由于迁移了VPS，从新部署了Let’s Encrypt，支持HTTPS链接，参考安装和更新Let’s Encrypt证书，发现一个有趣的现象，Let’s Encrypt更新证书有了一个限制。

[root@kaifashuo letsencrypt]# ./letsencrypt-auto certonly --standalone --email kf@kaifashuo.com -d kaifashuo.com -d kaifashuo.com
Saving debug log to /var/log/letsencrypt/letsencrypt.log
Cert not yet due for renewal

You have an existing certificate that has exactly the same domains or certificate name you requested and isn't close to expiry.
(ref: /etc/letsencrypt/renewal/kaifashuo.com.conf)

What would you like to do?
-------------------------------------------------------------------------------
1: Keep the existing certificate for now
2: Renew & replace the cert (limit ~5 per 7 days) 
多了一个限制选项：续订和更换证书（每7天限制约5次），另外一层意思，离证书到期5-7前，自动续订和更换证书，鸟语差的掉渣~
-------------------------------------------------------------------------------
Select the appropriate number [1-2] then [enter] (press 'c' to cancel): 2（此处选择2）
Renewing an existing certificate
Performing the following challenges:
tls-sni-01 challenge for kaifashuo.com
tls-sni-01 challenge for kaifashuo.com
Waiting for verification...
Cleaning up challenges
Generating key (2048 bits): /etc/letsencrypt/keys/0004_key-certbot.pem
Creating CSR: /etc/letsencrypt/csr/0004_csr-certbot.pem

IMPORTANT NOTES:
 - Congratulations! Your certificate and chain have been saved at
   /etc/letsencrypt/live/kaifashuo.com/fullchain.pem. Your cert will
   expire on 2017-07-30. To obtain a new or tweaked version of this
   certificate in the future, simply run letsencrypt-auto again. To
   non-interactively renew *all* of your certificates, run
   "letsencrypt-auto renew"
 - If you like Certbot, please consider supporting our work by:

   Donating to ISRG / Let's Encrypt:   https://letsencrypt.org/donate
   Donating to EFF:                    https://eff.org/donate-le