从服务器读取一些偏移量的文件 [英] read file from server with some offset

问题描述

如何从服务器读取文件，从一些偏移开始（与 wget -c 类似的行为）？我必须向服务器发送哪些标头？服务器支持哪些期货？

How can I read file from server starting with some offset (Similar behavior to wget -c)? What headers I must send to server? What futures must server support?

推荐答案

您应该使用范围标题在请求中。但是，只有当服务器通过 Accept-Ranges 响应标题通知您它接受范围请求时，您才可以使用它。

You should use the Range header in the request. But you may use it only if the server informs you that it accept range request by Accept-Ranges response header.

这是一个示例会话。假设我们有兴趣获得此图片的一部分。首先，我们发送HTTP HEAD 请求以确定：a）服务器是否支持字节范围，b）内容长度：

This is an example session. Suppose we are interested in getting a part of this picture. First, we send a HTTP HEAD request to determine: a) if the server supports byte ranges, b) the content-length:

> HEAD /2238/2758537173_670161cac7_b.jpg HTTP/1.1
> Host: farm3.static.flickr.com
> Accept: */*
> 
< HTTP/1.1 200 OK
< Date: Thu, 08 Jul 2010 12:22:12 GMT
< Content-Type: image/jpeg
< Connection: keep-alive
< Server: Apache/2.0.52 (Red Hat)
< Expires: Mon, 28 Jul 2014 23:30:00 GMT
< Last-Modified: Wed, 13 Aug 2008 06:13:54 GMT
< Accept-Ranges: bytes
< Content-Length: 350015

接下来，我们发送 GET 请求范围标头询问图片的前11个字节：

Next, we send a GET request with the Range header asking for the first 11 bytes of the picure:

> GET /2238/2758537173_670161cac7_b.jpg HTTP/1.1
> Host: farm3.static.flickr.com
> Accept: */*
> Range: bytes=0-10
> 
< HTTP/1.1 206 Partial Content
< Date: Thu, 08 Jul 2010 12:26:54 GMT
< Content-Type: image/jpeg
< Connection: keep-alive
< Server: Apache/2.0.52 (Red Hat)
< Expires: Mon, 28 Jul 2014 23:30:00 GMT
< Last-Modified: Wed, 13 Aug 2008 06:13:54 GMT
< Accept-Ranges: bytes
< Content-Range: bytes 0-10/350015
< Content-Length: 11
< 

这是前11个字节的十六进制转储：

This is a hex dump of the first 11 bytes:

00000000  ff d8 ff e0 00 10 4a 46  49 46 00                 |......JFIF.|
0000000b

有关详细信息，请参阅范围标题规范。

For more info see the Range header specification in HTTP RFC 2616.

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