
- HTTP Status Codes
508 Loop Detected: Proxy and Redirect Loops
508 Loop Detected means the server found an infinite loop while processing a request. Learn the WebDAV origin, proxy loops, CDN-Loop and how to find the cycle.
- HTTP Status Codes
511 Network Authentication Required: Captive Portals
511 is returned by a network gateway, such as hotel or airport Wi-Fi, that requires login before granting internet access. Learn how apps should handle it.
- HTTP Status Codes
521 Web Server Is Down
Cloudflare-specific status code indicating the origin server refused the connection. Learn about this proxy error and how to troubleshoot it.
- HTTP Headers
Forwarded
Learn how the Forwarded header preserves original client information (IP, protocol, host) that would otherwise be lost when requests pass through proxies.
- HTTP Status Codes
HTTP 502 Bad Gateway: nginx, ALB and Cloudflare Fixes
Fix 502 Bad Gateway: decode nginx error-log lines, php-fpm sockets, ALB keep-alive mismatches and Cloudflare 502 vs 52x, with curl checks.
- HTTP Methods
HTTP CONNECT Method: Proxy Tunnels Explained
HTTP CONNECT opens a TCP tunnel through a proxy so HTTPS can pass untouched. See the exact request and 200 response, 407 proxy auth, and extended CONNECT.
- Debug Guides
nginx 413 Request Entity Too Large: client_max_body_size
Fix nginx 413 Request Entity Too Large: raise client_max_body_size, then check Cloudflare limits, ingress-nginx proxy-body-size and PHP upload limits.
- Debug Guides
nginx 502 Bad Gateway: Causes and Fixes by Error Log
Fix nginx 502 Bad Gateway by matching the error log: connection refused, prematurely closed connection, php-fpm socket permissions, too big header, keepalive.
- Debug Guides
nginx 504 Gateway Timeout: Fix Upstream Timed Out (110)
Fix nginx 504 Gateway Time-out and 'upstream timed out (110)': proxy_read_timeout, fastcgi_read_timeout, ALB idle timeout, and Cloudflare 524 compared.
- HTTP Headers
Proxy-Authenticate Header
Learn how the Proxy-Authenticate header challenges clients for credentials when accessing resources through a proxy. Understand proxy authentication schemes.
- HTTP Headers
Proxy-Authorization Header
Learn how Proxy-Authorization provides credentials to access resources through a proxy server. Understand proxy authentication schemes and security.
- Glossary
Reverse Proxy
A reverse proxy sits in front of origin servers and forwards client requests to them. Learn the headers it adds, nginx proxy_pass pitfalls and 502/504 causes.
- HTTP Headers
Via Header
Learn how the Via header tracks the path of HTTP requests through proxies and gateways. Debug routing issues and understand your network infrastructure.
- HTTP Headers
X-Forwarded-For
X-Forwarded-For carries the client IP through proxies and load balancers, but clients can forge it. Trust it safely in nginx, Express, Cloudflare and AWS.
- HTTP Headers
X-Forwarded-Proto
Learn how the X-Forwarded-Proto header identifies the original protocol (HTTP/HTTPS) used by clients connecting through proxies or load balancers.
More topics
- 63 pages
Security
Study HTTP security controls for transport, cookies, browser isolation, authentication, content policy, and response hardening.
- 24 pages
Performance
Improve HTTP delivery with caching, compression, connection reuse, timing signals, and response metadata.
- 22 pages
Caching
Learn how browsers and intermediaries store, validate, and reuse HTTP responses with cache directives, validators, and conditional requests.
- 22 pages
Cookies
Understand cookie storage, scope, transport, expiration, and security across HTTP headers, attributes, and session flows.