Linux System Initialization
Now take a look at the sample Debian inittab. While similar to Red Hat's inittab, it also has some differences. First, you'll notice that while Red Hat used runlevel 3 for non-graphical mode and runlevel 5 for graphical mode, Debian uses runlevel 2 for both (see Listing 4, line 5). The difference is in Debian's use of a start/kill script for XDM.
I'd also like to draw your attention to a very special line, line 12. The line begins with “~~” (two tildes). Note that in single-user mode (state 1 or S), sulogin is called. This prevents someone from just booting the system and becoming root. While it doesn't prevent other tricks from being used to “back door” the system and isn't a substitute for physical security of the system, it does prevent the casual user from obtaining root access simply by rebooting. The use of the command:
boot from c: only, vice boot a: then c:
combined with password protection of the BIOS setup screens, and a lock on the case to prevent someone from resetting the BIOS on the motherboard, and finally setting LILO to 0 seconds, the computer is almost 50% of the way to being secured from unauthorized tampering. (You can get almost another 45% from the system itself, but note that the last 5% is effectively out of reach.)
Just below the script calls for each runlevel is another line to put a login screen up for root in runlevel 6. This is only for emergencies, should something go wrong with the kill scripts in runlevel 6 and the system does not halt properly. It should never run. (See Listing 3, lines 22 to 30).
The Debian inittab also includes some examples to enable gettys on modem and serial lines, should you find a use for them. The line that invokes mgetty, however, will obviously not work unless you've installed the mgetty package.
Following the logic through a boot-up, during a normal boot init knows it will run in state 2. Armed with this information and not overridden during boot-up, init first runs the /etc/init.d/boot script. Once this script has run, init then executes /etc/init.d/rc with an argument of 2. init also runs the commands associated with ca, kb, pf, pn and po. If you read up on powerfail, you'll see that nothing will happen until a change occurs with the power. Next, we see that init spawns gettys on the virtual terminals. In this case (runlevel 2), it will spawn six (see Listing 4, lines 50-55). The rest of the lines are commented out, and not used.
Looking at the /etc/init.d/rc script, you can see how it determines what to run to achieve a state change or to bring the system to the initial state.
Editing inittab or any of the rc scripts requires some degree of caution. Even the best tests cannot simulate a complete system reboot, and a script may appear to function properly after a system has initialized but fail during system initialization. The reasons are diverse, but usually involve getting things out of order.
In Caldera's Network Desktop, which ran on a 1.2.13 kernel and used modules, I had modified a script to start the kerneld process early in the boot sequence. When I upgraded the system to Caldera's OpenLinux v1.0 which ran a 2.0.25 kernel, I made the exact same changes to the same script, tested it and when I was satisfied all was well, I rebooted. Much to my dismay, the boot process hung, and guess where—yes, loading kerneld. I found that in the newer kernels, kerneld needed to know the host name of the computer, which was not yet available. Things like this can happen to anyone. Something as simple as typing the wrong key or forgetting to give the full path name of a file can leave you in the lurch.
Fortunately, you can pass boot-time parameters to init. When the system boots and you see: LILO:, you can press the shift key, then the tab key to see the kernel labels available for booting. You can then add a kernel label and follow it by any required parameters to boot the system. Any parameters the kernel needs are used and discarded. For example, if you have more than 64MB of RAM, you need to pass that information to the kernel in the form mem=96MB. If you pass the -b switch, the kernel won't use it, but will pass it on to init. The same goes for any single-digit number or the letters S or Q in either upper or lower case.
By passing any of the numbers or letters to init, we are overriding the defaults in inittab, as I stated earlier. Most of these numbers or letters do exactly what they would do if passed from a command line on a running system. However, the -b is special: it is the emergency boot parameter. This parameter tells init to read the inittab, but for some special exceptions not to execute any of the commands, just drop into maintenance mode. Thus, no rc scripts will be executed. You may mount the system read-write and fix it. One exception to not executing any inittab commands is the process id ~~ that should have as its process sulogin. This will give you a prompt for root's password so no unauthorized person can alter system files such as /etc/passwd or /etc/shadow.
What if you've made a mistake in the inittab file? Can the system be saved? Yes, but I must warn you not to do this unless absolutely necessary. Coded into the kernel is the instruction to start init once it is completely loaded and in memory. If the /etc/inittab is corrupted to the point that init can't run, not even with the -b switch (I've personally never seen this), it is possible to tell the Linux kernel to run a different program at bootup instead of init. Instead of issuing the -b switch, substitute init=/bin/sh after the kernel name. This will cause the kernel to run the bash shell, and you will be logged in as root. Be careful here, as nothing else is running, e.g., system logging or the update daemon. This is not a normal mode of operation for the system. Fix whatever is necessary and reboot.
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