Hello,
I was under the impression the Linux (in my case the Fedora OS) is very secure. However I've learnt with deep concern that that one can have access to the system during system startup i.e one can give various startup directives and bypass the normal login UI to have direct root access.

Is there a way to disble this so that the directives during startup are fixed and cannot be altered. I would like to make the system secure to the maximum extent possible.
Any suggestions on this will be very helpful.

Regards

1 -- use password protection in your bootloader (lilo, grub, etc..). This prevents people from changing the boot command line (partition can still be accessed using live media - see #2).

2 -- consider an encrypted install if you're worried about physical machine security. This would be most all-around secure, and will also prevent someone from booting the machine into "init=/bin/bash" maintenance mode, or accessing it using live media..

You should first protect your system with BIOS password(though,even that can be altered by removing BIOS battery)
Then password protect the GRUB so that no one can login by going in to Single User Mode.
GRUB Password protect link: http://forums.fedoraforum.org/showthread.php?t=216700
And to prevent by-pass using Rescue Mode you need to have disable boot from CD/DVD and USB and password protect BIOS.

To secure a linux installation, or any server/desktop, you can start by:

1) Restrict access to the computer via Locked doors, cabinets, access cards and such. Cameras are nice too.
2) Lock the case (because someone can remove the cmos battery to reset the bios password)
3) Put a password on the BIOS
4) For more security restrictive areas, remove the CDROM and any USB ports (Ive had to unsolder them from motherboards for high secure areas)
5) For less security restrictive areas, disabling the CDROM and USB in the BIOS will be sufficient.
6) Install an OS that supports complete OFE (on the fly encryption) and encrypt all drives.
7) Install GRUB with a password

Extra paranoia:
8) Set screensaver to logout the user after a specified period of time 2 min or something.
9) Disable any unneeded tty sessions
10) Disable the ctrl-alt-backspace shortcut to disallow someone from quitting X
11) Never log in as root into a graphical shell

This list is just a starting point.

Filling USB ports with epoxy is also sometimes used.

Ugh! That would be hard to tolerate!

and don't enable ctrl-alt-del (assuming that its not the default)

..or just walk away from a machine on which you are logged in as root, even if it is the Boss's Boss on the phone...

TBH, you can see why most of the time, most people don't do these; there is a level of inconvenience involved, But then, no pain, no gain...

Thanks guys....That was really informative. I will try it out and get back.
Regards and Best Wishes to all.