HTTP status codes are one of the most fundamental concepts in web development. Every time a browser sends a request to a server, the server responds with a three-digit status code that tells the client whether the request was successful, needs redirection, or encountered an error. This online HTTP status code lookup tool provides a complete list of status codes from 1xx to 5xx, covering all official codes defined by RFC standards as well as common extensions. Whether you are debugging an API, troubleshooting website access issues, or learning the HTTP protocol, this tool helps you quickly find the information you need. The tool supports searching by code number, English name, or description, and you can filter by the five major categories: 1xx (Informational), 2xx (Success), 3xx (Redirection), 4xx (Client Error), and 5xx (Server Error). Each status code includes a concise explanation to help developers quickly understand error causes and find solutions. Proper use of status codes is crucial for building high-quality web applications. In RESTful API design, returning appropriate status codes makes APIs more semantic and easier to understand. For example, successful data retrieval should return 200, creating a new resource should return 201, successful deletion can return 204, parameter errors should return 400, unauthorized access returns 401, forbidden access returns 403, and resource not found returns 404. Understanding these status codes and their use cases is an essential skill for every web developer.
Understanding the Five Categories of HTTP Status Codes
HTTP status codes are divided into five categories based on the first digit. 1xx (Informational) indicates the server has received the request and the client should continue, such as 100 Continue meaning the client can send the request body, and 101 Switching Protocols used for WebSocket upgrades. 2xx (Success) indicates the request was successfully received and processed, with 200 OK being the most common success response, 201 Created indicating resource creation, and 204 No Content indicating success with no response body. 3xx (Redirection) indicates further action is needed, with 301 permanent and 302 temporary redirects being crucial for SEO, and 304 Not Modified indicating cached content can be used. 4xx (Client Error) indicates request problems: 400 for malformed requests, 401 for unauthenticated, 403 for forbidden, 404 for not found, and 429 for rate limiting. 5xx (Server Error) indicates server-side failures: 500 internal error, 502 bad gateway, 503 service unavailable, and 504 gateway timeout.
Most Common Status Codes and Solutions
- 200 OK: Request successful - the ideal response status
- 301 Moved Permanently: Resource permanently moved, used for site redesigns or URL normalization
- 302 Found: Temporary redirect, used during maintenance or A/B testing
- 304 Not Modified: Resource unchanged, browser should use local cache
- 400 Bad Request: Request parameter format error, check JSON format or required fields
- 401 Unauthorized: No authentication provided or token expired, requires re-login
- 403 Forbidden: Authenticated but no permission, check user roles or API permissions
- 404 Not Found: Resource does not exist, verify URL path is correct
- 429 Too Many Requests: Rate limit exceeded, implement throttling or retry logic
- 500 Internal Server Error: Server error, check server logs to identify the issue
- 502 Bad Gateway: Upstream server not responding, check if backend service is running
- 503 Service Unavailable: Service temporarily unavailable, possibly maintenance or overload, retry later
RESTful API Status Code Best Practices
When designing RESTful APIs, proper status codes make interfaces more semantic and understandable. GET success returns 200, POST resource creation returns 201 with the new resource URL in the Location header, PUT/PATCH updates return 200 or 204, and DELETE success returns 204. For error responses, validation failures return 400 with specific error fields in the body, authentication failures return 401, permission denied returns 403, resource not found returns 404, resource conflicts (like duplicate unique keys) return 409, and server exceptions return 500. Avoid returning 200 for all responses and using a code field in the body to indicate errors - this approach violates HTTP semantics and makes it harder for HTTP clients and monitoring tools to properly handle responses.
HTTP Status Codes and SEO Optimization
HTTP status codes significantly impact website SEO. 301 permanent redirects transfer link equity from the original URL to the new one, making them ideal for site redesigns, domain changes, or URL restructuring. 302 temporary redirects do not transfer link equity and are only suitable for temporary situations. Too many 404 pages can signal quality issues to search engines, so regularly check for and fix broken links. Design user-friendly 404 pages that guide visitors back to useful content. The 503 status code tells search engines the site is temporarily unavailable, which helps avoid penalties during maintenance windows. Properly configured status codes help search engines better understand your site structure, improving indexing efficiency and rankings.
Common Status Code Troubleshooting Guide
When encountering 4xx errors, first verify the request is correct: check URL spelling, request method, required headers (Content-Type, Authorization), and request body format. The key difference between 401 and 403: 401 means unauthenticated (no credentials or invalid credentials), while 403 means authenticated but unauthorized (identity confirmed but access denied). For 5xx errors, the problem is server-side: 502 typically means the reverse proxy (like Nginx) cannot connect to the backend service - check if the backend is running and ports are correct. 503 may indicate server overload or active maintenance. 504 means the gateway timed out waiting for the backend - check if backend processing is too slow. Enable detailed error logging in development environments and set up proper error monitoring and alerting in production.
FAQ
Q: What is the difference between 301 and 302 redirects? How do I choose?
A: 301 is a permanent redirect, telling browsers and search engines the resource has permanently moved. Browsers cache this redirect, and search engines transfer link equity to the new URL. 302 is a temporary redirect, indicating the resource is temporarily elsewhere - browsers don't cache it, and search engines keep the original URL indexed. Choose 301 for site redesigns, domain changes, or permanent URL changes. Choose 302 for temporary maintenance, A/B testing, or login redirects. Modern browsers also support 307 (temporary) and 308 (permanent) redirects, which strictly preserve the original request method, while 301/302 may change POST to GET.
Q: Why am I getting 403 Forbidden and how do I fix it?
A: 403 means the server understood the request but refuses to execute it. Common causes include: 1) Not logged in or session expired - re-authenticate; 2) Authenticated but user role lacks permission for the resource; 3) Invalid API key or insufficient key permissions; 4) IP address blocked or restricted by the server; 5) Request triggered WAF (Web Application Firewall) security rules; 6) Directory access forbidden (like accessing a server directory directly); 7) Incorrect file permissions. Troubleshooting steps: verify authentication credentials, confirm user permission settings, check server access logs, review firewall rules, and verify file and directory permissions.
Q: What is the difference between 502 Bad Gateway and 503 Service Unavailable?
A: Although both are server errors, they have different causes. 502 Bad Gateway occurs when a proxy server (like Nginx or a load balancer) receives an invalid response from the upstream server - this usually means the backend application server has crashed, is not running, or returned an invalid response. Check backend service status immediately. 503 Service Unavailable indicates the server temporarily cannot handle requests due to overload, maintenance, or resource exhaustion - this is typically temporary and may resolve with retry. From an operations perspective: 502 requires immediate backend service health checks; 503 can include a Retry-After header to inform clients when to retry, providing a more graceful overload handling approach.
Q: How should I handle 404 pages for better UX and SEO?
A: Best practices for 404 pages: 1) Design a friendly 404 page with your logo, search box, popular content links, and a home button instead of just an error message; 2) Use server-side 404 status codes - don't return 200 and display 'not found' on the page; 3) Regularly check for and fix broken links, or set up 301 redirects; 4) Monitor 404 errors in Google Search Console and similar tools; 5) For deleted pages with external backlinks, set up 301 redirects to related content; 6) Ensure custom 404 pages return the correct 404 status code.
Q: When should I return 401 instead of 403?
A: 401 Unauthorized and 403 Forbidden are often confused. 401 means 'unauthenticated' - the request didn't include valid credentials, and the client should re-authenticate (log in, refresh token). 401 responses should include a WWW-Authenticate header indicating the authentication method. 403 means 'authenticated but unauthorized' - the server has confirmed the client's identity, but that user doesn't have permission for the requested resource. Re-authenticating won't help; the user needs elevated permissions from an administrator. Simple memory trick: 401 is 'Who are you?' (need to log in), 403 is 'You can't' (logged in but no permission).