https://superuser.com/questions/97201/how-to-save-a-remote-server-ssl-certificate-locally-as-a-file
openssl s_client -connect 10.222.20.15:27017 -showcerts
# 过滤并保存成 pem 文件
openssl s_client -showcerts -connect server.edu:443 </dev/null 2>/dev/null|openssl x509 -outform PEM >mycertfile.pem