https://blog.wongcw.com/2018/10/19/openbsd-6-4-%e5%8f%91%e5%b8%83-%e6%b7%bb%e5%8a%a0%e8%ae%b8%e5%a4%9a%e6%96%b0%e7%9a%84%e9%a9%b1%e5%8a%a8%e7%a8%8b%e5%ba%8f%e6%94%af%e6%8c%81/