code: func (m *HostKeyCallbackHelper) SetHostKeyCallback(cfg *ssh.ClientConfig) (*ssh.ClientConfig, error) {
  	var err error
  	if m.HostKeyCallback == nil {
  		if m.HostKeyCallback, err = NewKnownHostsCallback(); err != nil {
  			return cfg, err
  		}
  	}
  
  	cfg.HostKeyCallback = m.HostKeyCallback
  	return cfg, nil
  }
docs: // SetHostKeyCallback sets the field HostKeyCallback in the given cfg. If
  // HostKeyCallback is empty a default callback is created using
  // NewKnownHostsCallback.