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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
Figuren

If-Match HTTP 请求报头使得所述请求为条件。对于GETHEAD方法,服务器将只在与请求的资源匹配时发回请求的资源ETags。对于PUT和其他非安全方法,在这种情况下它只会上传资源。

与存储的ETag比较使用强比较算法,这意味着两个文件被认为是相同的字节到字节。当W/在前面使用前缀时,这被削弱了ETag

有两种常见用例:

  • 对于GETHEAD方法,与Range头部结合使用,可以保证请求的新范围来自与前一个相同的资源。如果不匹配,则416返回( Range Not Satisfiable )响应。

  • 对于其他方法,特别是对PUTIf-Match可用于防止丢失的更新问题。它可以检查用户想要上传的资源的修改是否会覆盖从原始资源获取以来完成的另一个更改。如果请求无法实现,412则返回( Precondition Failed )响应。

Header type

Request header

Forbidden header name

no

语法

If-Match: <etag_value>If-Match: <etag_value>, <etag_value>, …

指令

<etag_value> 只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 ASCII 字符串,可以加前缀W/表示应使用弱比较算法。*星号是表示任何资源的特殊值。

例子

If-Match: "bfc13a64729c4290ef5b2c2730249c88ca92d82d"If-Match: W/"67ab43", "54ed21", "7892dd"If-Match: *

规范

Specification

Title

RFC 7232, section 3.1: If-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)

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