由于要同时为多个域名申请ssl证书，为了方便所以要关闭占用80端口的程序
git clone https://github.com/letsencrypt/letsencrypt
cd letsencrypt
./letsencrypt-auto --agree-dev-preview --server https://acme-v01.api.letsencrypt.org/directory certonly
最后生成的证书在/etc/letsencrypt/live/yourdomain目录下
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