目录 搜索
Guides Access control CORS Authentication Browser detection using the user agent Caching Caching FAQ Compression Conditional requests Connection management in HTTP 1.x Content negotiation Content negotiation: List of default Accept values Cookies CSP Messages Overview Protocol upgrade mechanism Proxy servers and tunneling Proxy servers and tunneling: Proxy Auto-Configuration (PAC) file Public Key Pinning Range requests Redirections Resources and specifications Resources and URIs Response codes Server-Side Access Control Session Guides: Basics Basics of HTTP Choosing between www and non-www URLs Data URIs Evolution of HTTP Identifying resources on the Web MIME Types MIME types: Complete list of MIME types CSP Content-Security-Policy Content-Security-Policy-Report-Only CSP: base-uri CSP: block-all-mixed-content CSP: child-src CSP: connect-src CSP: default-src CSP: font-src CSP: form-action CSP: frame-ancestors CSP: frame-src CSP: img-src CSP: manifest-src CSP: media-src CSP: object-src CSP: plugin-types CSP: referrer CSP: report-uri CSP: require-sri-for CSP: sandbox CSP: script-src CSP: style-src CSP: upgrade-insecure-requests CSP: worker-src Headers Accept Accept-Charset Accept-Encoding Accept-Language Accept-Ranges Access-Control-Allow-Credentials Access-Control-Allow-Headers Access-Control-Allow-Methods Access-Control-Allow-Origin Access-Control-Expose-Headers Access-Control-Max-Age Access-Control-Request-Headers Access-Control-Request-Method Age Allow Authorization Cache-Control Connection Content-Disposition Content-Encoding Content-Language Content-Length Content-Location Content-Range Content-Type Cookie Cookie2 Date DNT ETag Expect Expires Forwarded From Headers Host If-Match If-Modified-Since If-None-Match If-Range If-Unmodified-Since Keep-Alive Large-Allocation Last-Modified Location Origin Pragma Proxy-Authenticate Proxy-Authorization Public-Key-Pins Public-Key-Pins-Report-Only Range Referer Referrer-Policy Retry-After Server Set-Cookie Set-Cookie2 SourceMap Strict-Transport-Security TE Tk Trailer Transfer-Encoding Upgrade-Insecure-Requests User-Agent User-Agent: Firefox Vary Via Warning WWW-Authenticate X-Content-Type-Options X-DNS-Prefetch-Control X-Forwarded-For X-Forwarded-Host X-Forwarded-Proto X-Frame-Options X-XSS-Protection Methods CONNECT DELETE GET HEAD Methods OPTIONS PATCH POST PUT Status 100 Continue 101 Switching Protocols 200 OK 201 Created 202 Accepted 203 Non-Authoritative Information 204 No Content 205 Reset Content 206 Partial Content 300 Multiple Choices 301 Moved Permanently 302 Found 303 See Other 304 Not Modified 307 Temporary Redirect 308 Permanent Redirect 400 Bad Request 401 Unauthorized 403 Forbidden 404 Not Found 405 Method Not Allowed 406 Not Acceptable 407 Proxy Authentication Required 408 Request Timeout 409 Conflict 410 Gone 411 Length Required 412 Precondition Failed 413 Payload Too Large 414 URI Too Long 415 Unsupported Media Type 416 Range Not Satisfiable 417 Expectation Failed 426 Upgrade Required 428 Precondition Required 429 Too Many Requests 431 Request Header Fields Too Large 451 Unavailable For Legal Reasons 500 Internal Server Error 501 Not Implemented 502 Bad Gateway 503 Service Unavailable 504 Gateway Timeout 505 HTTP Version Not Supported 511 Network Authentication Required Status
文字

If-None-Match HTTP 请求报头使得所述请求为条件。对于GETHEAD方法,200只有服务器没有ETag与给定资源匹配的情况下,服务器才会返回具有状态的请求资源。对于其他方法,仅当最终现有资源ETag不符合任何列出的值时才会处理该请求。

当条件GETHEAD方法失败时,服务器必须返回 HTTP 状态码304(未修改)。对于应用服务器端更改的方法,将使用状态码412( Precondition Failed )。请注意,生成304响应的服务器必须生成以下头域中的任何一个头域,这些域头域应该发送到同一个请求的200(OK)响应中:Cache-Control,Content-Location,Date,ETag,Expires 和 Vary。

与存储的比较ETag使用弱比较算法,这意味着不仅两个文件是相同的字节到字节,而且如果内容是等同的,则认为两个文件是相同的。例如,只有页脚生成日期不同的两页才会被视为相同。

当与其结合使用时If-Modified-Since,它具有优先权(如果服务器支持它)。

有两种常见用例:

  • 对于GETHEAD方法,更新具有关联的缓存实体ETag

  • 对于其他方法,特别是对PUTIf-None-Match与所使用的*值可以用来保存不知道存在的文件,保证另一个上载并没有发生之前,失去了以前放的数据; 此问题是丢失更新问题的变体。

Header type

Request header

Forbidden header name

no

语法

If-None-Match: <etag_value>If-None-Match: <etag_value>, <etag_value>, …
If-None-Match: *

指令

<etag_value> 唯一表示请求资源的实体标签。它们是放置在双引号(像 "675af34563dc-tr34")之间的一串 ASCII 字符,可以加前缀W/以表示应该使用弱比较算法(这是无用的,If-None-Match因为它只使用该算法)。*星号是表示任何资源的特殊值。它们只有在上传资源时才有用,通常PUT用于检查具有该标识的另一资源是否已经上传过。

例子

If-None-Match: "bfc13a64729c4290ef5b2c2730249c88ca92d82d"If-None-Match: W/"67ab43", "54ed21", "7892dd"If-None-Match: *

规范

Specification

Title

RFC 7232, section 3.2: If-None-Match

Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests

浏览器兼容性

Feature

Chrome

Firefox

Edge

Internet Explorer

Opera

Safari

Basic Support

(Yes)

(Yes)

(Yes)

(Yes)

(Yes)

(Yes)

Feature

Android

Chrome for Android

Edge mobile

Firefox for Android

IE mobile

Opera Android

iOS Safari

Basic Support

(Yes)

(Yes)

(Yes)

(Yes)

(Yes)

(Yes)

(Yes)

上一篇: 下一篇: