Let’s Encrypt

https://letsencrypt.org/

Let’s Encrypt is a free, automated, and open Certificate Authority.

Certbot

Automatically enable HTTPS on your website with EFF's Certbot, deploying Let's Encrypt certificates.

Install:

On Ubuntu systems, the Certbot team maintains a PPA. Once you add it to your list of repositories all you'll need to do is apt-get the following packages.

$ sudo apt-get update

$ sudo apt-get install software-properties-common

$ sudo add-apt-repository ppa:certbot/certbot

$ sudo apt-get update

$ sudo apt-get install python-certbot-nginx

$ sudo certbot --nginx

$ sudo certbot renew --dry-run #自动更新

执行该命令会创建系统级 cron 任务：/etc/cron.d/certbot，会每天检查两次。