HTTP Redirection

Summary

HTTP redirection is a mechanism that instructs a client (typically a web browser) to automatically request a different URL than the one originally requested. It is a fundamental part of the HTTP protocol, used for tasks such as moving content, handling URL normalization, load balancing, and enforcing secure connections.

Key Points

• Redirection is signaled by HTTP status codes in the 3xx range, most commonly 301 (Moved Permanently) and 302 (Found).
• The target URL is provided in the Location header of the server's response.
• Clients follow redirections automatically, often with a limit on the number of hops to prevent infinite loops.
• Redirection can be transparent to the user but may affect SEO, latency, and caching behavior.
• Different status codes indicate different semantics (permanent vs. temporary, cacheability, method preservation).

Concepts

• 301 Moved Permanently: Indicates the resource has a new permanent URL. Browsers and search engines update their records. The request method is usually changed to GET (for POST requests).
• 302 Found (or 307 Temporary Redirect): Indicates a temporary move. Clients should continue using the original URL for future requests. 307 preserves the original HTTP method; 302 may change it to GET.
• 303 See Other: Forces a GET request to the new URL, often used after form submission to avoid duplicate submissions.
• 308 Permanent Redirect: Like 301 but preserves the HTTP method (e.g., POST stays POST).
• Location header: The response header that contains the target URL for redirection.
• Redirect chain: A series of redirections (e.g., A → B → C). Too long chains degrade performance.
• SEO impact: 301 redirects transfer link equity; 302 does not. Improper redirects can cause ranking loss.

Details

HTTP redirection works via a request–response exchange:

1. The client makes an HTTP request to a URL.
2. The server responds with a 3xx status code and a Location header pointing to the new URL.
3. The client (browser or HTTP library) automatically issues a new request to the URL in the Location header.
4. The process may repeat if the new URL also returns a redirection.

Common use cases include:

• URL migration: When a website changes its domain or URL structure, 301 redirects preserve bookmarks and search rankings.
• Protocol enforcement: Redirecting http:// to https:// (often with a 301 or 302).
• Route canonicalization: Redirecting example.com to www.example.com or vice versa.
• Temporary maintenance: Using 302 to direct users to a maintenance page.
• Load balancing: Some load balancers return a 302 to direct clients to a specific backend.
• A/B testing: Redirecting a percentage of traffic to an alternative version.

Best practices:

• Use permanent (301) redirects only when truly permanent, as browsers cache them aggressively.
• Avoid redirect chains (more than 2–3 hops) to minimize latency.
• Ensure the Location URL is absolute and properly encoded.
• For API clients, respect the method‑preserving codes (307, 308) to avoid breaking POST/PUT semantics.

The diagram shows a simple permanent redirect: the client follows the Location header and retrieves the new resource.