HTTP Status Codes Lookup
High Frequency Status Codes
Search Results (77)
| Code | Group | Name | Description |
|---|---|---|---|
| 100 | 1xx | Continue | 已收到请求头，客户端应继续发送请求体 |
| 101 | 1xx | Switching Protocols | 服务器正按 Upgrade 切换协议（如 WebSocket） |
| 102 | 1xx | Processing | 已接受请求，处理尚未结束（WebDAV） |
| 103 | 1xx | Early Hints | 在最终响应前返回 Link 等提示以预加载资源 |
| 104 | 1xx | Upload Resumption Supported | 表明支持可恢复上传（IANA 临时注册，详见 httpbis 可恢复上传草案） |
| 200 | 2xx | OK | 请求成功 |
| 201 | 2xx | Created | 已创建资源 |
| 202 | 2xx | Accepted | 已接受，尚未处理完成（异步任务常见） |
| 203 | 2xx | Non-Authoritative Information | 成功，表示可能来自代理/缓存 |
| 204 | 2xx | No Content | 成功，无响应体 |
| 205 | 2xx | Reset Content | 成功，客户端应重置文档或表单视图 |
| 206 | 2xx | Partial Content | 成功返回部分内容（Range） |
| 207 | 2xx | Multi-Status | 多状态 XML响应（WebDAV） |
| 208 | 2xx | Already Reported | DAV 绑定成员已在先前响应中列出（WebDAV） |
| 226 | 2xx | IM Used | 已应用实例操作（Delta 编码，RFC 3229） |
| 300 | 3xx | Multiple Choices | 多种表示可选，需选择或重定向 |
| 301 | 3xx | Moved Permanently | 资源永久迁移至新 URI |
| 302 | 3xx | Found | 资源临时位于其他 URI |
| 303 | 3xx | See Other | 应使用 GET 访问另一 URI 获取结果 |
| 304 | 3xx | Not Modified | 缓存仍有效 |
| 305 | 3xx | Use Proxy | 须经由代理（已弃用，不建议实现） |
| 306 | 3xx | (Unused) | 历史占位，HTTP/1.1 起未使用 |
| 307 | 3xx | Temporary Redirect | 临时重定向，方法与正文不变 |
| 308 | 3xx | Permanent Redirect | 永久重定向，方法与正文不变 |
| 400 | 4xx | Bad Request | 语法无效，无法解析 |
| 401 | 4xx | Unauthorized | 需身份验证（常配合 WWW-Authenticate） |
| 402 | 4xx | Payment Required | 预留，极少实现 |
| 403 | 4xx | Forbidden | 拒绝访问（与401 不同：未必缺凭证） |
| 404 | 4xx | Not Found | 未找到资源 |
| 405 | 4xx | Method Not Allowed | 该 URI 不支持此 HTTP 方法 |
| 406 | 4xx | Not Acceptable | 无法生成 Accept 可接受的表示 |
| 407 | 4xx | Proxy Authentication Required | 需先通过代理认证 |
| 408 | 4xx | Request Timeout | 等待请求超时 |
| 409 | 4xx | Conflict | 与资源当前状态冲突 |
| 410 | 4xx | Gone | 资源曾存在且已永久移除 |
| 411 | 4xx | Length Required | 缺少有效 Content-Length 等 |
| 412 | 4xx | Precondition Failed | 前置条件（如 If-*）不满足 |
| 413 | 4xx | Content Too Large | 请求体过大（RFC 9110 用语） |
| 414 | 4xx | URI Too Long | URI 过长 |
| 415 | 4xx | Unsupported Media Type | 不支持的媒体类型 |
| 416 | 4xx | Range Not Satisfiable | 无法满足 Range |
| 417 | 4xx | Expectation Failed | 无法满足 Expect 头 |
| 418 | 4xx | (Unused) | RFC 9110 标记未使用；典故为 RFC 2324「I'm a teapot」 |
| 421 | 4xx | Misdirected Request | 请求未指向能响应的服务器/连接（如 HTTP/2） |
| 422 | 4xx | Unprocessable Content | 语义可理解但无法处理（校验失败等） |
| 423 | 4xx | Locked | 资源被锁定（WebDAV） |
| 424 | 4xx | Failed Dependency | 依赖请求失败（WebDAV） |
| 425 | 4xx | Too Early | 拒绝可能重放的过早请求 |
| 426 | 4xx | Upgrade Required | 需升级到 Upgrade 指定的协议 |
| 428 | 4xx | Precondition Required | 源站要求条件请求以防冲突 |
| 429 | 4xx | Too Many Requests | 请求过于频繁，应限速 |
| 431 | 4xx | Request Header Fields Too Large | 请求头整体或单项过大 |
| 444 | vendor | No Response (nginx) | Nginx 直接关连接不返回状态行（常见于限流/屏蔽） |
| 451 | 4xx | Unavailable For Legal Reasons | 因法律原因不可提供 |
| 495 | vendor | SSL Certificate Error (nginx) | Nginx：客户端证书校验失败 |
| 496 | vendor | SSL Certificate Required (nginx) | Nginx：需要客户端证书 |
| 497 | vendor | HTTP to HTTPS Port (nginx) | Nginx：向 HTTPS 端口发送明文 HTTP |
| 499 | vendor | Client Closed Request (nginx) | 客户端在响应完成前关闭连接 |
| 500 | 5xx | Internal Server Error | 服务器内部错误 |
| 501 | 5xx | Not Implemented | 不支持该方法或能力 |
| 502 | 5xx | Bad Gateway | 网关从上游收到无效响应 |
| 503 | 5xx | Service Unavailable | 过载或维护，暂不可用 |
| 504 | 5xx | Gateway Timeout | 网关等待上游超时 |
| 505 | 5xx | HTTP Version Not Supported | 不支持请求中的 HTTP 版本 |
| 506 | 5xx | Variant Also Negotiates | 透明协商配置错误 |
| 507 | 5xx | Insufficient Storage | 存储空间不足（WebDAV） |
| 508 | 5xx | Loop Detected | 检测到循环（WebDAV） |
| 510 | 5xx | Not Extended | 需扩展（RFC 2774 实验状态，IANA 标为 OBSOLETED，可忽略） |
| 511 | 5xx | Network Authentication Required | 需在网络层认证（如强制门户） |
| 520 | vendor | Unknown Error (Cloudflare) | 源站返回异常或空响应 |
| 521 | vendor | Web Server Is Down (Cloudflare) | 无法与源站建立 TCP |
| 522 | vendor | Connection Timed Out (Cloudflare) | 与源站握手或连接超时 |
| 523 | vendor | Origin Is Unreachable (Cloudflare) | 无法路由到源站 |
| 524 | vendor | A Timeout Occurred (Cloudflare) | 源站已连接但未及时返回完整响应 |
| 525 | vendor | SSL Handshake Failed (Cloudflare) | 与源站 SSL 握手失败 |
| 526 | vendor | Invalid SSL Certificate (Cloudflare) | 源站证书无效 |
| 527 | vendor | Railgun Error (Cloudflare) | Railgun 边缘到源监听错误（Cloudflare 专用） |
Free HTTP status code reference tool with complete coverage of 1xx Informational/2xx Success/3xx Redirection/4xx Client Error/5xx Server Error categories plus vendor extensions. Supports category filtering and keyword search, essential for development and debugging.
Related
What are HTTP Status Codes?
HTTP Status Codes are three-digit numeric codes returned by servers when responding to HTTP requests, indicating the result of processing — did it succeed or fail? If it failed, was it a client problem or a server problem? Is a redirect needed? This is one of the most fundamental and important diagnostic pieces of information in web development and operations.
Status codes are divided into 5 categories by their first digit: **1xx Informational** (request received, continuing process, less common in practice); **2xx Success** (request processed successfully, most common is 200 OK); **3xx Redirection** (further client action needed, such as 301 permanent redirect, 302 temporary redirect, 304 cache hit); **4xx Client Error** (problem with request, such as 400 bad parameters, 401 not logged in, 403 no permission, 404 not found, 429 rate limited); **5xx Server Error** (server failed to process, such as 500 internal error, 502 bad gateway, 503 unavailable, 504 timeout).
**Why distinguish between 4xx and 5xx?** This is the first priority judgment in troubleshooting: seeing 4xx you should first check your own request — is the URL correct, are parameters complete, did you include a Token, do you have sufficient permissions, are you making requests too frequently; seeing 5xx you need to look at server logs — are backend processes healthy, is the database connected, is configuration correct, is the server overloaded. Getting this direction wrong wastes significant debugging time.
**The differences between redirect status codes are a high-frequency exam topic**: 301 is permanent redirect (SEO authority transfer, browser cached), 302 is temporary redirect (no authority transfer), 307/308 strictly preserve request method (POST won't be changed to GET). In practice, use 301 for HTTPS redirects and domain migrations, 302 for temporary maintenance, 303 for redirecting after POST form submission (PRG pattern).
This tool **completely includes** all standard HTTP status codes (including easily confused ones like 303/307/308, 405/409/422/429 etc.) as well as commonly used vendor extensions from Cloudflare/Nginx (520-527, 499). It supports category filtering, keyword search, and quick-access cards — an essential everyday reference tool for development, API debugging, and operations troubleshooting.
Use Cases
- Web/API development debugging: Quickly look up status code meanings when APIs report errors to determine troubleshooting direction (frontend params or backend bug)
- API integration troubleshooting: Distinguish 401 auth failure, 403 insufficient permissions, 404 wrong address, 500 server error
- Operations troubleshooting: Quickly locate causes and solutions when encountering 502/503/504/522/524 errors
- SEO work: Verify 301/302 redirect types are correct, check 404 error page handling is proper
- HTTP protocol learning: Beginners systematically learn HTTP status code categories and each code's purpose
- Interview prep: Frontend/backend interviews often test 301 vs 302, 401 vs 403, 502 vs 504 differences
- Log analysis: Understand status code meanings when analyzing Nginx/Apache/CDN access logs
How to Use
- Simply type a status code number (e.g., 404) or keyword (e.g., redirect, forbidden, timeout) in the search box
- Or click category buttons (1xx/2xx/3xx/4xx/5xx/vendor) to browse by category
- Click quick-access status code cards to quickly filter for that code
- View the status code's name, category color, and detailed description
- Click the copy button to copy current filtered summary for troubleshooting records
Features
- Comprehensive coverage: Standard 1xx-5xx status codes + Cloudflare/Nginx vendor extensions (5xx vendor), all in one place
- Quick-access cards: 12 most commonly used codes (200/301/302/304/400/401/403/404/429/500/502/503) — click cards to view details directly
- Category filtering: One-click filtering by 1xx/2xx/3xx/4xx/5xx/vendor categories to quickly locate problem types
- Keyword search: Supports fuzzy search by status code number, English name, or description to quickly locate target codes
- Color coding: 2xx green (success), 3xx/4xx yellow (caution/client issues), 5xx red (server issues) for intuitive visual judgment
- Desktop table/mobile cards: Full table view on desktop, responsive card view on mobile for great experience on all devices
- One-click copy of filtered summary: Copy current filter conditions and result count for tickets/documentation
FAQ
What are HTTP status codes and what are they used for?
HTTP status codes are three-digit numeric codes returned by servers in response to HTTP requests, indicating whether the request succeeded, failed, or needs further action. They fall into 5 categories: 1xx Informational, 2xx Success, 3xx Redirection, 4xx Client Error, 5xx Server Error. During development and debugging, status codes quickly point you in the right direction: 2xx success means no action needed; 4xx means check request parameters/authentication/permissions/rate limits; 5xx means check server logs.
What exactly is the difference between 301 and 302 redirects?
Core difference: 301 is a **permanent redirect** — browsers/search engines cache the new address and SEO authority transfers to the new URL; 302 is a **temporary redirect** — not cached and no authority transfer. There's also an important method change difference: historically 301/302 would change POST to GET (loose behavior), while 307/308 strictly preserve the original request method. Use 301 for HTTPS redirects, domain migrations, permanent URL changes; use 302 or 307 for temporary maintenance redirects and campaign pages.
What's the difference between 401 and 403?
401 Unauthorized means 'not authenticated' — the server doesn't know who you are, you need to log in/provide a Token first (usually accompanied by WWW-Authenticate header); 403 Forbidden means 'authenticated but no permission' — the server knows who you are, but you don't have access to this resource. In plain terms: 401 is 'you didn't bring your access card', 403 is 'you have an access card but you're not allowed in this room'.
What's the difference between 502, 503, and 504?
All are server errors but with different causes: 502 Bad Gateway means the reverse proxy received an **invalid response** from upstream (backend crash/protocol error); 503 Service Unavailable means the server is **temporarily unavailable** (overloaded/maintenance/service not running); 504 Gateway Timeout means the proxy server **timed out waiting for upstream** (backend processing too slow/database slow queries). When troubleshooting: for 502 check if backend processes are alive and error logs; for 503 check service status/rate limits/maintenance pages; for 504 check backend performance/slow queries/timeout configuration.
When should I use 200 OK vs 204 No Content?
200 OK is the standard success response and can return a response body (e.g., query results, created resource data); 204 No Content indicates success but **returns no response body**, commonly used for successful DELETE operations, PUT/POST updates where no data needs to be returned. A 204 response must not contain a body.
How to handle 429 rate limit status code?
429 Too Many Requests means the client has exceeded the server's request frequency limit. The response usually includes a Retry-After header telling how long to wait before retrying. Clients should implement Exponential Backoff: wait 1 second on first retry, 2 seconds on second, 4 seconds on third... Also check X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset headers to understand rate limit quotas.
What are vendor extension status codes, like Cloudflare's 520-527?
Standard HTTP status codes are only defined in the 100-599 range and mostly registered with IANA, but CDN/proxy server vendors define custom extensions: Cloudflare defines the 520-527 series for origin connection issues (521 origin refused, 522 connection timeout, 524 response timeout, etc.); Nginx defines 499 (client disconnected). These codes aren't standard HTTP codes but appear frequently in actual operations and troubleshooting, and this tool includes commonly used vendor extension codes.
Glossary
- HTTP
- HyperText Transfer Protocol, the foundational protocol for World Wide Web data communication, based on TCP/IP, using a request-response model. Current mainstream versions are HTTP/1.1 and HTTP/2, with HTTP/3 based on QUIC.
- Idempotence
- Executing the same request multiple times has the same effect as executing it once. GET/HEAD/PUT/DELETE/OPTIONS/TRACE are idempotent; POST is not idempotent (multiple submissions create multiple resources). One important reason 307/308 preserve method is to maintain idempotence.
- CORS (Cross-Origin Resource Sharing)
- Cross-Origin Resource Sharing, a mechanism for cross-origin access under the browser's same-origin policy. CORS preflight requests (OPTIONS) may return 405 Method Not Allowed or CORS-related errors, one of the most common issues in frontend development.
- PRG Pattern (Post/Redirect/Get)
- Best practice for web form submission: After POSTing a form, return a 303 See Other redirect to the result page, preventing duplicate submissions when users refresh. This is the most typical use case for 303.
- Retry-After
- HTTP response header used with 503 Service Unavailable or 429 Too Many Requests to tell clients how many seconds to wait before retrying the request.
- WWW-Authenticate
- HTTP response header used with 401 Unauthorized to tell clients which authentication scheme to use (Basic/Bearer/Digest etc.) and the authentication realm.
- ETag / If-None-Match
- Cache validation mechanism. The server returns an ETag identifying the resource version; clients send the If-None-Match header on subsequent requests, and if the resource hasn't changed the server returns 304 Not Modified (no body, saving bandwidth).
- Reverse Proxy
- A proxy server (such as Nginx, Cloudflare, HAProxy) situated between clients and backend servers, receiving client requests and forwarding them to backends, then returning backend responses to clients. 502/503/504 errors all occur between the reverse proxy and backend.
Overview of Five HTTP Status Code Categories
| Class | Meaning | Purpose | Common Codes |
|---|---|---|---|
1xx | Informational | Request received, continuing process (intermediate protocol state) | 100 Continue, 101 Switching Protocols |
2xx | Success | Request successfully received, understood, and accepted by server | 200 OK, 201 Created, 204 No Content, 206 Partial Content |
3xx | Redirection | Further action needed by client to complete request | 301 Moved Permanently, 302 Found, 304 Not Modified, 307/308 |
4xx | Client Error | Request has syntax error or cannot be fulfilled, client is at fault | 400 Bad Request, 401 Unauthorized, 403 Forbidden, 404 Not Found, 429 Too Many Requests |
5xx | Server Error | Server failed to fulfill an apparently valid request, server is at fault | 500 Internal Server Error, 502 Bad Gateway, 503 Service Unavailable, 504 Gateway Timeout |
Comparison of Common Redirection Status Codes (301/302/303/307/308)
| Code | Name | Permanent | Method Change | Cacheable | Typical Use Case |
|---|---|---|---|---|---|
301 | Moved Permanently | ✅ Permanent | POST may change to GET (historical compat) | ✅ Cached by default | Site redesign, domain migration, HTTP→HTTPS redirect, deprecated URLs |
302 | Found | ❌ Temporary | POST may change to GET (historical compat) | ❌ Not cached by default | Temporary maintenance redirect, campaign pages, A/B testing (non-persistent) |
303 | See Other | ❌ Temporary | POST→GET (explicit change) | ❌ Not cached by default | Redirect to result page after POST form submission (PRG pattern) |
307 | Temporary Redirect | ❌ Temporary | Strictly preserves original method | ❌ Not cached by default | Temporary redirect that strictly preserves request method and body |
308 | Permanent Redirect | ✅ Permanent | Strictly preserves original method | ✅ Cached by default | Permanent redirect that strictly preserves POST from being changed to GET |
Common Client Error Status Code Troubleshooting Reference
| Code | Meaning | Common Causes | Troubleshooting Direction |
|---|---|---|---|
400 | Bad Request | Request syntax error/missing parameters/JSON format error/headers too large | Check request parameters, Content-Type, URL encoding |
401 | Unauthorized | Not authenticated/Token expired/missing Authorization header | Check login status, Token expiration, Bearer header format |
403 | Forbidden | Authenticated but no permission/IP blocked/CSRF Token invalid/WAF blocked | Check user permissions, IP whitelist, WAF rules, CSRF Token |
404 | Not Found | URL doesn't exist/resource deleted/routing misconfiguration | Check URL spelling, routing configuration, whether resource was deleted |
405 | Method Not Allowed | Wrong request method (e.g., using POST on GET-only endpoint) | Check HTTP methods (GET/POST/PUT/DELETE etc.), CORS preflight |
408 | Request Timeout | Client sending request too slow/network lag/server timeout too short | Check network connection, increase timeout, upload in batches |
409 | Conflict | Resource conflict (e.g., duplicate creation, version conflict, concurrent modification) | Check for duplicate submissions, use optimistic locking/idempotency keys |
413 | Payload Too Large | Upload file/request body exceeds server limit | Check nginx client_max_body_size, compress files, use chunked upload |
422 | Unprocessable Entity | Request format correct but semantic error (e.g., field validation failure) | RESTful API parameter validation, check required fields, data types, formats |
429 | Too Many Requests | Rate limit triggered/request frequency too high | Reduce request frequency, add retry backoff, apply for higher quota |
Common Server Errors and Cloudflare Extension Codes
| Code | Meaning | Common Causes |
|---|---|---|
500 | Internal Server Error | Server code exception/uncaught error/database connection failure/configuration error |
501 | Not Implemented | Server doesn't support requested functionality (e.g., unsupported HTTP method) |
502 | Bad Gateway | Reverse proxy (nginx/CDN) received invalid response from upstream/backend process crashed/PHP-FPM not running |
503 | Service Unavailable | Server overloaded/under maintenance/backend service not running/connection pool exhausted |
504 | Gateway Timeout | Reverse proxy timed out waiting for upstream response/backend processing too slow/database slow queries |
520 | Web Server Returned Unknown Error (CF) | Cloudflare: Origin returned unknown/abnormal response (protocol error/empty response/no status code) |
521 | Web Server Is Down (CF) | Cloudflare: Origin server refused connection (origin down/firewall blocking CF IPs) |
522 | Connection Timed Out (CF) | Cloudflare: Cannot establish TCP connection to origin (origin firewall/security groups/iptables) |
523 | Origin Is Unreachable (CF) | Cloudflare: Origin IP unreachable (origin server doesn't exist/routing issues) |
524 | A Timeout Occurred (CF) | Cloudflare: TCP connection succeeded but origin timed out returning HTTP response (>100s), common with slow queries/long tasks |
499 | Client Closed Request (Nginx) | Nginx extension code: Client closed connection before server responded (user cancelled/frontend timeout) |
Authoritative References
- ReferenceMDN - HTTP Response Status Codes
- ReferenceRFC 9110 - HTTP Semantics Standard
- ReferenceWikipedia - HTTP Status Codes
- ReferenceCloudflare 5xx Error Troubleshooting
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