1 文件元数据
我先确认一下这个下载链接应该返回的内容。> curl --head http://download.qt-project.org/official_releases/qt/5.0/5.0.2/qt-linux-opensource-5.0.2-x86_64-offline.run HTTP/1.1 200 OK Date: Mon, 13 May 2013 03:25:33 GMT Server: Apache/2.2.15 (CentOS) X-Prefix: 122.240.0.0/13 X-AS: 4134 Last-Modified: Tue, 09 Apr 2013 11:53:01 GMT ETag: "120117-189c7765-4d9ec31fcc140" Accept-Ranges: bytes Content-Length: 412907365 Connection: close Content-Type: text/plain; charset=UTF-8很明显是个文件，将近400MB，符合网页上显示的大小。那metalink怎么可能出来呢？还是抓包看看吧，用的是Wireshark。
2 请求头、响应头、响应内容
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GET /official_releases/qt/5.0/5.0.2/qt-linux-opensource-5.0.2-x86_64-offline.run HTTP/1.1 Host: download.qt-project.org User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:20.0) Gecko/20100101 Firefox/20.0 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 Accept-Language: en-us,en;q=0.8,zh-cn;q=0.5,zh;q=0.3 Accept-Encoding: gzip, deflate DNT: 1 Referer: http://qt-project.org/downloads Connection: keep-alive HTTP/1.1 302 Found Date: Mon, 13 May 2013 04:09:22 GMT Server: Apache/2.2.15 (CentOS) X-Prefix: 122.240.0.0/13 X-AS: 4134 X-MirrorBrain-Mirror: mirrors.ustc.edu.cn X-MirrorBrain-Realm: country Link: <http://download.qt-project.org/official_releases/qt/5.0/5.0.2/qt-linux-opensource-5.0.2-x86_64-offline.run.meta4>; rel=describedby; type="application/metalink4+xml" Link: <http://download.qt-project.org/official_releases/qt/5.0/5.0.2/qt-linux-opensource-5.0.2-x86_64-offline.run.torrent>; rel=describedby; type="application/x-bittorrent" Link: <http://mirrors.ustc.edu.cn/qtproject/official_releases/qt/5.0/5.0.2/qt-linux-opensource-5.0.2-x86_64-offline.run>; rel=duplicate; pri=1; geo=cn Link: <http://mirrors.neusoft.edu.cn/qt/official_releases/qt/5.0/5.0.2/qt-linux-opensource-5.0.2-x86_64-offline.run>; rel=duplicate; pri=2; geo=cn Link: <http://ftp.jaist.ac.jp/pub/qtproject/official_releases/qt/5.0/5.0.2/qt-linux-opensource-5.0.2-x86_64-offline.run>; rel=duplicate; pri=3; geo=jp Link: <http://master.qt-project.org/official_releases/qt/5.0/5.0.2/qt-linux-opensource-5.0.2-x86_64-offline.run>; rel=duplicate; pri=4; geo=fi Link: <http://www.nic.funet.fi/pub/mirrors/download.qt-project.org/official_releases/qt/5.0/5.0.2/qt-linux-opensource-5.0.2-x86_64-offline.run>; rel=duplicate; pri=5; geo=fi Digest: MD5=bbJjkMgQIwimVWhOyVetyQ== Digest: SHA=HJA5DwAdhRTN1/VuDF9tfwY2Bwg= Digest: SHA-256=eSPqSjbYzSA3nd9Pt5JkbzSZMjhBcX30DwJANdV9GsM= Location: http://mirrors.ustc.edu.cn/qtproject/official_releases/qt/5.0/5.0.2/qt-linux-opensource-5.0.2-x86_64-offline.run Content-Length: 385 Connection: close Content-Type: text/html; charset=iso-8859-1 <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"> <html><head> <title>302 Found</title> </head><body> <h1>Found</h1> <p>The document has moved <a href="http://mirrors.ustc.edu.cn/qtproject/official_releases/qt/5.0/5.0.2/qt-linux-opensource-5.0.2-x86_64-offline.run">here</a>.</p> <hr> <address>Apache/2.2.15 (CentOS) Server at download.qt-project.org Port 80</address> </body></html>
这是个重定向响应（状态码302），重定向的位置在Location，地址是一个国内的下载链接，这是通过请求对应的ip地址计算出来的，X-Prefix响应头还返回了ip所在区域。
有很多Link响应头，包括metalink、bittorrent，以及一些带优先级跟所在国家的http下载地址。
还有几个Digest响应头，对应的值应该是二进制数据的Base64表示形式。我一开始没查到这个响应头，后来在Google搜索http response "Digest: SHA" -auth查到的。
有一个DNT请求头，代表do not track。
3 深入阅读
* http://www.w3.org/Protocols/rfc2616/rfc2616-sec6.html* https://en.wikipedia.org/wiki/List_of_HTTP_header_fields
* RFC3230 Instance Digests in HTTP - tools.ietf.org
* https://en.wikipedia.org/wiki/HTTP_ETag
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