为什么浏览器不发送“If-None-Match”头？ [英] Why browser does not send "If-None-Match" header?

问题描述

我正在尝试下载（并希望缓存）PHP中动态加载的图像。以下是发送和接收的标题：

请求：

  GET /url:resource/Pomegranate/resources/images/logo.png HTTP / 1.1 
主机：pome.local 
连接：keep-alive 
缓存控制：max-age = 0 
接受：text / html，application / xhtml + xml，application / xml; q = 0.9，* / *; q = 0.8 
 User-Agent：Mozilla / 5.0（X11; Linux x86_64）AppleWebKit / 537.22（ KHTML，如Gecko）Ubuntu Chromium / 25.0.1364.160 Chrome / 25.0.1364.160 Safari / 537.22 
接受编码：gzip，deflate，sdch 
接受语言：en-US，en; q = 0.8 
 Accept-Charset：ISO-8859-1，utf-8; q = 0.7，*; q = 0.3 
 Cookie：PHPSESSID = fb8ghv9ti6v5s3ekkmvtacr9u5 
  

回复：

  HTTP / 1.1 200 OK 
日期：星期二，2013年4月9日11:00:36 GMT 
服务器：Apache / 2.2.22（Ubuntu）
 X-Powered-By：PHP / 5.3.14 ZendServer / 5.0 
到期： 1981年11月19日星期四08:52:00 GMT 
缓存控制：无存储，无缓存，必须重新验证，后检查= 0，预检查= 0 
 Pragma：no- cache 
 Content-Disposition：inline; filename =logo
 ETag：1355829295
 Last-Modified：Tue，18 Dec 2012 14:44:55 Asia / Tehran 
 Keep-Alive：timeout = 5，max = 98 
连接：Keep-Alive 
转移编码：chunked 
内容类型：image / png 
  

当我重新加载URL时，会发送和接收完全相同的标头。我的问题是我应该在回复中发送什么来查看后续请求中的 If-None-Match 标题？

<注意：我相信这些标题在不久前做得很好，即使我不能确定，但​​我认为浏览器已更改为不发送 If-None-Match 标题了（我以前看过那个标题）。我正在使用Chrome和Firefox进行测试，但都无法发送标题。

解决方案

你说缓存-Control：no-store，no-cache - 但期望缓存发生？

删除这些值（我认为 must-revalidate，post-check = 0，pre-check = 0 可以/应该保留 - 他们告诉浏览器检查服务器是否有变化。

我会坚持使用 Last-Modified （如果仅使用此标准可以检测到对资源的​​更改） - ETag 是一个更复杂的事情要处理（特别是如果你想在自己的PHP脚本中处理它），Google PageSpeed / YSlow也建议不要这样做。

I'm trying to download (and hopefully cache) a dynamically loaded image in PHP. Here are the headers sent and received:

Request:

GET /url:resource/Pomegranate/resources/images/logo.png HTTP/1.1
Host: pome.local
Connection: keep-alive
Cache-Control: max-age=0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.22 (KHTML, like Gecko) Ubuntu Chromium/25.0.1364.160 Chrome/25.0.1364.160 Safari/537.22
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3
Cookie: PHPSESSID=fb8ghv9ti6v5s3ekkmvtacr9u5

Response:

HTTP/1.1 200 OK
Date: Tue, 09 Apr 2013 11:00:36 GMT
Server: Apache/2.2.22 (Ubuntu)
X-Powered-By: PHP/5.3.14 ZendServer/5.0
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
Content-Disposition: inline; filename="logo"
ETag: "1355829295"
Last-Modified: Tue, 18 Dec 2012 14:44:55 Asia/Tehran
Keep-Alive: timeout=5, max=98
Connection: Keep-Alive
Transfer-Encoding: chunked
Content-Type: image/png

When I reload the URL, the exact same headers are sent and received. My question is what should I send in my response to see the If-None-Match header in the consequent request?

NOTE: I believe these headers were doing just fine not long ago, even though I can not be sure but I think browsers are changed not to sent the If-None-Match header anymore (I used to see that header). I'm testing with Chrome and Firefox and both fail to send the header.

解决方案

You say Cache-Control: no-store, no-cache – but expect caching to happen?

Remove those values (I think must-revalidate, post-check=0, pre-check=0 could/should be kept – they tell the browser to check with the server if there was a change).

And I would stick with Last-Modified alone (if the changes to your resources can be detected using this criterion alone) – ETag is a more complicated thing to handle (especially if you want to deal with it in your PHP script yourself), and Google PageSpeed/YSlow advise against this one too.

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