浏览器用户代理

Your browser sends its user agent to every website you connect to. We’ve written about changing your browser’s user agent before – but what exactly is a user agent, anyway?

您的浏览器会将其用户代理发送到您连接到的每个网站。 我们之前曾写过有关更改浏览器用户代理的文章-但是，究竟用户代理到底是什么？

A user agent is a “string” – that is, a line of text – identifying the browser and operating system to the web server. This sounds simple, but user agents have become a mess over time.

用户代理是一个“字符串”(即一行文本)，用于标识Web服务器的浏览器和操作系统。 这听起来很简单，但是随着时间的流逝，用户代理变得一团糟。

基础 (The Basics)

When your browser connects to a website, it includes a User-Agent field in its HTTP header. The contents of the user agent field vary from browser to browser. Each browser has its own, distinctive user agent. Essentially, a user agent is a way for a browser to say “Hi, I’m Mozilla Firefox on Windows” or “Hi, I’m Safari on an iPhone” to a web server.

当您的浏览器连接到网站时，它的HTTP标头中包含一个User-Agent字段。 用户代理字段的内容因浏览器而异。 每个浏览器都有自己的独特用户代理。 本质上，用户代理是浏览器向Web服务器说“嗨，我是Windows上的Mozilla Firefox”或“嗨，我是iPhone上的Safari”的一种方式。

The web server can use this information to serve different web pages to different web browsers and different operating systems. For example, a website could send mobile pages to mobile browsers, modern pages to modern browsers, and a “please upgrade your browser” message to Internet Explorer 6.

Web服务器可以使用此信息将不同的网页提供给不同的Web浏览器和不同的操作系统。 例如，一个网站可以将移动页面发送到移动浏览器，将现代页面发送到现代浏览器，并将“请升级您的浏览器”消息发送到Internet Explorer 6。

检查用户代理 (Examining User Agents)

For example, here’s Firefox’s user agent on Windows 7:

例如，这是Windows 7上Firefox的用户代理：

Mozilla/5.0 (Windows NT 6.1; WOW64; rv:12.0) Gecko/20100101 Firefox/12.0

Mozilla / 5.0(Windows NT 6.1; WOW64; rv：12.0)Gecko / 20100101 Firefox / 12.0

This user agent tells the web server quite a bit: The operating system is Windows 7 (code name Windows NT 6.1), it’s a 64-bit version of Windows (WOW64), and the browser itself is Firefox 12.

该用户代理告诉Web服务器很多信息：操作系统是Windows 7(代号Windows NT 6.1)，它是Windows的64位版本(WOW64)，浏览器本身是Firefox 12。

Now let’s take a look at Internet Explorer 9’s user agent, which is:

现在，让我们看一下Internet Explorer 9的用户代理：

Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0)

Mozilla / 5.0(兼容； MSIE 9.0； Windows NT 6.1； WOW64； Trident / 5.0)

The user agent string identifies the browser as IE 9 with the Trident 5 rendering engine. However, you might spot something confusing – IE identifies itself as Mozilla.

用户代理字符串使用Trident 5渲染引擎将浏览器标识为IE 9。 但是，您可能会发现一些令人困惑的地方-IE将自己标识为Mozilla。

We’ll come back to that in a minute. First, let’s examine Google Chrome’s user agent, too:

一分钟后，我们会再谈。 首先，我们还要检查一下Google Chrome浏览器的用户代理：

Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/536.5 (KHTML, like Gecko) Chrome/19.0.1084.52 Safari/536.5

Mozilla / 5.0(Windows NT 6.1; WOW64)AppleWebKit / 536.5(KHTML，Gecko一样)Chrome / 19.0.1084.52 Safari / 536.5

The plot thickens: Chrome is pretending to be both Mozilla and Safari. To understand why, we’ll have to examine the history of user agents and browsers.

剧情变厚了：Chrome假装既是Mozilla又是Safari。 要了解原因，我们必须检查用户代理和浏览器的历史记录。

用户代理字符串混乱 (The User Agent String Mess)

Mosaic was one of the first browsers. Its user agent string was NCSA_Mosaic/2.0. Later, Mozilla came along (later renamed Netscape), and its user agent was Mozilla/1.0. Mozilla was a more advanced browser than Mosaic – in particular, it supported frames. Web servers checked to see that the user agent contained the word Mozilla and sent pages containing frames to Mozilla browsers. To other browsers, web servers sent the old pages without frames.

Mosaic是最早的浏览器之一。 它的用户代理字符串是NCSA_Mosaic / 2.0。 后来，Mozilla出现了(后来更名为Netscape)，其用户代理为Mozilla / 1.0。 Mozilla是比Mosaic更高级的浏览器-特别是它支持框架。 Web服务器检查发现用户代理包含Mozilla一词，并将包含框架的页面发送到Mozilla浏览器。 Web服务器向其他浏览器发送了没有框架的旧页面。

Eventually, Microsoft’s Internet Explorer came along and it supported frames, too. However, IE didn’t receive web pages with frames, because web servers just sent those to Mozilla browsers. To fix this problem, Microsoft added the word Mozilla to their user agent and threw in additional information (the word “compatible” and a reference to IE.) Web servers were happy to see the word Mozilla and sent IE the modern web pages. Other browsers that came later did the same thing.

最终，Microsoft的Internet Explorer出现了，它也支持框架。 但是，IE没有收到带有框架的网页，因为Web服务器只是将其发送到Mozilla浏览器。 为了解决此问题，Microsoft在其用户代理中添加了Mozilla一词，并添加了其他信息(“兼容”一词和对IE的引用。)Web服务器很高兴看到Mozilla一词并向IE发送了现代网页。 后来出现的其他浏览器也做同样的事情。

Eventually, some servers looked for the word Gecko – Firefox’s rendering engine – and served Gecko browsers different pages than older browsers. KHTML – originally developed for Konquerer on Linux’s KDE desktop – added the words “like Gecko” so they’d get the modern pages designed for Gecko, too. WebKit was based on KHTML – when it was developed, they added the word WebKit and kept the original “KHTML, like Gecko” line for compatibility purposes. In this way, browser developers kept adding words to their user agents over time.

最终，一些服务器使用了Gecko这个词(Firefox的渲染引擎)，并为Gecko浏览器提供了与旧版浏览器不同的页面。 KHTML最初是为Linux的KDE桌面上的Konquerer开发的，添加了“ like Gecko”一词，因此他们也可以获得专为Gecko设计的现代页面。 WebKit基于KHTML –在开发时，他们添加了WebKit一词，并保留了原始的“ KHTML，如Gecko”这一行，以实现兼容性。 这样，浏览器开发人员就不断在其用户代理中添加单词。

Web servers don’t really care what the exact user agent string is – they just check to see if it contains a specific word.

Web服务器实际上并不关心确切的用户代理字符串是什么，它们只是检查它是否包含特定的单词。

用途 (Uses)

Web servers use user agents for a variety of purposes, including:

Web服务器将用户代理用于多种目的，包括：

• Serving different web pages to different web browsers. This can be used for good – for example, to serve simpler web pages to older browsers – or evil – for example, to display a “This web page must be viewed in Internet Explorer” message.将不同的网页提供给不同的Web浏览器。 这可能有益(例如，将较简单的网页提供给较旧的浏览器)，也可能有害(例如，用于显示“必须在Internet Explorer中查看此网页”)消息。
• Displaying different content to different operating systems – for example, by displaying a slimmed-down page on mobile devices.向不同的操作系统显示不同的内容-例如，通过在移动设备上显示缩小的页面。
• Gathering statistics showing the browsers and operating systems in use by their users. If you ever see browser market-share statistics, this is how they’re acquired.收集统计信息，显示其用户正在使用的浏览器和操作系统。 如果您看到过浏览器市场份额的统计数据，这就是它们的获取方式。

Web-crawling bots use user agents, too. For example, Google’s web crawler identifies itself as:

爬网机器人也使用用户代理。 例如，Google的网络爬虫将自己标识为：

Googlebot/2.1 (+http://www.google.com/bot.html)

Googlebot / 2.1(+ http：//www.google.com/bot.html)

Web servers can give bots special treatment – for example, by allowing them through mandatory registration screens. (Yes, this means that you can sometimes bypass registration screens by setting your user agent to Googlebot.)

Web服务器可以给漫游器以特殊待遇，例如，允许它们通过强制注册屏幕。 (是的，这意味着您有时可以通过将用户代理设置为Googlebot来绕过注册屏幕。)

Web servers can also give orders to specific bots (or all bots) using the robots.txt file. For example a web server could tell a specific bot to go away, or tell another bot to only index certain areas of the website. In the robots.txt file, the bots are identified by their user agent strings.

Web服务器还可以使用robots.txt文件向特定的机器人(或所有机器人)下达命令。 例如，Web服务器可以告诉某个特定的漫游器消失，或者告诉另一个漫游器仅对网站的某些区域编制索引。 在robots.txt文件中，漫游器由其用户代理字符串标识。

All major browsers contain ways to set custom user agents, so you can see what web servers send to to different browsers. For example, set your desktop browser to a mobile browser’s user agent string and you’ll see the mobile versions of web pages on your desktop.

所有主要的浏览器都包含设置自定义用户代理的方法，因此您可以查看Web服务器发送到不同浏览器的内容。 例如，将桌面浏览器设置为移动浏览器的用户代理字符串，您将在桌面上看到网页的移动版本。

翻译自: https://www.howtogeek.com/114937/htg-explains-whats-a-browser-user-agent/

浏览器用户代理