Mastering Linux Security and Hardening

Password-protecting boot options

For any given Linux system, you'll have at least two boot options. You'll have the option to boot normally and the option to boot into recovery mode. Red Hat-type and Ubuntu-type operating systems are unique, in that they don't overwrite the old kernel when you do an operating system update. Instead, they install the new kernel along with the old one, and all the installed kernels have their own boot menu entries. On Red Hat-type systems, you'll never have more than five installed kernels because once you have five kernels installed, the oldest kernel will automatically get deleted the next time a new kernel is available in a system update. With Ubuntu-type systems, you'll need to manually delete the old kernels ...