It is not secure to run with OAUTHBEARER mechanism in SASL_PLAINTEXT, we need the SSL to encrypt the data transportation.

certstrap https://github.com/square/certstrap 是一个自签名证书生成工具，相比 openssl 来说更简易好用。

简单来说，Certbot (https://certbot.eff.org/) 就是在服务器能够自动部署免费HTTPS证书的工具。