HTTP Status Code Lookup
Searchable directory and troubleshooting guide for RFC 9110 HTTP status codes, headers, and reverse proxy errors.
About HTTP Status Code Lookup
HTTP status codes are standardized three-digit integers issued by web servers in response to client requests. Governed by IETF RFC 9110, RFC 6585, and RFC 4918, they indicate whether an operation succeeded, redirected, or encountered client or server errors.
This workstation provides an interactive reference catalog of over 60 standardized and edge status codes across 1xx Informational, 2xx Success, 3xx Redirection, 4xx Client Error, 5xx Server Error, and proxy codes from Cloudflare and Nginx.
Beyond simple definitions, this engine includes RFC 9111 caching rules, expected response headers (Location, Retry-After, WWW-Authenticate), root cause diagnosis, and incident debugging checklists.
For network debugging, inspect server headers with the HTTP headers inspector or generate API code using the cURL to Fetch converter.
Step-by-step practical guide
- Type any status code (404, 502, 301), keyword (timeout, gateway), or header into the search input.
- Filter response classes using category buttons: 1xx, 2xx, 3xx, 4xx, 5xx, or Edge.
- Click any status card to open the Detail Inspector showing RFC specifications, caching rules, and root causes.
- Review the developer checklist to identify whether failures stem from code, network, or proxy configuration.
- Use Copy cURL to test mock endpoints in your terminal, or export the catalog to JSON or CSV.
Engineering specifications and execution details
Covers status codes from RFC 9110 (HTTP Semantics), RFC 6585, RFC 4918 (WebDAV), RFC 8297, and RFC 8470.
Provides RFC 9111 HTTP caching classifications: cacheable by default, heuristic, or non-cacheable.
Includes edge and proxy codes: Cloudflare (520 through 526) and Nginx 499.
In-memory search engine matches code numbers, names, descriptions, and headers with zero network round trips.
Client-side export generators produce clean RFC-compliant JSON schemas and CSV spreadsheets.
Data privacy and memory isolation
All searches, filtering, and data inspections run completely inside client browser memory.
No queries or diagnostic parameters are transmitted across the network or logged to external servers.
Common production use cases
- Backend and API engineers selecting correct semantic response codes for RESTful web services.
- Site Reliability Engineers (SREs) and DevOps teams diagnosing reverse proxy crashes (502 Bad Gateway, 504 Gateway Timeout).
- Frontend developers interpreting failed fetch and axios requests in browser network consoles.
- SEO professionals evaluating 301 permanent redirects versus 302 temporary redirects and 410 Gone directives.
- Security auditors investigating 401 Unauthorized versus 403 Forbidden permission boundaries.
Edge cases and operational boundaries
- 401 Unauthorized vs 403 Forbidden: 401 signals missing or invalid credentials (must log in). 403 indicates authentication succeeded, but the user lacks access rights.
- 301 Moved Permanently vs 308 Permanent Redirect: Browsers historically transformed 301 POST requests to GET. RFC 9110 specifies 308 to guarantee preservation of the HTTP method and body.
- 400 Bad Request vs 422 Unprocessable Content: 400 denotes structural syntax invalidity. 422 means JSON syntax is valid, but the values fail domain validation rules.
- 502 Bad Gateway vs 504 Gateway Timeout: 502 occurs when an upstream server answers with invalid data or terminates the TCP socket. 504 occurs when the upstream fails to reply before timeout.
Frequently Asked Questions
What is the fundamental difference between 401 Unauthorized and 403 Forbidden?
401 Unauthorized means authentication is missing or invalid, requiring valid credentials. 403 Forbidden means the server recognized the client identity but refuses access due to insufficient privileges.
How do 301 Moved Permanently and 302 Found redirects differ in SEO and browser caching?
A 301 response informs search engines and browsers that the resource has permanently relocated, passing search equity and caching aggressively. A 302 response indicates a temporary diversion without transferring search rankings.
When should modern REST APIs use 307 Temporary Redirect instead of 302?
Early browsers transformed redirected POST requests into GET requests upon receiving a 302 code. RFC 9110 introduced 307 and 308 to strictly guarantee user agents preserve the original HTTP method and body.
What causes a 502 Bad Gateway error and how can developers resolve it?
A 502 occurs when a proxy (Nginx, Cloudflare) receives an invalid or empty response from an upstream backend. Common causes include backend process crashes, pool exhaustion, or internal firewall drops.
What is the difference between 502 Bad Gateway and 504 Gateway Timeout?
A 502 indicates that the upstream server answered with invalid data or closed the socket immediately. A 504 indicates that the reverse proxy waited the full timeout duration without receiving any response.
What causes a 429 Too Many Requests status code?
Status code 429 is returned under RFC 6585 when a client exceeds rate limits. APIs typically include a Retry-After header indicating how many seconds the client must wait before retrying.
What is the difference between 400 Bad Request and 422 Unprocessable Content?
A 400 represents structural syntax failure (malformed JSON). A 422 (RFC 9110 Section 15.5.21) means JSON syntax is valid, but the data fails business validation rules (such as an invalid email).
Which HTTP status codes are cacheable by default under RFC 9111?
Under RFC 9111, status codes 200, 203, 204, 206, 300, 301, 404, 405, 410, 414, and 501 are cacheable by default unless overridden by explicit Cache-Control headers.
Are Cloudflare 520 through 526 and Nginx 499 official HTTP status codes?
No. Codes 520 through 526 are proprietary Cloudflare edge errors for origin connection or SSL issues. Code 499 is an internal Nginx status indicating the client closed the TCP connection before response completion.