之前写了篇文章用openresty实现了一个动态路由，虽然说是动态的，但是实际上还是需要将upstream在配置文件中写好，还是相当于静态的。

最近工作中有这方面的需求，upstream是完全动态，由客户端来指定，开始的时候有一些错误，最后通过resolver指定dns服务来完成，具体流程如下。

1. 开始踩坑

nginx的配置如下：

worker_processes  1;

events {
    worker_connections  1024;
}

http {
    server {
        listen       8001;
        server_name  localhost;

        location / {
                set $upstream_host $http_upstream_host;
                echo $http_upstream_host;
                proxy_pass http://$upstream_host;
        }

    }

}
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当我发送这种请求：

curl "127.0.0.1:8001/" -H "upstream-host:www.baidu.com" -v
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报了如下错误：

access.log:
127.0.0.1 - - [14/Sep/2017:23:37:10 +0800] "GET / HTTP/1.1" 502 179 "-" "curl/7.29.0"

error.log:
2017/09/14 23:38:31 [error] 25307#25307: *48 no resolver defined to resolve www.baidu.com, client: 127.0.0.1, server: localhost, request: "GET / HTTP/1.1", host: "127.0.0.1:8001"
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很明显这个问题说明没有指定resolver导致自定义upstreamwww.baidu.com。

2. 使用resolver定义域名解析

修改之后的nginx配置如下：

worker_processes  1;


events {
    worker_connections  1024;
}

http {
    resolver 114.114.114.114;
    server {
        listen       8001;
        server_name  localhost;

        location / {
                set $upstream_host $http_upstream_host;
                echo $http_upstream_host;
                proxy_pass http://$upstream_host;
        }

    }

}
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这下请求都正常200了：

req:
curl "127.0.0.1:8001/" -H "upstream-host:www.baidu.com" -voa
curl "127.0.0.1:8001/" -H "upstream-host:www.qq.com" -voa

access.log:
127.0.0.1 - - [14/Sep/2017:23:44:01 +0800] "GET / HTTP/1.1" 200 2381 "-" "curl/7.29.0"
127.0.0.1 - - [14/Sep/2017:23:44:07 +0800] "GET / HTTP/1.1" 200 244182 "-" "curl/7.29.0"
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3. resolver使用说明

resolver的语法如下：

Syntax: resolver address ... [valid=time] [ipv6=on|off];
Default:    —
Context:    http, server, location
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可以配置多个dns服务，nginx会采用轮询的方式去访问dns服务，nginx会缓存dns对域名解析的结果，缓存的时间由valid指定，ipv6用于显示开启或者关闭ipv6。

Syntax: resolver_timeout time;
Default:    
resolver_timeout 30s;
Context:    http, server, location
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resolver_timeout用于指定dns解析的超时时间。

4. ref

http://nginx.org/en/docs/http/ngx_http_core_module.html#resolver