server {
    listen       80;
    server_name  .a.com;

    charset utf-8;
    access_log  /home/a.com.access.log  main;

    location /(css|js|fonts|img)/ {
        access_log off;
        expires 1d;

        root "/path/to/app_a/static"
        try_files $uri @backend
    }

    location / {
        try_files /_not_exists_ @backend;
    }

    location @backend {
        proxy_set_header X-Forwarded-For $remote_addr;
        proxy_set_header Host            $http_host;

        proxy_pass http://127.0.0.1:8080;
    }
}

iptables禁止外网访问本机8080端口：

iptables -I INPUT -p tcp --dport 81 -j DROP

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