解决id_rsa权限不够的问题

错误描述：

It is recommended that your private key files are NOT accessible by others.

This private key will be ignored.

bad permissions: ignore key: /home/robin/.ssh/id_rsa

解决方案：

1. chmod 755 ~/.ssh/

2. chmod 600 ~/.ssh/id_rsa ~/.ssh/id_rsa.pub

3. chmod 644 ~/.ssh/known_hosts