code: func NewKnownHostsCallback(files ...string) (ssh.HostKeyCallback, error) {
  	var err error
  
  	if len(files) == 0 {
  		if files, err = getDefaultKnownHostsFiles(); err != nil {
  			return nil, err
  		}
  	}
  
  	if files, err = filterKnownHostsFiles(files...); err != nil {
  		return nil, err
  	}
  
  	return knownhosts.New(files...)
  }
docs: // NewKnownHostsCallback returns ssh.HostKeyCallback based on a file based on a
  // known_hosts file. http://man.openbsd.org/sshd#SSH_KNOWN_HOSTS_FILE_FORMAT
  //
  // If list of files is empty, then it will be read from the SSH_KNOWN_HOSTS
  // environment variable, example:
  //   /home/foo/custom_known_hosts_file:/etc/custom_known/hosts_file
  //
  // If SSH_KNOWN_HOSTS is not set the following file locations will be used:
  //   ~/.ssh/known_hosts
  //   /etc/ssh/ssh_known_hosts