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<HTML
><HEAD
><TITLE
>Building a root filesystem</TITLE
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="BUILDROOT"
>4. Building a root filesystem</A
></H1
><P
>Creating the root filesystem involves selecting files necessary for the
system to run. In this section we describe how to build a
<EM
>compressed root filesystem</EM
>. A less common option is to
build an uncompressed filesystem on a diskette that is directly mounted as
root; this alternative is described in <A
HREF="x1014.html#NONRAMDISKROOT"
>Section 9.1</A
>.</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN195"
>4.1. Overview</A
></H2
><P
>A root filesystem must contain everything needed to support a full Linux
system. To be able to do this, the disk must include the minimum requirements
for a Linux system:</P
><P
>&#13;<P
></P
><UL
><LI
><P
>The basic file system structure,</P
></LI
><LI
><P
>&#13;Minimum set of directories: <TT
CLASS="FILENAME"
>/dev</TT
>,
<TT
CLASS="FILENAME"
>/proc</TT
>,
<TT
CLASS="FILENAME"
>/bin</TT
>,
<TT
CLASS="FILENAME"
>/etc</TT
>,
<TT
CLASS="FILENAME"
>/lib</TT
>,
<TT
CLASS="FILENAME"
>/usr</TT
>,
<TT
CLASS="FILENAME"
>/tmp</TT
>,</P
></LI
><LI
><P
>Basic set of utilities: <TT
CLASS="FILENAME"
>sh</TT
>, <TT
CLASS="FILENAME"
>ls</TT
>, <TT
CLASS="FILENAME"
>cp</TT
>,
<TT
CLASS="FILENAME"
>mv</TT
>, etc.,</P
></LI
><LI
><P
>Minimum set of config files: <TT
CLASS="FILENAME"
>rc, inittab, fstab</TT
>, etc.,</P
></LI
><LI
><P
>Devices: <TT
CLASS="FILENAME"
>/dev/hd*, /dev/tty*, /dev/fd0</TT
>, etc.,</P
></LI
><LI
><P
>Runtime library to provide basic functions used by utilities.</P
></LI
></UL
>&#13;</P
><P
>Of course, any system only becomes useful when you can run something
on it, and a root diskette usually only becomes useful when you
can do something like:</P
><P
>&#13;<P
></P
><UL
><LI
><P
>&#13;Check a file system on another drive, for example to check your root file
system on your hard drive, you need to be able to boot Linux from another
drive, as you can with a root diskette system. Then you can run
<B
CLASS="COMMAND"
>fsck</B
> on your original root drive while it is not
mounted.&#13;</P
></LI
><LI
><P
> Restore all or part of your original root drive from backup using
archive and compression utilities such as <TT
CLASS="FILENAME"
>cpio</TT
>,
<TT
CLASS="FILENAME"
>tar</TT
>, <TT
CLASS="FILENAME"
>gzip</TT
> and
<TT
CLASS="FILENAME"
>ftape</TT
>.&#13;</P
></LI
></UL
>&#13;</P
><P
>&#13;We describe how to build a <EM
>compressed</EM
> filesystem, so
called because it is compressed on disk and, when booted, is uncompressed onto
a ramdisk. With a compressed filesystem you can fit many files (approximately
six megabytes) onto a standard 1440K diskette. Because the filesystem is much
larger than a diskette, it cannot be built on the diskette. We have to build
it elsewhere, compress it, then copy it to the diskette.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="CREATINGROOTFS"
>4.2. Creating the filesystem</A
></H2
><P
>In order to build such a root filesystem, you need a spare device that
is large enough to hold all the files before compression. You will
need a device capable of holding about four megabytes. There are
several choices:</P
><P
>&#13;<P
></P
><UL
><LI
><P
> Use a <EM
>ramdisk</EM
> (<SPAN
CLASS="SYMBOL"
>DEVICE</SPAN
> =
<TT
CLASS="FILENAME"
>/dev/ram0</TT
>). In this case, memory is used to simulate
a disk drive. The ramdisk must be large enough to hold a filesystem of the
appropriate size. If you use LILO, check your configuration file
(<TT
CLASS="FILENAME"
>/etc/lilo.conf</TT
>) for a line like <TT
CLASS="LITERAL"
>RAMDISK =
nnn</TT
> which determines the maximum RAM that can be allocated to a
ramdisk. The default is 4096K, which should be sufficient. You should
probably not try to use such a ramdisk on a machine with less than 8MB of
RAM.
Check to make sure you have a device like <TT
CLASS="FILENAME"
>/dev/ram0,
/dev/ram</TT
> or <TT
CLASS="FILENAME"
>/dev/ramdisk</TT
>. If not, create
<TT
CLASS="FILENAME"
>/dev/ram0</TT
> with <B
CLASS="COMMAND"
>mknod</B
> (major number
1, minor 0).&#13;</P
></LI
><LI
><P
>If you have an unused hard disk partition that is large enough (several
megabytes), this is acceptable.&#13;</P
></LI
><LI
><P
>Use a <EM
>loopback device</EM
>, which allows a disk file to be
treated as a device. Using a loopback device you can create a three
megabyte file on your hard disk and build the filesystem on it.</P
><P
>Type <B
CLASS="COMMAND"
>man losetup</B
> for instructions on using loopback
devices. If you don't have <B
CLASS="COMMAND"
>losetup</B
>, you can get it
along with compatible versions of <B
CLASS="COMMAND"
>mount</B
> and
<B
CLASS="COMMAND"
>unmount</B
> from the <TT
CLASS="FILENAME"
>util-linux</TT
> package
in the directory <A
HREF="ftp://ftp.win.tue.nl/pub/linux-local/utils/util-linux/"
TARGET="_top"
><TT
CLASS="FILENAME"
>ftp://ftp.win.tue.nl/pub/linux/utils/util-linux/</TT
></A
>.</P
><P
> If you do not have a loop device (<TT
CLASS="FILENAME"
>/dev/loop0</TT
>,
<TT
CLASS="FILENAME"
>/dev/loop1</TT
>, etc.) on your system, you will have to
create one with ``<B
CLASS="COMMAND"
>mknod /dev/loop0 b 7 0</B
>''. Once you've
installed these special <B
CLASS="COMMAND"
>mount</B
> and
<B
CLASS="COMMAND"
>umount</B
> binaries, create a temporary file on a hard disk
with enough capacity (eg, <TT
CLASS="FILENAME"
>/tmp/fsfile</TT
>). You can use a
command like:
<TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
> dd if=/dev/zero of=/tmp/fsfile bs=1k count=<TT
CLASS="VARNAME"
>nnn</TT
></PRE
></FONT
></TD
></TR
></TABLE
>
to create an <TT
CLASS="VARNAME"
>nnn</TT
>-block file.</P
><P
>Use the file name in place of <SPAN
CLASS="SYMBOL"
>DEVICE</SPAN
> below. When you issue a
mount command you must include the option <TT
CLASS="OPTION"
>-o loop</TT
> to tell
mount to use a loopback device.
&#13;</P
></LI
></UL
></P
><P
>After you've chosen one of these options, prepare the <SPAN
CLASS="SYMBOL"
>DEVICE</SPAN
>
with:
<TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
> dd if=/dev/zero of=<SPAN
CLASS="SYMBOL"
>DEVICE</SPAN
> bs=1k count=4096</PRE
></FONT
></TD
></TR
></TABLE
></P
><P
>This command zeroes out the device.</P
><DIV
CLASS="IMPORTANT"
><P
></P
><TABLE
CLASS="IMPORTANT"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="../images/important.gif"
HSPACE="5"
ALT="Important"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Zeroing the device is critical because the filesystem will be compressed
later, so all unused portions should be filled with zeroes to achieve maximum
compression. Keep this in mind whenever you move or delete files on the
filesystem. The filesystem will correctly de-allocate the blocks,
<EM
>but it will not zero them out again</EM
>. If you do a lot of
deletions and copying, your compressed filesystem may end up much larger than
necessary.</P
></TD
></TR
></TABLE
></DIV
><P
>&#13;
Next, create the filesystem. The Linux kernel recognizes two file system
types for root disks to be automatically copied to ramdisk. These are minix
and ext2, of which ext2 is preferred. If using ext2, you may find it useful
to use the <TT
CLASS="LITERAL"
>-N</TT
> option to specify more inodes than the
default; <TT
CLASS="LITERAL"
>-N 2000</TT
> is suggested so that you don't run out of
inodes. Alternatively, you can save on inodes by removing lots of unnecessary
<TT
CLASS="FILENAME"
>/dev</TT
> files. <B
CLASS="COMMAND"
>mke2fs</B
> will by default
create 360 inodes on a 1.44Mb diskette. I find that 120 inodes is ample on my
current rescue root diskette, but if you include all the devices in
<TT
CLASS="FILENAME"
>/dev</TT
> you will easily exceed 360. Using a compressed root
filesystem allows a larger filesystem, and hence more inodes by default, but
you may still need to either reduce the number of files or increase the number
of inodes. </P
><P
>So the command you use will look like:
<TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
> mke2fs -m 0 -N 2000 <SPAN
CLASS="SYMBOL"
>DEVICE</SPAN
></PRE
></FONT
></TD
></TR
></TABLE
></P
><P
>(If you're using a loopback device, the disk file you're using should be
supplied in place of this <SPAN
CLASS="SYMBOL"
>DEVICE</SPAN
>.)</P
><P
>The <B
CLASS="COMMAND"
>mke2fs</B
> command will automatically detect the
space available and configure itself accordingly. The
``<TT
CLASS="LITERAL"
>-m<>0</TT
>'' parameter prevents it from reserving space
for root, and hence provides more usable space on the disk.</P
><P
>Next, mount the device:
<TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
> mount -t ext2 <SPAN
CLASS="SYMBOL"
>DEVICE</SPAN
> /mnt</PRE
></FONT
></TD
></TR
></TABLE
>
(You must create a mount point <TT
CLASS="FILENAME"
>/mnt</TT
> if it
does not already exist.) In the remaining sections, all destination
directory names are assumed to be relative to <TT
CLASS="FILENAME"
>/mnt</TT
>.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN315"
>4.3. Populating the filesystem</A
></H2
><P
>Here is a reasonable minimum set of directories for your root filesystem
<A
NAME="AEN318"
HREF="#FTN.AEN318"
>[1]</A
>:
<P
></P
><UL
><LI
><P
><TT
CLASS="FILENAME"
>/dev</TT
> -- Device files, required to perform I/O</P
></LI
><LI
><P
><TT
CLASS="FILENAME"
>/proc</TT
> -- Directory stub required by the
proc filesystem</P
></LI
><LI
><P
><TT
CLASS="FILENAME"
>/etc</TT
> -- System configuration files</P
></LI
><LI
><P
><TT
CLASS="FILENAME"
>/sbin</TT
> -- Critical system binaries</P
></LI
><LI
><P
><TT
CLASS="FILENAME"
>/bin</TT
> -- Essential binaries considered part of the
system</P
></LI
><LI
><P
><TT
CLASS="FILENAME"
>/lib</TT
> -- Shared libraries to provide run-time
support</P
></LI
><LI
><P
><TT
CLASS="FILENAME"
>/mnt</TT
> -- A mount point for maintenance on other
disks</P
></LI
><LI
><P
><TT
CLASS="FILENAME"
>/usr</TT
> -- Additional utilities and applications</P
></LI
></UL
></P
><P
>Three of these directories will be empty on the root filesystem, so they
only need to be created with <B
CLASS="COMMAND"
>mkdir</B
>. The
<TT
CLASS="FILENAME"
>/proc</TT
> directory is basically a stub under which the
proc filesystem is placed. The directories <TT
CLASS="FILENAME"
>/mnt</TT
> and
<TT
CLASS="FILENAME"
>/usr</TT
> are only mount points for use after the boot/root
system is running. Hence again, these directories only need to be created.</P
><P
>The remaining four directories are described in the following sections.</P
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN352"
>4.3.1. /dev</A
></H3
><P
> A <TT
CLASS="FILENAME"
>/dev</TT
> directory containing a special file
for all devices to be used by the system is mandatory for any Linux
system. The directory itself is a normal directory, and can be
created with <TT
CLASS="LITERAL"
>mkdir</TT
> in the normal way. The device
special files, however, must be created in a special way, using the
<B
CLASS="COMMAND"
>mknod</B
> command.</P
><P
> There is a shortcut, though &#8212; copy devices files from your
existing hard disk <TT
CLASS="FILENAME"
>/dev</TT
> directory. The only requirement
is that you copy the device special files using <TT
CLASS="LITERAL"
>-R</TT
> option.
This will copy the directory without attempting to copy the contents of the
files. Be sure to use an <EM
>upper case <TT
CLASS="OPTION"
>R</TT
></EM
>.
For example:
<TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
> cp -dpR /dev/fd[01]* /mnt/dev
cp -dpR /dev/tty[0-6] /mnt/dev</PRE
></FONT
></TD
></TR
></TABLE
>
assuming that the diskette is mounted at <TT
CLASS="FILENAME"
>/mnt</TT
>. The
<TT
CLASS="LITERAL"
>dp</TT
> switches ensure that symbolic links are copied as links,
rather than using the target file, and that the original file attributes are
preserved, thus preserving ownership information.</P
><P
>If you want to do it the hard way, use <TT
CLASS="LITERAL"
>ls -l</TT
> to display the
major and minor device numbers for the devices you want, and create them on
the diskette using <TT
CLASS="LITERAL"
>mknod</TT
>.</P
><P
>However the devices files are created, check that any special devices
you need have been placed on the rescue diskette. For example,
<TT
CLASS="LITERAL"
>ftape</TT
> uses tape devices, so you will need to copy all of
these if you intend to access your floppy tape drive from the bootdisk.</P
><P
>Note that one inode is required for each device special file, and inodes
can at times be a scarce resource, especially on diskette filesystems. You'll
need to be selective about the device files you include. For example, if you
do not have SCSI disks you can safely ignore <TT
CLASS="LITERAL"
>/dev/sd*</TT
>; if
you don't intend to use serial ports you can ignore
<TT
CLASS="LITERAL"
>/dev/ttyS*</TT
>. </P
><P
>If, in building your root filesystem, you get the error <TT
CLASS="LITERAL"
>No
space left on device</TT
> but a <TT
CLASS="LITERAL"
>df</TT
> command shows space
still available, you have probably run out of inodes. A <TT
CLASS="LITERAL"
>df
-i</TT
> will display inode usage.</P
><DIV
CLASS="IMPORTANT"
><P
></P
><TABLE
CLASS="IMPORTANT"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="../images/important.gif"
HSPACE="5"
ALT="Important"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Be sure to include the following files from this directory:
<TT
CLASS="FILENAME"
>console</TT
>, <TT
CLASS="FILENAME"
>kmem</TT
>, <TT
CLASS="FILENAME"
>mem</TT
>, <TT
CLASS="FILENAME"
>null</TT
>,
<TT
CLASS="FILENAME"
>ram0</TT
> and <TT
CLASS="FILENAME"
>tty1</TT
>.</P
></TD
></TR
></TABLE
></DIV
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN388"
>4.3.2. /etc</A
></H3
><P
>The /etc directory contains configuration files. What it should contain
depends on what programs you intend to run.
On most systems, these can be divided into three groups:
<P
></P
><OL
TYPE="1"
><LI
><P
>Required at all times, <EM
>e.g.</EM
> <TT
CLASS="FILENAME"
>rc</TT
>,
<TT
CLASS="FILENAME"
>fstab</TT
>, <TT
CLASS="FILENAME"
>passwd</TT
>.</P
></LI
><LI
><P
>May be required, but no one is too sure.</P
></LI
><LI
><P
>Junk that crept in. </P
></LI
></OL
>
Files which are not essential can usually be identified with the command:
<TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
> ls -ltru</PRE
></FONT
></TD
></TR
></TABLE
>
This lists files in reverse order of date last accessed, so if any
files are not being accessed, they can be omitted from a root diskette.</P
><P
>On my root diskettes, I have the number of config files down to
15. This reduces my work to dealing with three sets of files:
<P
></P
><OL
TYPE="1"
><LI
><P
>The ones I must configure for a boot/root system:
<P
></P
><OL
TYPE="a"
><LI
><P
><TT
CLASS="FILENAME"
>rc.d/*</TT
> -- system startup and run level change scripts</P
></LI
><LI
><P
><TT
CLASS="FILENAME"
>fstab</TT
> -- list of file systems to be mounted </P
></LI
><LI
><P
><TT
CLASS="FILENAME"
>inittab</TT
> -- parameters for the
<B
CLASS="COMMAND"
>init</B
> process, the first process started at boot time.</P
></LI
><LI
><P
><TT
CLASS="FILENAME"
>gettydefs</TT
> -- parameters for the
<B
CLASS="COMMAND"
>init</B
> process, the first process started at boot time.</P
></LI
></OL
></P
></LI
><LI
><P
>The ones I should tidy up for a boot/root system:
<P
></P
><OL
TYPE="a"
><LI
><P
><TT
CLASS="FILENAME"
>passwd</TT
> -- Critical list of users, home directories, etc.</P
></LI
><LI
><P
><TT
CLASS="FILENAME"
>group</TT
> -- user groups. &#13;</P
></LI
><LI
><P
><TT
CLASS="FILENAME"
>shadow</TT
> -- passwords of users. You may not have this.&#13;</P
></LI
><LI
><P
><TT
CLASS="FILENAME"
>termcap</TT
> -- the terminal capability database.</P
></LI
></OL
></P
><P
>If security is important, <TT
CLASS="FILENAME"
>passwd</TT
> and
<TT
CLASS="FILENAME"
>shadow</TT
> should be pruned to avoid copying user passwords
off the system, and so that unwanted logins are rejected when you boot from
diskette.</P
><P
>&#13;
Be sure that <TT
CLASS="FILENAME"
>passwd</TT
> contains at least
<TT
CLASS="LITERAL"
>root</TT
>. If you intend other users to login, be sure their
home directories and shells exist.</P
><P
><TT
CLASS="FILENAME"
>termcap</TT
>, the terminal database, is typically several
hundred kilobytes. The version on your boot/root diskette should be pruned
down to contain only the terminal(s) you use, which is usually just the
<TT
CLASS="LITERAL"
>linux</TT
> or <TT
CLASS="LITERAL"
>linux-console</TT
> entry.</P
></LI
><LI
><P
>The rest. They work at the moment, so I leave them alone.</P
></LI
></OL
></P
><P
>Out of this, I only really have to configure two files, and what they
should contain is surprisingly small.
<P
></P
><UL
><LI
><P
><TT
CLASS="FILENAME"
>rc</TT
> should contain:
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
> #!/bin/sh
/bin/mount -av
/bin/hostname Kangaroo</PRE
></FONT
></TD
></TR
></TABLE
>
Be sure it is executable, be sure it has a "#!" line at the top, and be sure
any absolute filenames are correct. You don't really need to run
<B
CLASS="COMMAND"
>hostname</B
> &#8212; it just looks nicer if you do.&#13;</P
></LI
><LI
><P
><TT
CLASS="FILENAME"
>fstab</TT
> should contain at least:
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
> /dev/ram0 / ext2 defaults
/dev/fd0 / ext2 defaults
/proc /proc proc defaults</PRE
></FONT
></TD
></TR
></TABLE
>
You can copy entries from your existing <TT
CLASS="FILENAME"
>fstab</TT
>, but you
should not automatically mount any of your hard disk partitions; use the
<TT
CLASS="LITERAL"
>noauto</TT
> keyword with them. Your hard disk may be damaged
or dead when the bootdisk is used.</P
></LI
></UL
></P
><P
>Your <TT
CLASS="FILENAME"
>inittab</TT
> should be changed so that its
<TT
CLASS="LITERAL"
>sysinit</TT
> line runs <TT
CLASS="FILENAME"
>rc</TT
> or whatever
basic boot script will be used. Also, if you want to ensure that users on
serial ports cannot login, comment out all the entries for
<TT
CLASS="FILENAME"
>getty</TT
> which include a <TT
CLASS="FILENAME"
>ttys</TT
> or
<TT
CLASS="FILENAME"
>ttyS</TT
> device at the end of the line. Leave in the
<TT
CLASS="FILENAME"
>tty</TT
> ports so that you can login at the console.</P
><P
>A minimal <TT
CLASS="FILENAME"
>inittab</TT
> file looks like this:
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
> id:2:initdefault:
si::sysinit:/etc/rc
1:2345:respawn:/sbin/getty 9600 tty1
2:23:respawn:/sbin/getty 9600 tty2</PRE
></FONT
></TD
></TR
></TABLE
>
The <TT
CLASS="FILENAME"
>inittab</TT
> file defines what the system will run in
various states including startup, move to multi-user mode, etc.
Check carefully the filenames mentioned in <TT
CLASS="FILENAME"
>inittab</TT
>; if
<TT
CLASS="LITERAL"
>init</TT
> cannot find the program mentioned the bootdisk will
hang, and you may not even get an error message.</P
><P
>Note that some programs cannot be moved elsewhere because other programs have
hardcoded their locations. For example, on my system,
<TT
CLASS="FILENAME"
>/etc/shutdown</TT
> has hardcoded in it
<TT
CLASS="FILENAME"
>/etc/reboot</TT
>. If I move <TT
CLASS="FILENAME"
>reboot</TT
> to
<TT
CLASS="FILENAME"
>/bin/reboot</TT
>, and then issue a <TT
CLASS="LITERAL"
>shutdown</TT
>
command, it will fail because it cannot find the <TT
CLASS="LITERAL"
>reboot</TT
>
file.</P
><P
>For the rest, just copy all the text files in your
<TT
CLASS="FILENAME"
>/etc</TT
> directory, plus all the executables in your
<TT
CLASS="FILENAME"
>/etc</TT
> directory that you cannot be sure you do not need.
As a guide, consult the sample listing in <A
HREF="listings.html"
>Appendix C</A
>. Probably
it will suffice to copy only those files, but systems differ a great deal, so
you cannot be sure that the same set of files on your system is equivalent to
the files in the list. The only sure method is to start with
<TT
CLASS="LITERAL"
>inittab</TT
> and work out what is required.</P
><P
>Most systems now use an <TT
CLASS="FILENAME"
>/etc/rc.d/</TT
> directory
containing shell scripts for different run levels. The minimum is a single
<TT
CLASS="FILENAME"
>rc</TT
> script, but it may be simpler just to copy
<TT
CLASS="FILENAME"
>inittab</TT
> and the <TT
CLASS="FILENAME"
>/etc/rc.d</TT
>
directory from your existing system, and prune the shell scripts in the
<TT
CLASS="FILENAME"
>rc.d</TT
> directory to remove processing not relevent to a
diskette system environment.</P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN523"
>4.3.3. /bin and /sbin</A
></H3
><P
>&#13;The <TT
CLASS="FILENAME"
>/bin</TT
> directory is a convenient place for extra
utilities you need to perform basic operations, utilities such as
<B
CLASS="COMMAND"
>ls</B
>, <B
CLASS="COMMAND"
>mv</B
>, <B
CLASS="COMMAND"
>cat</B
> and
<B
CLASS="COMMAND"
>dd</B
>. See <A
HREF="listings.html"
>Appendix C</A
> for an example list
of files that go in a <B
CLASS="COMMAND"
>/bin</B
> and
<TT
CLASS="FILENAME"
>/sbin</TT
> directories. It does not include any of the
utilities required to restore from backup, such as <B
CLASS="COMMAND"
>cpio</B
>,
<B
CLASS="COMMAND"
>tar</B
> and <B
CLASS="COMMAND"
>gzip</B
>. That is because I
place these on a separate utility diskette, to save space on the boot/root
diskette. Once the boot/root diskette is booted, it is copied to the
ramdisk leaving the diskette drive free to mount another diskette, the
utility diskette. I usually mount this as <TT
CLASS="FILENAME"
>/usr</TT
>.</P
><P
> Creation of a utility diskette is described below in <A
HREF="x1014.html#UTILITYDISK"
>Section 9.2</A
>. It is probably desirable to maintain a copy of the
same version of backup utilities used to write the backups so you don't
waste time trying to install versions that cannot read your backup tapes.</P
><DIV
CLASS="IMPORTANT"
><P
></P
><TABLE
CLASS="IMPORTANT"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="../images/important.gif"
HSPACE="5"
ALT="Important"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Be sure to include the following programs: <TT
CLASS="FILENAME"
>init</TT
>,
<TT
CLASS="FILENAME"
>getty</TT
> or equivalent, <TT
CLASS="FILENAME"
>login</TT
>, <TT
CLASS="FILENAME"
>mount</TT
>, some
shell capable of running your rc scripts, a link from <TT
CLASS="FILENAME"
>sh</TT
> to the
shell.</P
></TD
></TR
></TABLE
></DIV
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN561"
>4.3.4. /lib</A
></H3
><P
>In <TT
CLASS="FILENAME"
>/lib</TT
> you place necessary shared libraries and
loaders. If the necessary libraries are not found in your
<TT
CLASS="FILENAME"
>/lib</TT
> directory then the system will be unable to boot.
If you're lucky you may see an error message telling you why.</P
><P
>&#13;Nearly every program requires at least the <TT
CLASS="FILENAME"
>libc</TT
>
library, <TT
CLASS="FILENAME"
>libc.so.</TT
><EM
>N</EM
>, where
<EM
>N</EM
> is the current version number. Check your
<TT
CLASS="FILENAME"
>/lib</TT
> directory. The file
<TT
CLASS="FILENAME"
>libc.so.N</TT
> is usually a symlink to a filename with a
complete version number:</P
><P
>&#13;<TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
>% ls -l /lib/libc*
-rwxr-xr-x 1 root root 4016683 Apr 16 18:48 libc-2.1.1.so*
lrwxrwxrwx 1 root root 13 Apr 10 12:25 libc.so.6 -&#62; libc-2.1.1.so*</PRE
></FONT
></TD
></TR
></TABLE
>&#13;</P
><P
> In this case, you want <TT
CLASS="FILENAME"
>libc-2.1.1.so</TT
>. To find
other libraries you should go through all the binaries you plan to include
and check their dependencies with <B
CLASS="COMMAND"
>ldd</B
>. For example:
<TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
> % ldd /sbin/mke2fs
libext2fs.so.2 =&#62; /lib/libext2fs.so.2 (0x40014000)
libcom_err.so.2 =&#62; /lib/libcom_err.so.2 (0x40026000)
libuuid.so.1 =&#62; /lib/libuuid.so.1 (0x40028000)
libc.so.6 =&#62; /lib/libc.so.6 (0x4002c000)
/lib/ld-linux.so.2 =&#62; /lib/ld-linux.so.2 (0x40000000)</PRE
></FONT
></TD
></TR
></TABLE
>
Each file on the right-hand side is required. The file may be a symbolic
link.</P
><P
> Note that some libraries are <EM
>quite large</EM
> and
will not fit easily on your root filesystem. For example, the
<TT
CLASS="FILENAME"
>libc.so</TT
> listed above is about 4 meg. You will
probably need to strip libraries when copying them to your root filesystem.
See <A
HREF="slimfast.html"
>Section 8</A
> for instructions.</P
><P
>In <TT
CLASS="FILENAME"
>/lib</TT
> you must also include a loader for the libraries.
The loader will be either <TT
CLASS="FILENAME"
>ld.so</TT
> (for A.OUT libraries,
which are no longer common) or <TT
CLASS="FILENAME"
>ld-linux.so</TT
> (for
ELF libraries). Newer versions of <B
CLASS="COMMAND"
>ldd</B
> tell you exactly
which loader is needed, as in the example above, but older versions may not.
If you're unsure which you need, run the <B
CLASS="COMMAND"
>file</B
> command on
the library. For example:
<TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
>% file /lib/libc.so.4.7.2 /lib/libc.so.5.4.33 /lib/libc-2.1.1.so
/lib/libc.so.4.7.2: Linux/i386 demand-paged executable (QMAGIC), stripped
/lib/libc.so.5.4.33: ELF 32-bit LSB shared object, Intel 80386, version 1, stripped
/lib/libc-2.1.1.so: ELF 32-bit LSB shared object, Intel 80386, version 1, not stripped</PRE
></FONT
></TD
></TR
></TABLE
>
The <TT
CLASS="LITERAL"
>QMAGIC</TT
> indicates that <TT
CLASS="LITERAL"
>4.7.2</TT
> is for
A.OUT libraries, and <TT
CLASS="LITERAL"
>ELF</TT
> indicates that
<TT
CLASS="LITERAL"
>5.4.33</TT
> and <TT
CLASS="LITERAL"
>2.1.1</TT
> are for ELF.
&#13;</P
><P
>Copy the specific loader(s) you need to the root filesystem you're building.
Libraries and loaders should be checked <EM
>carefully</EM
> against
the included binaries. If the kernel cannot load a necessary library, the
kernel may hang with no error message.</P
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PAMANDNSS"
>4.4. Providing for PAM and NSS</A
></H2
><P
>Your system may require dynamically loaded libraries that are not visible to
<TT
CLASS="FILENAME"
>ldd</TT
>. If you don't provide for these, you may have
trouble logging in or using your bootdisk.</P
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN612"
>4.4.1. PAM (Pluggable Authentication Modules)</A
></H3
><P
>If your system uses PAM (Pluggable Authentication Modules), you must make some
provision for it on your bootdisk. Briefly, PAM is a sophisticated modular
method for authenticating users and controlling their access to services. An
easy way to determine if your system uses PAM is run <TT
CLASS="FILENAME"
>ldd</TT
>
on your <TT
CLASS="FILENAME"
>login</TT
> executable; if the output includes
<TT
CLASS="FILENAME"
>libpam.so</TT
>, you need PAM.</P
><P
>Fortunately, security is usually of no concern with bootdisks since
anyone who has physical access to a machine can usually do anything they
want anyway. Therefore, you can effectively disable PAM by creating a
simple <TT
CLASS="FILENAME"
>/etc/pam.conf</TT
> file in your root filesystem that
looks like this:
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>OTHER auth optional /lib/security/pam_permit.so
OTHER account optional /lib/security/pam_permit.so
OTHER password optional /lib/security/pam_permit.so
OTHER session optional /lib/security/pam_permit.so</PRE
></FONT
></TD
></TR
></TABLE
>
Also copy the file <TT
CLASS="FILENAME"
>/lib/security/pam_permit.so</TT
> to
your root filesystem. This library is only about 8K so it imposes minimal
overhead.</P
><P
>This configuration allows anyone complete access to the files and
services on your machine. If you care about security on your bootdisk for
some reason, you'll have to copy some or all of your hard disk's PAM setup to
your root filesystem. Be sure to read the PAM documentation carefully, and
copy any libraries needed in <TT
CLASS="FILENAME"
>/lib/security</TT
> onto your root
filesystem.</P
><P
>You must also include <TT
CLASS="FILENAME"
>/lib/libpam.so</TT
> on your bootdisk.
But you already know this since you ran ldd on
<TT
CLASS="FILENAME"
>/bin/login</TT
>, which showed this dependency.</P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN627"
>4.4.2. NSS (Name Service Switch)</A
></H3
><P
>If you are using glibc (aka libc6), you will have to make provisions
for name services or you will not be able to login. The file
<TT
CLASS="FILENAME"
>/etc/nsswitch.conf</TT
> controls database lookups for
various servies. If you don't plan to access services from the
network (eg, DNS or NIS lookups), you need only prepare a simple
<TT
CLASS="FILENAME"
>nsswitch.conf</TT
> file that looks like this:
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
> passwd: files
shadow: files
group: files
hosts: files
services: files
networks: files
protocols: files
rpc: files
ethers: files
netmasks: files
bootparams: files
automount: files
aliases: files
netgroup: files
publickey: files</PRE
></FONT
></TD
></TR
></TABLE
>
This specifies that every service be provided only by local files.
You will also need to include
<TT
CLASS="FILENAME"
>/lib/libnss_files.so.</TT
><EM
>X</EM
>,
where <EM
>X</EM
> is 1 for glibc 2.0 and 2 for glibc 2.1.
This library will be loaded dynamically to handle the file lookups.</P
><P
>If you plan to access the network from your bootdisk, you may want to create a
more elaborate <TT
CLASS="FILENAME"
>nsswitch.conf</TT
> file. See the
<TT
CLASS="FILENAME"
>nsswitch</TT
> man page for details. You must include a file
<TT
CLASS="FILENAME"
>/lib/libnss_</TT
><EM
>service</EM
><TT
CLASS="FILENAME"
>.so.1</TT
>
for each <EM
>service</EM
> you specify.</P
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN643"
>4.5. Modules</A
></H2
><P
> If you have a modular kernel, you must consider which modules you
may want to load from your bootdisk after booting. You might want to
include <B
CLASS="COMMAND"
>ftape</B
> and <B
CLASS="COMMAND"
>zftape</B
> modules if
your backup tapes are on floppy tape, modules for SCSI devices if you have
them, and possibly modules for PPP or SLIP support if you want to access
the net in an emergency.</P
><P
>These modules may be placed in <TT
CLASS="FILENAME"
>/lib/modules</TT
>. You should also
include <B
CLASS="COMMAND"
>insmod</B
>, <B
CLASS="COMMAND"
>rmmod</B
> and <B
CLASS="COMMAND"
>lsmod</B
>. Depending on whether you
want to load modules automatically, you might also include <B
CLASS="COMMAND"
>modprobe</B
>,
<B
CLASS="COMMAND"
>depmod</B
> and <B
CLASS="COMMAND"
>swapout</B
>. If you use
<B
CLASS="COMMAND"
>kerneld</B
>, include it along
with <TT
CLASS="FILENAME"
>/etc/conf.modules</TT
>.</P
><P
>However, the main advantage to using modules is that you can move non-critical
modules to a utility disk and load them when needed, thus using less space on
your root disk. If you may have to deal with many different devices, this
approach is preferable to building one huge kernel with many drivers built in.</P
><DIV
CLASS="IMPORTANT"
><P
></P
><TABLE
CLASS="IMPORTANT"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="../images/important.gif"
HSPACE="5"
ALT="Important"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>In order to boot a compressed ext2 filesystem, you must have ramdisk and
ext2 support built-in. <EM
>They cannot be supplied as
modules.</EM
></P
></TD
></TR
></TABLE
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN664"
>4.6. Some final details</A
></H2
><P
> Some system programs, such as <B
CLASS="COMMAND"
>login</B
>, complain if
the file <TT
CLASS="FILENAME"
>/var/run/utmp</TT
> and the directory
<TT
CLASS="FILENAME"
>/var/log</TT
> do not exist. So:</P
><TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
> mkdir -p /mnt/var/{log,run}
touch /mnt/var/run/utmp</PRE
></FONT
></TD
></TR
></TABLE
><P
>Finally, after you have set up all the libraries you need, run
<B
CLASS="COMMAND"
>ldconfig</B
> to remake <TT
CLASS="FILENAME"
>/etc/ld.so.cache</TT
> on
the root filesystem. The cache tells the loader where to find the libraries.
You can do this with:
<TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
> ldconfig -r /mnt</PRE
></FONT
></TD
></TR
></TABLE
>
&#13;</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="WRAPPINGITUP"
>4.7. Wrapping it up</A
></H2
><P
>When you have finished constructing the root filesystem, unmount it, copy it
to a file and compress it:
<TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
> umount /mnt
dd if=<SPAN
CLASS="SYMBOL"
>DEVICE</SPAN
> bs=1k | gzip -v9 &#62; rootfs.gz</PRE
></FONT
></TD
></TR
></TABLE
>
When this finishes you will have a file <TT
CLASS="FILENAME"
>rootfs.gz</TT
>. This
is your compressed root filesystem. You should check its size to make sure it
will fit on a diskette; if it doesn't you'll have to go back and remove some
files. Some suggestions for reducing root filesystem size appear in <A
HREF="slimfast.html"
>Section 8</A
>.</P
></DIV
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN318"
HREF="buildroot.html#AEN318"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>The directory structure presented here is for root diskette use only.
Real Linux systems have a more complex and disciplined set of policies, called
the <A
HREF="http://www.pathname.com/fhs/2.0/fhs-toc.html"
TARGET="_top"
>Filesystem
Hierarchy Standard</A
>, for determining where files should go.)</P
></TD
></TR
></TABLE
><DIV
CLASS="NAVFOOTER"
><HR
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WIDTH="100%"
BORDER="0"
CELLPADDING="0"
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><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="x88.html"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
>Home</A
></TD
><TD
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><A
HREF="x688.html"
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></TD
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><TR
><TD
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><TD
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