如何调试“在上游 app_server 中未找到主机”？

Question

我在 ubuntu 14.04 上创建了一个新的 django（digitalocean）droplet，将我的域名注册商指向 DO 名称服务器，使用 sslmate 在服务器上购买和存储相关的 ssl 文件，并根据需要更新 nginx 配置文件。

当我尝试启动（或重新启动）nginx 时，在 /var/log/nginx/error.log 中我得到：

[emerg] 1656#0: host not found in upstream "app_server" in /etc/nginx/sites-enabled/django:56

我不知道如何调试这个问题，而且我在谷歌上也没有找到任何有用的信息。我甚至不确定错误消息指的是什么。有知道我应该从哪里开始吗？

这是整个配置文件：

upstream app_server {
    server 127.0.0.1:9000 fail_timeout=0;
}

server {
    listen 80;
    server_name www.domain.com domain.com;
    rewrite ^ https://domain.com$request_uri? permanent;
}

server {
    listen 80 default_server;
    listen [::]:80 default_server ipv6only=on;

    listen 443 ssl;

    root /usr/share/nginx/html;
    index index.html index.htm;

    client_max_body_size 4G;
    server_name domain.com;
    ssl_certificate_key /etc/sslmate/www.domain.com.key;
    ssl_certificate /etc/sslmate/www.domain.com.chained.crt;

    # Recommended security settings from https://wiki.mozilla.org/Security/Server_Side_TLS
    ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
    ssl_ciphers 'ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:kEDH+AESGCM:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA256:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:AES:CAMELLIA:DES-CBC3-SHA:!aNULL:!eNULL:!EXPORT:!DES:!RC4:!MD5:!PSK:!aECDH:!EDH-DSS-DES-CBC3-SHA:!EDH-RSA-DES-CBC3-SHA:!KRB5-DES-CBC3-SHA';
    ssl_prefer_server_ciphers on;
    ssl_dhparam /usr/share/sslmate/dhparams/dh2048-group14.pem;
    ssl_session_timeout 5m;
    ssl_session_cache shared:SSL:5m;
    # Enable this if you want HSTS (recommended)
    # add_header Strict-Transport-Security max-age=15768000;

    keepalive_timeout 5;

    # Your Django project's media files - amend as required
    location /media  {
        alias /home/django/media;
    }

    # your Django project's static files - amend as required
    location /static {
        alias /home/django/static;
    }

    # Proxy the static assests for the Django Admin panel
    location /static/admin {
       alias /usr/lib/python2.7/dist-packages/django/contrib/admin/static/admin/;
    }

    location / {
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $http_host;
        proxy_redirect off;
        proxy_pass http://app_server;
    }
}