2017-4月 免费ssl证书，申请安装。无需注册账号。

首先安装apache ssl模块

git clone https://github.com/letsencrypt/letsencrypt
cd letsencrypt
./letsencrypt-auto -d www.youdomain.com

报错：

Error: Protected multilib versions: openssl-1.0.1e-48.el6_8.4.x86_64 != openssl-1.0.1e-42.el6_7.4.i686

yum remove openssl-1.0.1e-42.el6_7.4.i686 -y

再次：

./letsencrypt-auto -d www.youdomain.com

报错：

Type:   connection
   Detail: Failed to connect to 221.236.173.67:443 for TLS-SNI-01
   challenge

删除/etc/httpd/conf.d/ssl.conf里面的443的默认主机。

再次：

./letsencrypt-auto -d www.youdomain.com

警告：

IMPORTANT NOTES:
 - Unable to install the certificate
 - Congratulations! Your certificate and chain have been saved at
   /etc/letsencrypt/live/www.hualike.net/fullchain.pem. Your cert will
   expire on 2017-07-05. To obtain a new or tweaked version of this
   certificate in the future, simply run letsencrypt-auto again with
   the "certonly" option. To non-interactively renew *all* of your
   certificates, run "letsencrypt-auto renew"

忽略他，自己增加一个端口为443的虚拟机，并增加类似下面的配置到这个虚拟机：

SSLEngine on
SSLCertificateFile /etc/letsencrypt/live/www.youdomain.com/fullchain.pem
SSLCertificateKeyFile /etc/letsencrypt/live/www.youdomain.com/privkey.pem


重启apache，访问你的网站：

没ssl证书警告了，如果你的图和js css之类的也走https的话，浏览器地址栏将显示绿色的锁。

证书有效期3个月，3个月后你需要renew。

已经可以用于adhoc安装ipa了，safari不会警告 无法连接到“www.youdomain.com”了。