1. 按照自动设置工具

$ apt-get update
$ sudo apt-get install certbot
$ apt-get install python3-certbot-nginx

2. 配置Nginx
   /etc/nginx/conf.d

server {
    listen 80 default_server;
    listen [::]:80 default_server;
    root /var/www/html;
    server_name example.com www.example.com;
}

3. 更新配置

nginx -t && nginx -s reload

4. 自动生成SSL并配置

$ sudo certbot --nginx -d example.com -d www.example.com

5. 出现下面消息代表配置成功

Congratulations! You have successfully enabled https://example.com and https://www.example.com 

-------------------------------------------------------------------------------------
IMPORTANT NOTES: 

Congratulations! Your certificate and chain have been saved at: 
/etc/letsencrypt/live/example.com/fullchain.pem 
Your key file has been saved at: 
/etc/letsencrypt/live/example.com//privkey.pem
Your cert will expire on 2017-12-12.

6. 查看更新后的配置文件

server {
    listen 80 default_server;
    listen [::]:80 default_server;
    root /var/www/html;
    server_name  example.com www.example.com;

    listen 443 ssl; # managed by Certbot

    # RSA certificate
    ssl_certificate /etc/letsencrypt/live/example.com/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem; # managed by Certbot

    include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot

    # Redirect non-https traffic to https
    if ($scheme != "https") {
        return 301 https://$host$request_uri;
    } # managed by Certbot
}

7. 到期自动更新设置
   每天0点check是不是剩下30天期限，然后更新

crontab -e
0 12 * * * /usr/bin/certbot renew --quiet

Nginx免费SSL证明书自动设置