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设备检测基于请求中的User-agent找出哪种内容返回给客户端。
使用Device detection情况的一个例子，发送给小屏幕的手机客户端或者复杂的网络环境，减少发送文件的数量，或者提供给不能解码的客户端一个video解码器。
有些使用这种方案的典型场景：
重写url
使用不同的后端给客户端
改变后端请求为了让后端发送裁剪的内容
可能为了便于理解，下文中假定req.http.X-UA-Device头表示当前前客户端，同时每种客户端是唯一的。
简单的像这样设置头：
sub vcl_recv {
if (req.http.User-Agent ~ "(?i)iphone" {
set req.http.X-UA-Device = "mobile-iphone";
}
}
不同的处理请求，同时免费提供组织和验证更详细的的客户端。基本且通用的是使用正则表达式来设置。查看https://github.com/varnish/varnish-devicedetect/。
这个戏法表明：
验证客户端（相当简单，只需要包含devicedetect.vcl，然后调用它）
根据客户端的类型找出怎样发送请求到后端。这包含了些例子，设置header，改变header，甚至改变发送到后端的url。
修改后端响应，增加miss标志‘Vary‘头。
修改发送到客户端的输出，这样如果任何cache不受控，可以不提供错误的内容给客户端。
所有这些都需要确定我们每种设备只会获得一个缓存的对象。
基本情况是Varnish增加X-UA-Device头在发送到后端的请求上，然后后端在响应header中添加Vary头，响应的内容依赖发送到后端的header。
从Varnish角度看一切开箱即用。
VCL：
sub vcl_recv {
# call some detection engine that set req.http.X-UA-Device
# 调用一些检测机来设置req.http.X-UA-Device。
}
# req.http.X-UA-Device is copied by Varnish into bereq.http.X-UA-Device
# req.http.X-UA-Device 被复制到bereq.http.X-UA-Device，当请求传递到后端时。
# so, this is a bit counterintuitive. The backend creates content based on
# the normalized User-Agent, but we use Vary on X-UA-Device so Varnish will
# use the same cached object for all U-As that map to the same X-UA-Device.
#
# 因此这是有点反直觉的。后端创建响应内容基于正常的User-Agent，但是在X-UA-Device使用
# Vary，varnish将使用相同的已缓存的对象，对所有的U-As映射到相同的X-UA-Device。
# If the backend does not mention in Vary that it has crafted special
# content based on the User-Agent (==X-UA-Device), add it.
# 如果基于User-Agent的后端响应不带有Vary，在header种增加它。
# If your backend does set Vary: User-Agent, you may have to remove that here.
# 如果后端设置了Vary：User-Agent，你可以在此删除它，或者设置成其他的。
sub vcl_backend_response {
if (bereq.http.X-UA-Device) {
if (!beresp.http.Vary) { # no Vary at all
set beresp.http.Vary = "X-UA-Device";
} elseif (beresp.http.Vary !~ "X-UA-Device") { # add to existing Vary
set beresp.http.Vary = beresp.http.Vary + ", X-UA-Device";
}
}
# comment this out if you don‘t want the client to know your
# classification
# 如果不想客户端知道你的分类，注释掉即可。
set beresp.http.X-UA-Device = bereq.http.X-UA-Device;
}
# to keep any caches in the wild from serving wrong content to client #2
# behind them, we need to transform the Vary on the way out.
sub vcl_deliver {
if ((req.http.X-UA-Device) && (resp.http.Vary)) {
set resp.http.Vary = regsub(resp.http.Vary, "X-UA-Device", "User-Agent");
# 在resp.http.Vary的值中将X-UA-Device替换为User-Agent。
}
}
标准化User-Agent字符。
另一种发送设备类型的目的是为了重写或者标准化发送到后端的User-Agent。
例如：
User-Agent: Mozilla/5.0 (Linux; U; Android 2.2; nb-no; HTC Desire Build/FRF91) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1
变成：
User-Agent: mobile-android
如果你在后端上不需要源http header的任何信息可以这样设置。一种可能性就是使用CGI脚本，只需小小设置下预定义的头信息在可以用的脚本中。
VCL：
sub vcl_recv {
# call some detection engine that set req.http.X-UA-Device
}
# override the header before it is sent to the backend
# 发送到后端之前重写header。
sub vcl_miss { if (req.http.X-UA-Device) { set bereq.http.User-Agent = req.http.X-UA-Device; } }
sub vcl_pass { if (req.http.X-UA-Device) { set bereq.http.User-Agent = req.http.X-UA-Device; } }
# standard Vary handling code from previous（之前的） examples.
sub vcl_backend_response {
if (bereq.http.X-UA-Device) {
if (!beresp.http.Vary) { # no Vary at all
set beresp.http.Vary = "X-UA-Device";
} elseif (beresp.http.Vary !~ "X-UA-Device") { # add to existing Vary
set beresp.http.Vary = beresp.http.Vary + ", X-UA-Device";
}
}
set beresp.http.X-UA-Device = bereq.http.X-UA-Device;
}
sub vcl_deliver {
if ((req.http.X-UA-Device) && (resp.http.Vary)) {
set resp.http.Vary = regsub(resp.http.Vary, "X-UA-Device", "User-Agent");
}
}
以GET参数作为设备类型
上面两种方式都不行的话你可使用GET参数来增加设备类型。
http://example.com/article/1234.html --> http://example.com/article/1234.html?devicetype=mobile-iphone
客户端不会查看分类，仅仅是后端请求被修改了。
VCL：
sub vcl_recv {
# call some detection engine that set req.http.X-UA-Device
}
sub append_ua {
if ((req.http.X-UA-Device) && (req.method == "GET")) {
# if there are existing GET arguments;
if (req.url ~ "\?") {
set req.http.X-get-devicetype = "&devicetype=" + req.http.X-UA-Device;
} else {
set req.http.X-get-devicetype = "?devicetype=" + req.http.X-UA-Device;
}
set req.url = req.url + req.http.X-get-devicetype;
# 重写url
unset req.http.X-get-devicetype;
# 移除req.http.X-get-devicetype
}
}
# do this after vcl_hash, so all Vary-ants can be purged in one go. (avoid ban()ing)
sub vcl_miss { call append_ua; }
sub vcl_pass { call append_ua; }
# Handle redirects, otherwise standard Vary handling code from previous
# examples.
sub vcl_backend_response {
if (bereq.http.X-UA-Device) {
if (!beresp.http.Vary) { # no Vary at all
set beresp.http.Vary = "X-UA-Device";
} elseif (beresp.http.Vary !~ "X-UA-Device") { # add to existing Vary
set beresp.http.Vary = beresp.http.Vary + ", X-UA-Device";
}
# if the backend returns a redirect (think missing trailing slash),
# we will potentially show the extra address to the client. we
# don‘t want that. if the backend reorders the get parameters, you
# may need to be smarter here. (? and & ordering)
if (beresp.status == 301 || beresp.status == 302 || beresp.status == 303) {
set beresp.http.location = regsub(beresp.http.location, "[?&]devicetype=.*$", "");
}
}
set beresp.http.X-UA-Device = bereq.http.X-UA-Device;
}
sub vcl_deliver {
if ((req.http.X-UA-Device) && (resp.http.Vary)) {
set resp.http.Vary = regsub(resp.http.Vary, "X-UA-Device", "User-Agent");
}
}
对手机客户端使用不同的后端。
如果对于手机客户端你有不同的后端来提供页面，或者在VCL种有任何特殊需求，你可以使‘X-UA-Device‘头像这样：
backend mobile {
.host = "10.0.0.1";
.port = "80";
}
sub vcl_recv {
# call some detection engine
if (req.http.X-UA-Device ~ "^mobile" || req.http.X-UA-device ~ "^tablet") {
set req.backend_hint = mobile;
}
}
sub vcl_hash {
if (req.http.X-UA-Device) {
hash_data(req.http.X-UA-Device);
}
}
如果想重定向手机客户端可以使用下面的代码片段：
sub vcl_recv {
# call some detection engine
if (req.http.X-UA-Device ~ "^mobile" || req.http.X-UA-device ~ "^tablet") {
return(synth(750, "Moved Temporarily"));
}
}
sub vcl_synth {
if (obj.status == 750) {
set obj.http.Location = "http://m.example.com" + req.url;
set obj.status = 302;
return(deliver);
}
}
sub vcl_recv {
if (req.http.host == "www.example.com" && req.http.User-Agent ~ "(?i)MIDP|WAP|UP.Browser|Smartphone|Obigo|Mobile|AU.Browser|wxd.Mms|WxdB.Browser|CLDC|UP.Link|KM.Browser|UCWEB|SEMC\-Browser|Mini|Symbian|Palm|Nokia|Panasonic|MOT|SonyEricsson|NEC|Alcatel|Ericsson|BENQ|BenQ|Amoisonic|Amoi|Capitel|PHILIPS|SAMSUNG|Lenovo|Mitsu|Motorola|SHARP|WAPPER|LG|EG900|CECT|Compal|kejian|Bird|BIRD|G900/V1.0|Arima|CTL|TDG|Daxian|DAXIAN|DBTEL|Eastcom|EASTCOM|PANTECH|Dopod|Haier|HAIER|KONKA|KEJIAN|LENOVO|Soutec|SOUTEC|SAGEM|SEC|SED|EMOL|INNO55|ZTE|iPhone|Android|Windows CE|Wget|Java|Opera") {
set req.backend_hint = mobile;
return(pass);
}
}
varnish 4.0 官方文档翻译20-Device detection
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原文地址：http://my.oschina.net/monkeyzhu/blog/469304
