Есть секция в man smb.conf. Называется "NOTE ABOUT USERNAME/PASSWORD VALIDATION"
Обязательна к прочтению.

chmod  2777 /mnt/fileserver
chown -R nobody:nobody /mnt/fileserver
find /mnt/fileserver -type f -exec chmod 666 {} \;

[global]
        workgroup = WORKGROUP1
        server string = file server
        security = share
        guest account = nobody
        default service = fileserver
        guest ok = yes

[fileserver]
        path = /mnt/fileserver
        guest ok = yes
        writable = yes
        force user = nobody
        force group = nobody
        read only = No
        force create mode = 0666
        force directory mode = 0777
        case sensitive = Yes