What is an HTTP status code?

An HTTP status code is a standardized 3-digit numeric code returned by a web server to indicate the outcome of a client's request (such as a web browser or API consumer) per RFC 9110 specifications.

Why can't client-side browser JavaScript read cross-origin response status codes directly?

Web browsers enforce the Same-Origin Policy (SOP). When making fetch requests across domains without explicit CORS headers (Access-Control-Allow-Origin), responses are opaque (status 0), preventing malicious scripts from inspecting private internal network headers.

What is the difference between a 301 and a 308 redirect?

Both indicate permanent redirects. However, under RFC specifications, a 301 redirect allows browsers to inadvertently switch the request method from POST to GET, whereas a 308 Permanent Redirect strictly forbids changing the HTTP request method.

What is the difference between a 502 Bad Gateway and a 504 Gateway Timeout?

A 502 Bad Gateway means a proxy or load balancer received an invalid or corrupt response from the upstream application server. A 504 Gateway Timeout means the proxy did not receive a response within its configured timeout limit.

HTTP Status Code Reference & Diagnostic Directory

🌐 Live Endpoint Reachability Probe

Test target server network response times. Note: Due to browser Same-Origin Policy (SOP), cross-origin responses without CORS headers are evaluated via network round-trip timing. For detailed header inspection, use the generated curl commands below.

Network Active Latency: 0ms

📖 Searchable HTTP Status Code Reference (RFC 9110)

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1. Understanding HTTP Response Code Architecture

The Hypertext Transfer Protocol (HTTP) utilizes standardized 3-digit response status codes grouped into five distinct semantic classes. These codes allow web clients, search engine crawlers, and API consumers to understand the outcome of an HTTP request without needing to parse the response body text.

Proper status code implementation is critical for both web development and technical SEO. For example, returning a 200 OK response on a page displaying a "Product Not Found" message (known as a Soft 404) confuses search engine crawlers and wastes crawl budget.

2. Standard HTTP Status Code Classes Reference

Class Range	Designation	Primary Semantic Meaning	Typical Example
1xx	Informational	Request received; protocol handshake or preliminary headers continuing	`101 Switching Protocols`, `103 Early Hints`
2xx	Success	The action requested by the client was received, understood, and accepted	`200 OK`, `201 Created`, `204 No Content`
3xx	Redirection	Further action needed; client must request a new URI target	`301 Moved Permanently`, `304 Not Modified`
4xx	Client Error	The request contains invalid syntax, missing auth, or requests non-existent resources	`401 Unauthorized`, `403 Forbidden`, `404 Not Found`
5xx	Server Error	The server failed to fulfill an apparently valid request due to internal failure	`500 Internal Error`, `502 Bad Gateway`, `503 Unavailable`

3. Technical SEO Impact of Common Redirects & Errors

301 vs 302 vs 307: 301 Moved Permanently transfers search ranking equity (PageRank) to the destination URL. 302 Found and 307 Temporary Redirect indicate a temporary change; search engines keep the original URL indexed.
404 vs 410: While 404 Not Found causes crawlers to re-check the URL periodically, 410 Gone explicitly tells search engines that the resource was permanently purged, leading to faster de-indexing.
429 Too Many Requests: Indicates API rate limits or crawler throttling. When returned to search crawlers with a Retry-After header, it prevents search engines from overwhelming your origin server.

4. Client-Side Privacy Guarantee

All reference lookups, filtering routines, and diagnostic probe calculations run locally within your web browser sandbox using JavaScript. No target URLs, internal server paths, or diagnostic investigations are ever uploaded, recorded, or logged on MicroToolStack servers.

5. Frequently Asked Questions

What is the difference between 401 Unauthorized and 403 Forbidden?

A 401 Unauthorized status indicates that authentication credentials are missing or invalid (the user is not authenticated). A 403 Forbidden status means the server knows who the user is, but they lack authorization to access the specific resource.

Why does 502 Bad Gateway occur behind Cloudflare or Nginx?

A 502 Bad Gateway occurs when a reverse proxy (like Nginx, HAProxy, or Cloudflare) connects to an upstream application server (such as Node.js, PHP-FPM, or Python Gunicorn) and receives an invalid response, crash, or dropped socket.

How can I inspect full HTTP response headers on my computer?

Use your operating system terminal to execute curl -I https://example.com. This sends an HTTP HEAD request and prints all response headers directly without downloading the body.

HTTP Status Code Directory & Diagnostic Guide