https(ssl) 免费证书

https://letsencrypt.org/getting-started/
https://certbot.eff.org/lets-encrypt/centosrhel7-nginx

这个比较灵活
https://acme.sh
https://github.com/acmesh-official/acme.sh/wiki/dnsapi#2-dnspodcn-option

$ echo "0 0,12 * * * root python -c 'import random; import time; time.sleep(random.random() * 3600)' && certbot renew -q" | sudo tee -a /etc/crontab > /dev/null