Let's Encrypt SSL 免费证书出来一段时间了，不过我试用了好几个第三方工具，都遇到不同的问题都没成功，后干脆换回官方的工具，这里就以官方的工具为例写个笔记。
官方证书工具下载：（如果还没装git就先安装）
git clone https://github.com/letsencrypt/letsencrypt.git
官方工具有点无语就是需要占用443/80端口，也就是要把网站暂停一下，把Nginx或Apache临时停掉
service nginx stop
或
service httpd stop
进入letsencrypt目录，运行：
./letsencrypt-auto certonly --standalone -d xxx.com -d www.xxx.com -m xxx@email.com --agree-tos
Note:上面xxx.com 和 xxx@email.com 请修改成自己的。邮箱不验证，不需要填你Whois的邮箱，这是用来丢失证书之后重置的。
稍等片刻，程序自动返回以下的成功信息
Requesting root privileges to run letsencrypt...
IMPORTANT NOTES:
Congratulations! Your certificate and chain have been saved at
/etc/letsencrypt/live/xxx.com/fullchain.pem. Your cert will
expire on 2016-07-14. To obtain a new version of the certificate in
the future, simply run Let's Encrypt again.If you lose your account credentials, you can recover through
e-mails sent to xxxx@email.com.Your account credentials have been saved in your Let's Encrypt
configuration directory at /etc/letsencrypt. You should make a
secure backup of this folder now. This configuration directory will
also contain certificates and private keys obtained by Let's
Encrypt so making regular backups of this folder is ideal.-
If you like Let's Encrypt, please consider supporting our work by:
Donating to ISRG / Let's Encrypt: https://letsencrypt.org/donate
Donating to EFF: https://eff.org/donate-le
证书是放在 /etc/letsencrypt/live/xxx.com 目录，其中fullchain.pem是拼好的完整证书链证书，privkey.pem是私钥，将它们引用即可。
/etc/letsencrypt/live/xxx.com/fullchain.pem
/etc/letsencrypt/live/xxx.com/privkey.pem
当然最后记得重启Nginx或Apache就大功告成了。
Note：Let's Encrypt SSL 免费证书是一种短期证书，只有90天期限，到期前记得重新运行一次命令即可续期90天了。
./letsencrypt-auto certonly --standalone -d xxx.com -d www.xxx.com -m xxx@email.com --agree-tos
当然，可以加进cron定时任务里，先在/root根目录写个脚本，比如命名为 ressl.sh 内容如下：
#!/bin/bash service nginx stop cd /xxxx/letsencrypt ./letsencrypt-auto certonly --standalone -d xxx.com -d www.xxx.com -m xxx@email.com --agree-tos service nginx start
然后使用rontab -e命令，加入以下内容，这样就可以定期每60天更新一次
0 0 1 */2 * /root/ressl.sh > /dev/null 2>&1
另：crontab -e
0 0 1 */2 * ./letsencrypt-auto renew >> /dev/null 2>&1
Note: 上面xxxx换成你所在的目录和域名邮箱等，例如你是在root目录下载的，就是 /root 目录了
