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406 lines
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
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<HTML>
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<HEAD>
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<title>Single-User Booting Under Linux Issue 19</title>
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</HEAD>
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<BODY BGCOLOR="#EEE1CC" TEXT="#000000" LINK="#0000FF" VLINK="#0020F0"
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ALINK="#FF0000">
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<!--endcut ============================================================-->
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<H4>
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"Linux Gazette...<I>making Linux just a little more fun!</I>"
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</H4>
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<P> <HR> <P>
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<!--===================================================================-->
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<center>
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<H2>Single-User Booting Under Linux</H2>
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<H4>By John Gatewood Ham,
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<a href="mailto:zappaman@alphabox.compsci.buu.ac.th">zappaman@alphabox.compsci.buu.ac.th</a></H4>
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</center>
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<P><HR>
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I was trained as a system administrator on HP, IBM, and Sun workstations
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while working as a DRT consultant assigned to Informix as an alpha-tester.
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There I learned the need for a true single-user operating mode in Unix.
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When I tried to use the single user mode with Linux, it did not work in the
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way that I expected. After many, many reboots I worked out the right
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configuration to support a true single-user mode on the distribution I was
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using, Slackware 3.2, by modifying the boot process.
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</P>
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<P>
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This article will now explain how to setup the bootup process for Linux so
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that single-user mode really works if you are using the Slackware 3.2
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distribution (or a derivative). I will begin by assuming that your kernel
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is correctly configured and that the <CODE>init</CODE> program starts successfully.
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See the Installation-HOWTO at
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<A HREF=ftp://sunsite.unc.edu/pub/Linux/docs/HOWTO/Installation-HOWTO><code>ftp://sunsite.unc.edu/pub/Linux/docs/HOWTO/Installation-HOWTO</code></A>
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for help to get this far. Once you have a system that boots, however, you
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have only begun. Why? Most distributions will give you a generic set of
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initialization scripts that are designed to work for an average installation.
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You will want to customize this in order to run extra things you want and/or
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to prevent running things you do not want. With the dozen or so standard
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startup scripts things can seem confusing, but after you read this article
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you should be able to understand enough to create a custom environment
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when you boot that exactly suits you.
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</P>
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<P>
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As I stated earlier, I will begin by assuming that <CODE>init</CODE> has started
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successfully. It will examine the file <CODE>/etc/inittab</CODE> to determine
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what to do. In that file are located the lines to activate your login
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devices such as terminals, modems, and your virtual consoles. Leave that
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stuff alone. What we are interested in are the lines which call the
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startup/shutdown scripts. These lines will look something like this:
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</P>
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<PRE>
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# Default runlevel.
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id:3:initdefault:
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# System initialization (runs when system boots).
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si::sysinit:/etc/rc.d/rc.S
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# Script to run when going single user (runlevel 1).
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l1:1:wait:/etc/rc.d/rc.K
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# Script to run when going single user (runlevel S or s)
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mm:S:wait:/etc/rc.d/rc.K2
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# Script to run when going multi user.
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rc:23456:wait:/etc/rc.d/rc.M
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# Runlevel 0 halts the system.
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l0:0:wait:/etc/rc.d/rc.0
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# Runlevel 6 reboots the system.
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l6:6:wait:/etc/rc.d/rc.6
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</PRE>
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<P>
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The comments are present and are very helpful. First you need to determine
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your default runlevel. In this case it is 3. The format of the <CODE>/etc/inittab</CODE>
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file section we are looking at is simple. Blank lines are ignored. Lines
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with '#' as the first character are comments and are ignored. Other lines
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have 4 parts separated by the colon character. These parts are 1. symbolic
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label, 2. runlevel, 3. action and 4.command to run. These are documented in
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the section 5 manual page for <CODE>/etc/inittab</CODE> (<CODE>man 5 inittab</CODE>). First
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we must find a line with an action of <CODE>initdefault</CODE>, and then see what runlevel
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it has. That will be the default runlevel. Obviously you should not have 2
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lines that have <CODE>initdefault</CODE> as the action in an <CODE>/etc/inittab</CODE> file. Once you know
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the default runlevel, you will be able to know what <CODE>/etc/inittab</CODE> entries
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will be processed by <CODE>init</CODE>. The 1 runlevel is considered single-user
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maintenance mode, but it supported multiple simultaneous logins in virtual
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terminals with the default <CODE>/etc/inittab</CODE> on my systems. You can prevent this by
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removing the 1 from the getty lines of the tty2, tty3, tty4, etc. The 3
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runlevel is considered the normal multi-user mode with full networking
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support. The S runlevel is supposed to be true single-user, and you can
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theoretically enter that runlevel using the <CODE>lilo</CODE> parameter
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<CODE>single</CODE>. However, for the Slackware 3.2 distribution, that does
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not put you in a single-user mode as you would expect, but instead you wind
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up in runlevel 3. The <CODE>/etc/inittab</CODE> file I show here does not have that problem
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however. Once you have read this article you can change the system to
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behave in the expected manner. So we know we will go to runlevel 3. That
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means <CODE>init</CODE> will perform every command in the <CODE>/etc/inittab</CODE> file that has a <CODE>sysinit</CODE>,
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then <CODE>boot</CODE>, or <CODE>bootwait</CODE>, and finally any entries for our runlevel 3. When you
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want to run a script when entering a runlevel, it doesn't make sense to have
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more than one script line in the <CODE>/etc/inittab</CODE> file for that level. Instead, you
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should put everything in 1 script, or call scripts from within the script
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mentioned in the <CODE>/etc/inittab</CODE> file using the dot method. Once thing to note is
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that field 2, the runlevel field, can have more than 1 runlevel specified.
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The <CODE>init</CODE> program will first run the <CODE>si</CODE> entry
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(and we will wait for it to finish running <CODE>/etc/rc.d/rc.S</CODE>) since it has
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<CODE>sysinit</CODE> (which implies <CODE>wait</CODE>) in the third field. Then it will run everything
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with 3 specified. So in our example file we will run the <CODE>si</CODE> target, then
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the <CODE>rc</CODE> target (and we will wait for it to finish running the
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<CODE>/etc/rc.d/rc.M</CODE> script since the third field is <CODE>wait</CODE>), and finally
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we it will do the <CODE>c1</CODE> through <CODE>c6</CODE> targets which set up the virtual ttys during
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a normal boot.
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</P>
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<P>
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If we boot (via <CODE>lilo</CODE>) and add the <CODE>single</CODE>
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parameter, we will still run the <CODE>si</CODE> target (<CODE>/etc/rc.d/rc.S</CODE>) and wait for it to
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complete, but then we will run the <CODE>mm</CODE> target (<CODE>/etc/rc.d/rc.K2</CODE>). Keep in mind
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that runlevel 1 and runlevel S are essentially the same when you enter them,
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but how you get there is very different. Runlevel 1 can be entered by using
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the command <CODE>/sbin/telinit 1</CODE>, but <CODE>/sbin/telinit s</CODE> will send you to runlevel 5
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often for some reason (some kind of bug). Runlevel 1 will give you a normal
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log in, and allows 1 user (<I>any</I> 1 user) to log in at the console.
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With this setup, runlevel S will give you a special root-only login that
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allows only root to use the console. Since only root can log in, only a
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special password prompt is displayed. If you press enter or ctl-D, the system
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will return to runlevel 3. This root-only login is accomplished by using
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the <CODE>/bin/sulogin</CODE> program. Runlevel S is probably what you want when you think
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single-user, but you have to reboot the machine and use <CODE>lilo</CODE> and have the
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single parameter to make it work. You can use runlevel 1 to accomplish the
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same things, but remember you will have to manually return to runlevel 3
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when you are done with another call to <CODE>/sbin/telinit 3</CODE> or a reboot, and you
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must insure that nobody else can get to the console but the root user.
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WARNING: The true single-user mode entered with the single parameter to
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<CODE>lilo</CODE> with my <CODE>/etc/inittab</CODE> and <CODE>/etc/rc.d/rc.K2</CODE> will support only 1 console and no other
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virtual terminals. Do not run anything that locks up the terminal!
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</P>
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<P>
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Ok, so what do we know now? We know what scripts <CODE>init</CODE> will call and when they
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will be called. But what can be in those scripts? The scripts should be
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written for bash unless you are a real guru and KNOW the other shell you
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wrote scripts for will be available during boot. There is nothing preventing
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you from using <CODE>perl</CODE> or <CODE>tcsh</CODE> or whatever, but traditionally most everyone uses
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<CODE>bash</CODE> scripts (ok, ok, Bourne shell scripts) for unix boot scripts. The <CODE>/etc/rc.d/rc.S</CODE>
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script which is called at system boot time should take care of things like
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<CODE>fsck</CODE>'ing your file systems, mounting them, and starting up swapping and other
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essential daemons. These are things that you need independent of runlevel.
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The <CODE>/etc/rc.d/rc.M</CODE> script which is called when you enter runlevel 3 should start all
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the processes that remain that you usually need during normal system
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operation EXCEPT things like <CODE>getty</CODE>. Processes that must be restarted whenever
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they stop running like <CODE>getty</CODE> should be placed in the <CODE>/etc/inittab</CODE> file instead of
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being started by a boot script. So what is in a typical <CODE>/etc/rc.d/rc.M</CODE> script?
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Usually configuring the network , starting web servers, sendmail, and
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anything else you want to always run like database servers, quota
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programs, etc.
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</P>
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<P>
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The only startup script I mention in my <CODE>/etc/inittab</CODE> that is not included in the
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Slackware 3.2 distribution is <CODE>/etc/rc.d/rc.K2</CODE>, and it is merely a modified version
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of <CODE>/etc/rc.d/rc.K</CODE> set up for single user mode. Remember this is the startup script
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that will be used if you choose to enter the <CODE>single</CODE> parameter to <CODE>lilo</CODE>. At
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the end of this file you will see a line:
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</P>
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<PRE>
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exec /bin/sulogin /dev/console
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</PRE>
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<P>
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This will replace the current process which is running the script with the
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<CODE>/bin/sulogin</CODE> program. This means, of course, that this has to be the last
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line in your script, since nothing after this line will be processed by <CODE>bash</CODE>.
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After that program starts, it displays a message to either enter the root
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password or press ctl-D. If you enter the correct root password, you will
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be logged in as root in a true single-user mode. Be careful, though,
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because when you exit that shell the machine will go into runlevel 3.
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If you want to reboot before entering runlevel 3 you must remember to do
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it (via <CODE>shutdown</CODE>) instead of just exiting the shell. If you press ctl-D
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instead of the root password, the system will enter runlevel 3. I have
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changed the incorrect calls to <CODE>kill</CODE> to use the <CODE>killall5</CODE> program, since
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the lines with <CODE>kill</CODE> caused <CODE>init</CODE> to be killed and a runlevel change was
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happening incorrectly.
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</P>
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<P>
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Well, I hope that this description of how I enabled my Linux machine to
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have a single-user mode similar to that of the big-name workstations
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proves helpful to you. Customizing your boot process is not too hard,
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once you understand something about how the <CODE>/etc/inittab</CODE>
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and <CODE>/etc/rc.d/*</CODE>
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scripts work. Be sure you 1. backup your entire system, 2. have a boot
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floppy, and 3. a rescue floppy that can restore the backup (or any
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individual files) you made in step 1 using the boot floppy from step 2 to
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boot the machine. If you make a 1 character typo you can prevent the
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machine from booting, so the backup steps, while tedious, are really
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necessary to protect yourself before you experiment.
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</P>
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<h1><CENTER>The Files</CENTER></h1>
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<P>
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Here are the files I used. Use at your own risk. They work for me, but
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may need to be modified to work for you.
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</P>
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<hr>
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<h2><Center><code>/etc/inittab</code></center></h2>
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<PRE>
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#
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# inittab This file describes how the INIT process should set up
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# the system in a certain run-level.
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#
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# Version: @(#)inittab 2.04 17/05/93 MvS
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# 2.10 02/10/95 PV
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#
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# Author: Miquel van Smoorenburg, <a href="mailto:miquels@drinkel.nl.mugnet.org">miquel@drinkel.nl.jugnet.org</a>
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# Modified by: Patrick J. Volkerding, <a href="mailto:volkerdi@ftp.cdrom.com">volkerdi@ftp.cdrom.com</a>
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# Modified by: John Gatewood Ham, <a href="mailto:zappaman@alphabox.compsci.buu.ac.th">zappaman@alphabox.compsci.buu.ac.th</a>
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#
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# Default runlevel.
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id:3:initdefault:
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# System initialization (runs when system boots).
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si::sysinit:/etc/rc.d/rc.S
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# Script to run when going maintenance mode (runlevel 1).
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l1:1:wait:/etc/rc.d/rc.K
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# Script to run when going single user (runlevel s)
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mm:S:wait:/etc/rc.d/rc.K2
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# Script to run when going multi user.
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rc:23456:wait:/etc/rc.d/rc.M
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# What to do at the "Three Finger Salute".
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# make the machine halt on ctl-alt-del
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ca::ctrlaltdel:/sbin/shutdown -h now "going down on ctl-alt-del"
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# Runlevel 0 halts the system.
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l0:0:wait:/etc/rc.d/rc.0
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# Runlevel 6 reboots the system.
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l6:6:wait:/etc/rc.d/rc.6
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# What to do when power fails (shutdown to single user).
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pf::powerfail:/sbin/shutdown -f +5 "THE POWER IS FAILING"
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# If power is back before shutdown, cancel the running shutdown.
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pg:0123456:powerokwait:/sbin/shutdown -c "THE POWER IS BACK"
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# If power comes back in single user mode, return to multi user mode.
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ps:S:powerokwait:/sbin/init 5
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# The getties in multi user mode on consoles an serial lines.
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#
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# NOTE NOTE NOTE adjust this to your getty or you will not be
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# able to login !!
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#
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# Note: for 'agetty' you use linespeed, line.
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# for 'getty_ps' you use line, linespeed and also use 'gettydefs'
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# we really don't want multiple logins in single user mode...
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c1:12345:respawn:/sbin/agetty 38400 tty1 linux
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c2:235:respawn:/sbin/agetty 38400 tty2 linux
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c3:235:respawn:/sbin/agetty 38400 tty3 linux
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c4:235:respawn:/sbin/agetty 38400 tty4 linux
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c5:235:respawn:/sbin/agetty 38400 tty5 linux
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c6:235:respawn:/sbin/agetty 38400 tty6 linux
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# Serial lines
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#s1:12345:respawn:/sbin/agetty 19200 ttyS0 vt100
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#s2:12345:respawn:/sbin/agetty 19200 ttyS1 vt100
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# Dialup lines
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#d1:12345:respawn:/sbin/agetty -mt60 38400,19200,9600,2400,1200 ttyS0 vt100
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#d2:12345:respawn:/sbin/agetty -mt60 38400,19200,9600,2400,1200 ttyS1 vt100
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# Runlevel 4 used to be for an X-window only system, until we discovered
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# that it throws init into a loop that keeps your load avg at least 1 all
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# the time. Thus, there is now one getty opened on tty1. Hopefully no one
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# will notice. ;^)
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# It might not be bad to have one text console anyway, in case something
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# happens to X.
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x1:4:wait:/etc/rc.d/rc.4
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# End of /etc/inittab
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</PRE>
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<hr>
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<h2><Center><code>/etc/rc.d/rc.K</code></center></h2>
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<PRE>
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# /bin/sh
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#
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# rc.K This file is executed by init when it goes into runlevel
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# 1, which is the administrative state. It kills all
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# deamons and then puts the system into single user mode.
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# Note that the file systems are kept mounted.
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#
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# Version: @(#)/etc/rc.d/rc.K 1.50 1994-01-18
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# Version: @(#)/etc/rc.d/rc.K 1.60 1995-10-02 (PV)
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#
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# Author: Miquel van Smoorenburg <a href="mailto:miquels@drinkel.nl.mugnet.org">miquels@drinkel.nl.mugnet.org</a>
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# Modified by: Patrick J. Volkerding <a href="mailto:volkerdi@ftp.cdrom.com">volkerdi@ftp.cdrom.com</a>
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# Modified by: John Gatewood Ham <a href="mailto:zappaman@alphabox.compsci.buu.ac.th">zappaman@alphabox.compsci.buu.ac.th</a>
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#
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# Set the path.
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PATH=/sbin:/etc:/bin:/usr/bin
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# Kill all processes.
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echo
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echo "Sending all processes the TERM signal."
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killall5 -15
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echo -n "Waiting for processes to terminate"
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for loop in 0 1 2 3 4 5 6 7 ; do
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sleep 1
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echo -n "."
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done
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echo
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echo "Sending all processes the KILL signal."
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killall5 -9
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# Try to turn off quota and accounting.
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if [ -x /usr/sbin/quotaoff ]
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then
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echo "Turning off quota.."
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/usr/sbin/quotaoff -a
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fi
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if [ -x /sbin/accton ]
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then
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echo "Turning off accounting.."
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/sbin/accton
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fi
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</PRE>
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<hr>
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<h2><Center><code>/etc/rc.d/rc.K2</code></center></h2>
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<PRE>
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# /bin/sh
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#
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# rc.K This file is executed by init when it goes into runlevel
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# 1, which is the administrative state. It kills all
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# deamons and then puts the system into single user mode.
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# Note that the file systems are kept mounted.
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#
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# Version: @(#)/etc/rc.d/rc.K 1.50 1994-01-18
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# Version: @(#)/etc/rc.d/rc.K 1.60 1995-10-02 (PV)
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#
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# Author: Miquel van Smoorenburg <a href="mailto:miquels@drinkel.nl.mugnet.org">miquels@drinkel.nl.mugnet.org</a>
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# Modified by: Patrick J. Volkerding <a href="mailto:volkerdi@ftp.cdrom.com">volkerdi@ftp.cdrom.com</a>
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# Modified by: John Gatewood Ham <a href="mailto:zappaman@alphabox.compsci.buu.ac.th">zappaman@alphabox.compsci.buu.ac.th</a>
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#
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# Set the path.
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PATH=/sbin:/etc:/bin:/usr/bin
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# Kill all processes.
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echo
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echo "Sending all processes the TERM signal."
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killall5 -15
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echo -n "Waiting for processes to terminate"
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for loop in 0 1 2 3 4 5 6 7 ; do
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sleep 1
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echo -n "."
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done
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echo
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echo "Sending all processes the KILL signal."
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killall5 -9
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# Try to turn off quota and accounting.
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if [ -x /usr/sbin/quotaoff ]
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then
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echo "Turning off quota.."
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/usr/sbin/quotaoff -a
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fi
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if [ -x /sbin/accton ]
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then
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echo "Turning off accounting.."
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/sbin/accton
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fi
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# Now go to the single user level
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exec /bin/sulogin /dev/console
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</PRE>
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<HR>
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<address>
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<A HREF="mailto:zappaman@alphabox.compsci.buu.ac.th">zappaman@alphabox.compsci.buu.ac.th</A>
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<BR>
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<A HREF="http://alphabox.compsci.buu.ac.th/~zappaman">Information about me.</A>
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</address>
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<center><H5>Copyright © 1997, John Gatewood Ham<BR>
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Published in Issue 19 of the Linux Gazette, July 1997</H5></center>
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