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Introduction to Linux - A Hands on Guide
This guide was created as an overview of the Linux Operating System, geared toward new users as an exploration tour and getting started guide, with exercises at the end of each chapter.
For more advanced trainees it can be a desktop reference, and a collection of the base knowledge needed to proceed with system and network administration. This book contains many real life examples derived from the author's experience as a Linux system and network administrator, trainer and consultant. They hope these examples will help you to get a better understanding of the Linux system and that you feel encouraged to try out things on your own.
Click Here to receive this Complete Guide absolutely free.
By mchirico at 2005-03-15 06:44
On system boot-up a filesystem will be created and the contents from the CD will be uncompressed and completely loaded into RAM -- the CD could be removed at this point for boot-up on a second computer. You can take over any PC assuming you have configured the kernel with the appropriate drivers and the PC can boot from a CD.
QUICK INSTALL: Confirmation that Everything Works - Before Rolling You Own
Quick step 1:
Download "cdrom_linux_boot_proj1.iso" and burn this iso on your cdrom. Next, reboot the system. This is a check to see if the current kernel has been compiled with the necessary network, video, and keyboard (USB included) drivers. If critical drivers are not recognized they can be added into the kernel.
Quick step 2:
Download the complete project environment "proj1.tar.gz." This includes everything used to build and customize "cdrom_linux_boot_proj1.iso", including the BusyBox source, libraries, and ssh. The directory "_install" contains the necessary libraries for ssh, init and tcpdump. The 2.6.11 kernel ".config" can be found in the documents directory. The staging area contains a copy of the kernel bzImage, along with the isolinux boot loader.
The "proj1.tar.gz" file is about 13M, since it includes everything you need, except the kernel source, which can be downloaded from the following link:
You must "tar -xzf" this file as root to get all the necessary character and block devices. Standard user accounts cannot create all the necessary block and character devices that will be needed. Try "tar -xzf" under an account without root privileges to see the differences in the files created under "_install/dev".
If you're curious about the difference between character devices and block devices, you may want to reference the Linux Journal article
In summary, only block devices can mount filesystems. But character devices, such as a tape drive can contain data found on a block device.
Checking the download:
a. Check the md5sum to make sure you got the full copy, then:
$ tar -xzf" the package.
$ md5sum proj1.tar.gz
b. Now as root, so that you will get all "dev/" devices, run the following commands:
[Must be done as root]
$ su -
# cd <to project directory>
# tar -xzf proj1.tar.gz
If you did the above as root, then you should have the following:
[root@third-fl-71 _install]# ls -l dev
crw------- 1 root root 5, 1 Feb 25 20:41 console
lrwxrwxrwx 1 root root 11 Mar 6 09:22 core -> /proc/kcore
brw-rw-rw- 1 root floppy 2, 0 Feb 20 17:37 fd0
brw-rw---- 1 root disk 3, 0 Mar 5 04:32 hda
brw-rw---- 1 root disk 3, 1 Mar 5 20:37 hda1
brw-rw---- 1 root disk 3, 2 Mar 5 20:38 hda2
brw-rw---- 1 root disk 3, 3 Mar 5 21:02 hda3
brw-rw---- 1 root disk 3, 4 Mar 5 21:02 hda4
drwxr-xr-x 2 root root 4096 Mar 5 21:13 mapper
crw-rw-rw- 1 root root 1, 3 Feb 20 17:37 null
crw-rw-rw- 1 root tty 5, 2 Feb 26 12:54 ptmx
drwxr-xr-x 2 root root 4096 Mar 4 05:33 pts
brw------- 1 root disk 1, 0 Feb 20 17:37 ram0
brw-rw-rw- 1 root disk 1, 1 Feb 20 17:37 ram1
brw-rw-rw- 1 root disk 1, 2 Feb 20 17:37 ram2
brw-rw-rw- 1 root disk 1, 3 Feb 20 17:37 ram3
crw-rw-rw- 1 root root 1, 8 Feb 26 03:23 random
brw-rw---- 1 root disk 8, 0 Mar 5 04:32 sda
brw-rw---- 1 root disk 8, 1 Mar 5 04:32 sda1
brw-rw---- 1 root disk 8, 2 Mar 5 04:32 sda2
brw-rw---- 1 root disk 8, 3 Mar 5 21:02 sda3
brw-rw---- 1 root disk 8, 4 Mar 5 21:02 sda4
lrwxrwxrwx 1 root root 15 Mar 6 09:22 stderr -> /proc/self/fd/2
lrwxrwxrwx 1 root root 15 Mar 6 09:22 stdin -> /proc/self/fd/0
lrwxrwxrwx 1 root root 15 Mar 6 09:22 stdout -> /proc/self/fd/1
crw-rw-rw- 1 root root 5, 0 Feb 21 18:32 tty
crw-rw-rw- 1 root root 4, 0 Feb 20 17:37 tty0
crw-rw-rw- 1 root root 4, 1 Feb 20 22:38 tty1
crw-rw-rw- 1 root root 4, 2 Feb 26 08:24 tty2
crw-rw-rw- 1 root root 4, 3 Feb 26 08:24 tty3
crw-rw-rw- 1 root root 4, 4 Feb 26 08:24 tty4
crw-rw-rw- 1 root root 4, 5 Feb 26 08:24 tty5
crw------- 1 root root 4, 6 Mar 1 19:34 tty6
crw-rw---- 1 root root 4, 7 Mar 1 14:33 tty7
crw-rw---- 1 root root 4, 8 Mar 1 14:33 tty8
crw-rw---- 1 root tty 4, 9 Mar 1 14:33 tty9
cr--r--r-- 1 root root 1, 9 Feb 26 03:23 urandom
crw------- 1 vcsa tty 7, 0 Mar 1 14:33 vcs
crw-rw-rw- 1 root root 1, 5 Feb 26 03:23 zero
If you repeat the steps above using a standard account, a lot of the files will be missing.
c. Take a look at the file "proj1/createiso". This will create a filesystem on a loop back device with the mount point "./cdrom". Copy the contents of "_install", which has already been compiled with the necessary busybox code. Once it is copied, umount "./cdrom" so that it can be compressed. This will then get copied to the staging area where "mkisofs" will create a bootable CD image. The boot loader isolinux, along with the needed config files can be found in "proj1/staging_iso_image/boot/isolinux/"
There is a bash script "createiso" that performs the above tasks.
$ cd proj1
Take a look at "createiso". This bash script creates a virtual filesystem. For details on creating a virtual filesystem reference the following article on Freshmeat [ http://freshmeat.net/articles/view/1387/ ]
COMPREHENSIVE TUTORIAL: Building Everything from Downloaded Source
You may want to keep the results of the quick install steps above in a separate directory to compare against the completion of each step below.
You should take a look at the options that I have set by running "make menuconfig" or take a look at "chiricobusybox.config" directly.
$ make menuconfig
In particular, "Support version 2.6.x Linux kernels" is checked. Under "Login/Password Management Utilities" everything is checked EXCEPT "Use internal password and group functions". Note, at this stage you do NOT want to select this option because "/etc/passwd", "/etc/shadow" and "/etc/shadow-", will be copied to the "_install/etc" directory, and used instead.
Also, "lsmod", "modprobe", "rmmod" are checked. Although not essential for the initial build, this system, you are building, will support networking. You may eventually want to scp in modules and load them, once you get the system running, of course. There is plenty of space for these modules.
After taking a look at ".config", run "make" and "make install", which by default will install everything under "_install".
$ make install
During the final stages of "make install" the following message will be displayed.
You will probably need to make your busybox binary
setuid root to ensure all configured applets will
The next command must be executed after each "make install" to setuid root on the BusyBox binary.
$ chmod 4755 ./_install/bin/busybox
STEP 3: Needed Directories
Several directories need to be created under "_install". Take a look at the current contents.
$ cd _install
bin linuxrc sbin usr
Next create "dev" for "device entries", which will be populated later, "etc", "lib", "proc", "tmp", "var" and "sys" with the following commands.
The "dev" directory needs to be populated with device entries. They can either be copied from the current system or created with the "mknod -m " command. The following will have to be done as root.
Since there are a lot of files, and you may want to rebuild you "_install", it is recommended that these be put in a file. Below the file "createdev" is created with the following contents:
# put this in a file called createdev
cp -avp /dev/console dev
cp -avp /dev/core dev
cp -avp /dev/fd0 dev
cp -avp /dev/null dev
cp -avp /dev/ptmx dev
cp -avp /dev/pts dev
cp -avp /dev/ram0 dev
cp -avp /dev/ram1 dev
cp -avp /dev/ram2 dev
cp -avp /dev/ram3 dev
cp -avp /dev/random dev
cp -avp /dev/stderr dev
cp -avp /dev/stdin dev
cp -avp /dev/stdout dev
cp -avp /dev/tty dev
cp -avp /dev/tty0 dev
cp -avp /dev/tty1 dev
cp -avp /dev/tty2 dev
cp -avp /dev/tty3 dev
cp -avp /dev/tty4 dev
cp -avp /dev/tty5 dev
cp -avp /dev/tty6 dev
cp -avp /dev/tty7 dev
cp -avp /dev/tty8 dev
cp -avp /dev/tty9 dev
cp -avp /dev/urandom dev
cp -avp /dev/vcs dev
cp -avp /dev/zero dev
The "ls -al" command now shows the following contents.
# ls -l dev
crw------- 1 root root 5, 1 Feb 17 14:49 console
crw------- 1 root root 1, 6 Jan 30 2003 core
brw-rw---- 1 root floppy 2, 0 Jan 30 2003 fd0
crw-rw-rw- 1 root root 1, 3 Jan 30 2003 null
crw-rw-rw- 1 root root 5, 2 Mar 5 17:16 ptmx
drwxr-xr-x 2 root root 4096 Feb 17 09:48 pts
brw-rw---- 1 root disk 1, 0 Jan 30 2003 ram0
brw-rw---- 1 root disk 1, 1 Jan 30 2003 ram1
brw-rw---- 1 root disk 1, 2 Jan 30 2003 ram2
brw-rw---- 1 root disk 1, 3 Jan 30 2003 ram3
crw-r--r-- 1 root root 1, 8 Jan 30 2003 random
lrwxr-xr-x 1 root root 17 Mar 5 17:16 stderr -> ../proc/self/fd/2
lrwxr-xr-x 1 root root 17 Mar 5 17:16 stdin -> ../proc/self/fd/0
lrwxr-xr-x 1 root root 17 Mar 5 17:16 stdout -> ../proc/self/fd/1
crw-rw-rw- 1 root root 5, 0 Mar 3 21:20 tty
crw--w---- 1 root root 4, 0 Jan 30 2003 tty0
crw------- 1 root root 4, 1 Feb 17 14:49 tty1
crw------- 1 root root 4, 2 Feb 17 14:49 tty2
crw------- 1 root root 4, 3 Feb 17 14:49 tty3
crw------- 1 root root 4, 4 Feb 17 14:49 tty4
crw------- 1 root root 4, 5 Feb 17 14:49 tty5
crw------- 1 root root 4, 6 Feb 17 14:49 tty6
crw--w---- 1 root root 4, 7 Oct 24 2003 tty7
crw--w---- 1 root root 4, 8 Jan 30 2003 tty8
crw--w---- 1 root tty 4, 9 Jan 30 2003 tty9
crw-r--r-- 1 root root 1, 9 Feb 17 14:49 urandom
crw--w---- 1 vcsa tty 7, 0 Jan 30 2003 vcs
crw-rw-rw- 1 root root 1, 5 Jan 30 2003 zero
These files could all have been created with the "mknod" command. Taking a look at "tty" above, about half way down, it is a character device with a major number of 5 and a minor number of 0. It has rights rw-rw-rw. So the "tty" device could have been created with the command "mknod -m 666 dev/tty c 5 0" .
So, if you want to mount disk drives, ide (hda) and scsi (sda) consider executing the following commands:
mknod -m 660 dev/hda b 3 0
mknod -m 660 dev/hda1 b 3 1
mknod -m 660 dev/hda2 b 3 2
mknod -m 660 dev/hda3 b 3 3
mknod -m 660 dev/hda4 b 3 4
chown root.disk dev/hda*
mknod -m 660 dev/sda b 8 0
mknod -m 660 dev/sda1 b 8 1
mknod -m 660 dev/sda2 b 8 2
mknod -m 660 dev/sda2 b 8 3
mknod -m 660 dev/sda2 b 8 4
chown root.disk dev/sda*
After the PC is booted from the CD, you can mount these devices after creating a directory as the mount point "mkdir /h", then, it gets mounted as "mount -t ext2 /dev/hda2 /h".
The VolGroup would be mounted as ext3, most likely by doing the following after creating a mount point "/v1", then, "mount -t ext3 /dev/VolGroup00/LogVol01 /v1". But, would require the proper device drivers to be loaded in the kernel module, and the needed configuration in "/etc/rc.sysinit", notably the section under "# LVM2 initialization". All of this will be discussed in a future update of this article.
STEP 5: Needed Files and Directories (files and directories in "etc" and "var")
Create the necessary files in "etc". Exit out of root at this point, so that there is no chance of over-writing you system "/etc" -- note disaster is only a "/" away. WARNING: Never copy anything into a directory that starts with "/", since that is your current running system.
[Exit out of root]
The "etc/passwd" file is shown below. Since the ssh daemon will run, an account will be created for it. Note for sshd that login is set to "/bin/false"
Below "root" and "sshd" have been added to the group.
Note below that there is a password for the account root. This encrypted password is "root". You could create your own password here by copying an existing account password from "/etc/shadow" and "/etc/shadow-". The account "sshd" should have "*" for the password.
Interesting question regarding sshd: Why is "/dev/pts" necessary when sshing into this computer? If you are uncertain, remove this line and observe the results of the command "ps aux", when attempting to ssh in.
The file "etc/inittab" is called by the init program. There are no run levels with BusyBox. The lines "tty2::respawn:/sbin/getty 38400 tty2" allow you to enter "ctl-alt-F2" and get a login screen.
# This is run first except when booting in single-user mode.
::respawn:/sbin/getty 38400 tty1
# /sbin/getty invocations for selected ttys
#tty1::respawn:/sbin/getty 38400 tty1
tty2::respawn:/sbin/getty 38400 tty2
tty3::respawn:/sbin/getty 38400 tty3
tty4::respawn:/sbin/getty 38400 tty4
tty5::respawn:/sbin/getty 38400 tty5
tty6::respawn:/sbin/getty 38400 tty6
tty7::respawn:/sbin/getty 38400 tty7
tty8::respawn:/sbin/getty 38400 tty8
tty9::respawn:/sbin/getty 38400 tty9
# Example of how to put a getty on a serial line (for a terminal)
#::respawn:/sbin/getty -L ttyS0 9600 vt100
#::respawn:/sbin/getty -L ttyS1 9600 vt100
# Example how to put a getty on a modem line.
#::respawn:/sbin/getty 57600 ttyS2
# Stuff to do when restarting the init process
# Stuff to do before rebooting
::shutdown:/bin/umount -a -r
Note above, "inittab" calls "etc/init.d/rcS". The ram drive must be remounted; otherwise, it will be read only. Also, when the system boots, DHCP will be enabled. If the computer is not going to be connected to the network, comment this out, since it will repeatedly attempt to acquire an IP address. Also, if the proper NIC (Network Interface Card) is not found, you will inundated with messages.
# below getting rid of ram being mounted ro
/bin/mount -o remount /
# The following is for dhcp
ifconfig eth0 0.0.0.0
# Instead, if you want static IP address
#ifconfig eth0 192.168.1.13 netmask 255.255.252.0
#route add default gw 192.168.1.1
# Run ssh daemon
The file below, along with libraries /lib/libnss_* are necessary for password authentication, since the recent version of GNU Libc (glibc) uses Name Service Switch (NSS). This file can probably be copied from your system's "/etc/nsswitch.conf" file. If you don't have this file on your system, take the necessary files from "proj1.tar.gz".
# An example Name Service Switch config file. This file should be
# sorted with the most-used services at the beginning.
# The entry '[NOTFOUND=return]' means that the search for an
# entry should stop if the search in the previous entry turned
# up nothing. Note that if the search failed due to some other reason
# (like no NIS server responding) then the search continues with the
# next entry.
# Legal entries are:
# nisplus or nis+ Use NIS+ (NIS version 3)
# nis or yp Use NIS (NIS version 2), also called YP
# dns Use DNS (Domain Name Service)
# files Use the local files
# db Use the local database (.db) files
# compat Use NIS on compat mode
# hesiod Use Hesiod for user lookups
# [NOTFOUND=return] Stop searching if not found so far
# To use db, put the "db" in front of "files" for entries you want to be
# looked up first in the databases
#passwd: db files nisplus nis
#shadow: db files nisplus nis
#group: db files nisplus nis
#hosts: db files nisplus nis dns
hosts: files dns
# Example - obey only what nisplus tells us...
#services: nisplus [NOTFOUND=return] files
#networks: nisplus [NOTFOUND=return] files
#protocols: nisplus [NOTFOUND=return] files
#rpc: nisplus [NOTFOUND=return] files
#ethers: nisplus [NOTFOUND=return] files
#netmasks: nisplus [NOTFOUND=return] files
bootparams: nisplus [NOTFOUND=return] files
aliases: files nisplus
This file contains device names that permit root logins. For now, it makes sense for root to have lots of capabilities, for testing. "vc/1","vc/2" stands for virtual consoles.
Below is a minimal hosts file. If you are running on a local LAN without a DNS server, then, add in additional names. Note, if you ping localhost and get nothing, then, lo may need to be configured: "ifconfig lo 127.0.0.1". If you are not sure what is defined where "ifconfig" will give you a listing.
# Additional names can be added
This is BusyBox's minimal conf.
su = ssx root.0 # applet su can be run by anyone and runs with euid=0/egid=0
su = ssx # exactly the same
The following is used for acquiring an IP address via dhcp. The important setting here is the interface, which should be set to "eth0". The script that gets run "_install/usr/share/udhcpc/default.script" is the default BusyBox script for acquiring a dhcp address. Note, "etc/init.d/rcS" runs "/sbin/udhcpc", on bootup. You could remove this and assign a static IP address instead.
# Sample udhcpd configuration file (/etc/udhcpd.conf)
# The start and end of the IP lease block
# The interface that udhcpd will use
interface eth0 #default: eth0
# The maximum number of leases (includes addresses reserved
# by OFFER's, DECLINE's, and ARP conficts
#max_leases 254 #default: 254
# If remaining is true (default), udhcpd will store the time
# remaining for each lease in the udhcpd leases file. This is
# for embedded systems that cannot keep time between reboots.
# If you set remaining to no, the absolute time that the lease
# expires at will be stored in the dhcpd.leases file.
#remaining yes #default: yes
# The time period at which udhcpd will write out a dhcpd.leases
# file. If this is 0, udhcpd will never automatically write a
# lease file. (specified in seconds)
#auto_time 7200 #default: 7200 (2 hours)
# The amount of time that an IP will be reserved (leased) for if a
# DHCP decline message is received (seconds).
#decline_time 3600 #default: 3600 (1 hour)
# The amount of time that an IP will be reserved (leased) for if an
# ARP conflict occurs. (seconds
#conflict_time 3600 #default: 3600 (1 hour)
# How long an offered address is reserved (leased) in seconds
#offer_time 60 #default: 60 (1 minute)
# If a lease to be given is below this value, the full lease time is
# instead used (seconds).
#min_lease 60 #default: 60
# The location of the leases file
#lease_file /var/lib/misc/udhcpd.leases #default: /var/lib/misc/udhcpd.leases
# The location of the pid file
#pidfile /var/run/udhcpd.pid #default: /var/run/udhcpd.pid
# Everytime udhcpd writes a leases file, the below script will be called.
# Useful for writing the lease file to flash every few hours.
#notify_file #default: (no script)
#notify_file dumpleases # <--- useful for debugging
# The following are bootp specific options, setable by udhcpd.
#siaddr 192.168.0.22 #default: 0.0.0.0
#sname zorak #default: (none)
#boot_file /var/nfs_root #default: (none)
# The remainder of options are DHCP options and can be specified with the
# keyword 'opt' or 'option'. If an option can take multiple items, such
# as the dns option, they can be listed on the same line, or multiple
# lines. The only option with a default is 'lease'.
Copying utmp for the "who" command.
$ cd proj1/_install/var/run/
$ cp -pav /var/run/utmp .
STEP 6: Libraries
This configuration uses GNU Libc (glibc). In contrast, "uClibc" is smaller and easier to setup with ssh, since ssh or any program that makes a C call to "getpwname" will fail under glibc, if the appropriate Name Service Switch (NSS) files are not copied. Specifically, "/etc/nsswitch.conf", and "/lib/libnss_*" must be copied. Otherwise, running the simple ssh command will produce the following error:
you don't exist, go away!
You will get this error even if you have "etc/passwd", "etc/shadow",and "etc/shadow-", because getpwname cannot extract the correct values from "passwd". Here is a sample program to test this behavior. Reference the following program "getpwuid.c" in the following link
[ http://prdownloads.sourceforge.net/cpearls/simple_but_common_cpp.tar.gz ] if you are curious about this behavior.
For the most part, you can find which libraries are dynamically linked into a program by issuing the "ldd" command. Try this on your own system.
Download ssh, configure and make. Next, all files must be installed in the into the respective "_install/*" directories, without changing the keys, configs, or other files installed on your running environment. Furthermore, you'll need to have all the libraries linked correct. Basically, this means running "make install" in a "chroot" environment. But, if you "chroot", "/bin/bash" cannot be found. Plus, there are a few lib's needed with make.
Download the latest version of openssh. Note you should check the signed keys.
[**** Special Security Note ****]
In general it's good to verify keys with a trusted signature. Not covered here; but, it is something like the following. You'll need gpg. See Linux Tips in the reference.
Note, do this in the "proj1/_install" directory as root.
$ su -
$ cd proj1/_install
$ cp <location of openssh> .
$ tar -xzf openssh-3.9p1.tar.gz
$ cd openssh-3.9p1
$ ./configure --prefix=
This should leave you with the following. Note the location of the User binaries and "System binaries" when using "--prefix=" without any directory specified.
OpenSSH has been configured with the following options:
User binaries: /bin
System binaries: /sbin
Configuration files: /etc
Askpass program: /libexec/ssh-askpass
Manual pages: /man/manX
PID file: /var/run
Privilege separation chroot path: /var/empty
sshd default user PATH: /usr/bin:/bin:/usr/sbin:/sbin
Manpage format: doc
PAM support: no
KerberosV support: no
Smartcard support: no
S/KEY support: no
TCP Wrappers support: no
MD5 password support: no
IP address in $DISPLAY hack: no
Translate v4 in v6 hack: yes
BSD Auth support: no
Random number source: OpenSSL internal ONLY
At this point ssh should be installed with the correct references to "etc", and all other files.
STEP 8: Compiling the Linux Kernel
You do not need to download the full kernel on each revision. Note, with the "linux-2.6.11.tar.gz" there is also a patch "patch-2.6.11.bz2". The patch get's applied to the "linux-2.6.10" kernel, assuming you had already downloaded it.
$ cd linux-2.6.10 <-- note this is my existing source
$ bzip2 -dc ../patch-2.6.11.bz2 | patch -p1
$ cd ..
[********************* End Special Note ********************************************]
You may want to copy by kernel ".config", or take a look at it if downloading the ""cdrom_linux_boot_proj1.iso"" found all the drivers on your computer.
$ make xconfig -- This will allow you to graphically look at all the options.
$ make bzImage -- Only do this
$ cp arch/i386/boot/bzImage
That is it for create the compress kernel image. It may be necessary to add in additional drivers. What driver modules are used by your current system? Check "/etc/modprobe.conf"
$ cat "/etc/modprobe.conf"
alias eth0 tg3
Notice above that "eth0", the network interface card on my current system is loading this module. If I look for "tg3" in all the kernel "Makefile's", I can see that "CONFIG_TIGON3" should compiled into the kernel.
Check your device drives and set them to yes in the ".config" file. As you can see, the ".config" used for proj1 has this option set to yes so when the kernel boot it will automatically find the needed network drivers. These steps should be performed for any NICs, or USB devices needed at boot time.
There are two directories here "boot" and "isolinux", with all of the files under "isolinux". A description of each file is shown below:
boot.cat - The El Torito specification requires a "boot catalog" to be created.
isolinux.bin - This is the ISOLINUX boot loader program
isolinux.cfg - Configuration file that contains the necessary instructions for loading the kernel as shown directly below. The kernel is the file "linux26" and after the kernel loads, the compressed image file "initrd.bin" will be mounted on "/dev/ram0". After it gets mounted the program "/sbin/init", will start.