http://nginx.org/en/docs/http/request_processing.html

按照优先级由上至下

1）匹配监听的端口

server {
    listen      80;
    ...
}

server {
    listen      8080;
    ...
}

2）匹配 request header （Host）

server {
    listen      80;
    server_name www.baidu.com;
    ...
}

server {
    listen      80;
    server_name zhidao.baidu.com;
    ...
}

3)  匹配到了listen 未匹配到 server_name

Nginx will route the request to the default server for this port.

① 隐式 default server （第一个）

server {
    listen      80;
    ...
}

server {
    listen      80;
    ...
}

②显示 default server

server {
    listen      80;
    ...
}

server {
    listen      80 default_server;
    ...
}