Study Guide for

Linux System Administration II

Lab work for LPI 102 (RPM)

released under the GFDL by LinuxIT

March 2004

Copyright (c) 2003 LinuxIT.

Permission is granted to copy, distribute and/or modify this document

under the terms of the GNU Free Documentation License, Version 1.2

or any later version published by the Free Software Foundation;

with the Invariant Sections being History, Acknowledgements, with the	Front-Cover Texts being “released under the GFDL by LinuxIT”.

GNU Free Documentation License

Version 1.2, November 2002

Copyright (C) 2000,2001,2002 Free Software Foundation, Inc.

59 Temple Place, Suite 330, Boston, MA 02111-1307 USA

Everyone is permitted to copy and distribute verbatim copies

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0. PREAMBLE

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5. COMBINING DOCUMENTS

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6. COLLECTIONS OF DOCUMENTS

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You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.

7. AGGREGATION WITH INDEPENDENT WORKS

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9. TERMINATION

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10. FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License "or any later version" applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.

Introduction:

Acknowledgements

The original material was made available by LinuxIT's technical training centre www.linuxit.com. Many thanks to Andrew Meredith for suggesting the idea in the first place. A special thanks to all the students who have helped dilute the technical aspects of Linux administration through their many questions, this has led to the inclusion of more illustrations attempting to introduce concepts in a userfriendly way. Finally, many thanks to Paul McEnery for the technical advice and for starting off some of the most difficult chapters such as the ones covering the X server (101), modems (102), security (102) and the Linux kernel (102).

The manual is available online at http://savannah.nongnu.org/projects/lpi-manuals/. Thank you to the Savannah Volunteers for assessing the project and providing us with the Web space.

History

First release (version 0.0) October 2003. Reviewed by Adrian Thomasset.

Second release (revision1) January 2003. Reviewed by Andrew Meredith

Release (version 0.1-test) March 2004. Reviewed by Adrian Thomasset.

Audience

This course is designed as a 3 to 4 days practical course preparing for the LPI 102 exam. It is recommended that candidates have at least one year experience doing Linux administration professionally. However for those who are ready for a challenge the training is designed to provide as much insight and examples as possible to help non specialists understand the basic concepts and command sets which form the core of Linux computing.

The LPI Certification Program

There are currently two LPI certification levels. The first level LPIC-1 is granted after passing both exams LPI 101 and LPI 102. Similarly passing the LPI 201 and LPI 202 exams will grant the second level certification LPIC-2.

There are no pre-requisites for LPI 101 and 102. However the exams for LPIC-2 can only be attempted once LPIC-1 has been obtained.

Exam Registration

In order to register for an LPI exam you first need to get a unique LPI at www.lpi.org. You will also need to register with one of the testing organisations such as www.vue.com or www.prometric.com

No Guarantee

The manual comes with no guarantee at all.

Resources

www.lpi.org

www.linux-praxis.de

www.lpiforums.com

www.tldp.org

www.fsf.org

www.linuxit.com

Notations

Commands and filenames will appear in the text in bold.

The <> symbols are used to indicate a non optional argument.

The [] symbols are used to indicate an optional argument

Commands that can be typed directly in the shell are highlighted as below

command

or

command

Table of Contents

The Linux Kernel	11

1. Kernel Concepts	11

2. The Modular Kernel	12

3. Routine Kernel Recompilation	14

4. Exercises	20

Booting Linux	22

1. Understanding Runlevels	22

2. The joys of inittab	24

3. LILO the Linux boot Loader	25

4. From boot to bash	26

5. Exercises	28

Managing Groups and Users	29

1. Creating new users	29

2. Working with groups	30

3. Configuration files	32

4. Command options	34

5. Modifying accounts and default settings	34

6. Exercises	36

Network Configuration	37

1. The Network Interface	37

2. Host Information	38

3. Stop and Start Networking	39

4. Routing	40

5. Common Network Tools	42

6. Exercises	44

TCP/IP Networks	45

1. Binary Numbers and the Dotted Quad	45

2. Broadcast Address, Network Address and Netmask	45

3. Network Classes	47

4. Subnets	48

5. The TCP/IP Suite	50

6. TCP/IP Services and Ports	51

7. Exercices	52

Network Services	54

1. The inetd daemon (old)	54

2. The xinetd Daemon	55

3. TCP wrappers	56

4. Setting up NFS	56

5. SMB and NMB	57

6. DNS services	59

7. Sendmail main Configuration	63

8. The Apache server	63

9. Exercises	65

Bash Scripting	67

1. The bash environment	67

2. Scripting Essentials	68

3. Logical evaluations	69

4. Flow Control and Loops	70

5. Expecting user input	72

6. Working with Numbers	72

7. Exercises	73

Basic Security	74

1. Local Security	74

2. Network Security	76

3. The Secure Shell	80

4. Time Configuration	81

5. Kernel security	82

Linux System Administration	85

1. Logfiles and configuration files	85

2. Log Utilities	87

3. Automatic Tasks	88

4. Backups and Compressions	90

5. Documentation	92

6. Exercises	95

Setting up PPP	96

1. Serial Modems	96

2. Dialup Configuration	97

3. pppd and chat	97

4. PPPD peers	98

5. Wvdial	99

Printing	100

1. Filters and gs	100

2. Printers and print queues	100

3. Printing Tools	101

4. The configuration files	102

5. Exercises	105

LPI 102 Objectives	106

Index	111

The Linux Kernel

1. Kernel Concepts

The two different types of Linux kernel are:

A:	Monolithic

A monolithic kernel is one which has support for all hardware, network, and filesystem

compiled into a single image file.

B:	Modular

A modular kernel is one which has some drivers compiled as object files, which the kernel can load and remove on demand. Loadable modules are kept in /lib/modules.

The advantage of a modular kernel is that it doesn’t always need to be recompiled when hardware is added or replaced on the system. Monolithic kernels boot slightly faster than modular kernels, but do not outperform the modular kernel

2. The Modular Kernel

Many components of the Linux kernel may be compiled as modules which the kernel can dynamically load and remove as required.

The modules for a particular kernel are stored in /lib/modules/<kernel-version>.

The best components to modularise are ones not required at boot time, for example peripheral devices and supplementary file systems.

Kernel modules are controlled by utilities supplied by the modutils package:

lsmod

rmmod

insmod

modprobe

modinfo

Many modules are dependant on the presence of other modules. A flat file database of module dependencies /lib/modules/<kernel-version>/modules.dep is generated by the depmod command. This command is run by the rc.sysinit script when booting the system.

-- modprobe will load any module and dependent modules listed in modules.dep

Search for example for modules that will be loaded at the same time as tvaudio.

grep tvaudio /lib/modules/kernel-version/modules.dep /lib/modules/kernel-version/kernel/drivers/media/video/tvaudio.o: \ /lib/modules/kernel-version/kernel/drivers/i2c/i2c-core.o

This means that the module i2c-core.o will also be loaded when using modprobe. This dependency is also apparent when listing the module with lsmod:

lsmod Module Size Used by Not tainted tvaudio 16796 0 (unused) i2c-core 19236 0 [tvaudio]

-- /etc/modules.conf is consulted for module parameters (IRQ and IO ports) but most often contains a list of aliases. These aliases allow applications to refer to a device using a common name. For example the first ethernet device is always referred to as eth0 and not by the name of the particular driver.

Fig1: Sample /etc/modules.conf file:

alias eth0 e100 alias usb-core usb-uhc alias sound-slot-0 i810_audio alias char-major-108 ppp_generic alias ppp-compress-18 ppp_mppe # 100Mbps full duplex options eth0 e100_speed_duplex=4

--modinfo will give information about modules.

modinfo tvaudio filename: /lib/modules/kernel-version/kernel/drivers/media/video/tvaudio.o description: "device driver for various i2c TV sound decoder / audiomux chips" author: "Eric Sandeen, Steve VanDeBogart, Greg Alexander, Gerd Knorr" license: "GPL" parm: debug int parm: probe short array (min = 1, max = 48), description "List of adapter,address pairs to scan additionally" parm: probe_range short array (min = 1, max = 48), description "List of adapter,start-addr,end-addr triples to scan additionally" parm: ignore short array (min = 1, max = 48), description "List of adapter,address pairs not to scan" parm: ignore_range short array (min = 1, max = 48), description "List of adapter,start-addr,end-addr triples not to scan" parm: force short array (min = 1, max = 48), description "List of adapter,address pairs to boldly assume to be present" parm: tda9874a_SIF int parm: tda9874a_AMSEL int parm: tda9874a_STD int parm: tda8425 int parm: tda9840 int

To get information only about parameter option use modinfo -p, to get information about the license type use modinfo -l , etc.

-- kmod is a mechanism that allows the kernel to automatically load modules as needed (one seldom needs to insert modules manually). This is in fact a statically compiled (resident) module that needs to be configured before compiling the kernel. The command used by the kernel to load the modules is defined in /proc/sys/kernel/modprobe.

3. Routine Kernel Recompilation

3.1 Source extraction

The kernel source is stored in the /usr/src/linux directory tree, which is a symbolic link to the

/usr/src/(kernel-version) directory. When extracting a new kernel source archive it is recommended to:

remove the symbolic link to the old kernel source directory tree

rm linux

Kernel sources which have been packaged as an RPM often create a link called linux-2-4

extract the new source archive (e.g linux-2.4.20.tar.bz2)

tar xjf linux-2.4.29.tar.bz2

Note: The archived 2.2 series kernels create a directory called linux instead of linux-version. This is why the first step is important, otherwise you may overwrite an old source tree with the new one. Since kernel 2.4 the name of the directory is linux-version.

create a symbolic link called linux from the newly created directory

ln -s linux-2.4.20 linux

The kernel is almost ready to be configured now, but first we need to make sure that all old binary files are cleared out of the source tree, and this is done with the make mrproper command.

Warning: this command will also delete the kernel configuration file .config discussed later.

cd /usr/src/linux make mrproper

Note: mrproper is a Scandinavian brand of cleaner that gets things “cleaner than clean”, it is one step beyond “make clean”.

3.2 Kernel Configuration

First edit the Makefile and make sure that the “EXTRAVERSION” variable is different from the existing version:

VERSION = 2

PATCHLEVEL = 4

SUBLEVEL = 20

EXTRAVERSION = -test

The kernel is now ready to be configured. This essentially means creating a configuration file called .config. This is done from the kernel source tree directory /usr/src/linux with any of the following

make menuconfig

make xconfig

make config

All these methods will save the configuration file as /usr/src/linux/.config

It is often easier to configure a new kernel using an older .config file by using the make oldconfig command. This will prompt the user only for new features in the kernel source tree (if the kernel is newer or has been patched).

Notice: Some distributions such as RedHat have a configs subdirectory containing files to be used as .config files with predefined configurations.

To enable kernel features (with make menuconfig) you will enter the top level category by moving with the arrow keys and pressing enter to access the desired category. Once in the particular category, pressing the space bar will change the kernel support for a feature or driver.

Possible support types are

supported (statically compiled) [*]

modular (dynamically compiled) [M]

not supported [ ]

The same choices are available with the other menu editors config and xconfig.

Troubleshooting: The make menuconfig target needs the ncurses header files. These are provided by the ncurses-devel package and must be installed for this target to work.

Fig 2: The make xconfig top level menu:

3.3 Kernel Compilation

make clean

The make command gets instructions from the Makefile and will build what is needed. If some files are already present make will use them as is. In particular files with *.o extensions. To make sure that all the configuration options in .config are used to rebuild the files needed one has to run make clean (this deletes *.o files)

Notice: you do not need to do “make clean” at this stage if you already prepared the source directory with “make mrproper”

make dep

Once the kernel configuration is complete, it is necessary to reflect these choices in all the subdirectories of the kernel source tree. This is done with the make dep command. Files called .depend containing paths to header files present in the kernel source tree (/usr/src/linux/include) are generated with the dep target..

The kernel itself is compiled compiled with one of the commands:

make zImage

make bzImage

When the command exits without any errors, there will be a file in the /usr/src/linux/ directory called vmlinux. This is the uncompressed kernel.

The two other commands will write an additional file in /usr/src/linux/arch/i386/boot/ called zImage and bzImage respectively. These are compressed kernels using gzip and bzip2. See the next section Installing the New Kernel to find out how to proceed with these files.

make modules

The modules are compiled with make modules.

make modules_install

Once the modules are compiled they need to be copied to the corresponding subdirectory in /lib/modules. The make modules_install command will do that.

The sequence of commands are depicted in Fig 3.

Fig 3: kernel compilation commands:

make dep make clean make bzImage make modules make modules_install

3.4 Installing a New Kernel

The new kernel can be found in /usr/src/linux/arch/i386/boot/bzImage, depending on your architecture of your system. This file must be copied to the /boot directory, and named vmlinuz-<full-kernel-version>

cp /usr/src/linux/arch/i386/boot/bzImage /boot/vmlinuz-<full-kernel-version>

Next the /etc/lilo.conf or /boot/grub/grub.conf file needs to be edited to add our newly compiled kernel to the boot menu. Copy the “image” section from your existing kernel and add a new image section at the bottom of the file, as shown below:

Editing the /etc/lilo.conf file

prompt timeout=50 message=/boot/message image=/boot/vmlinuz label=linux root=/dev/hda6 Existing section read-only image=/boot/vmlinuz-<full-kernel-version> label=linux-new Added section root=/dev/hda6 read-only ----------snip-------------------------------

The symbol table for the various kernel procedures can be copied to the /boot directory:

cp /usr/src/linux/System.map /boot/System.map-<full-kernel-version>

3.5 The full kernel version

On a system, the version of the running kernel can be printed out with

uname -r

This kernel version is also displayed on the virtual terminals if the \k option is present in /etc/issue.

3.5 Initial Ramdisks

If any dynamically compiled kernel modules are required at boot time (e.g a scsi driver, or the filesystem module for the root partition) they will be loaded using an initial ramdisk.

The initial ramdisk is created with the mkinitrd command which only takes two parameters: the filename, and the kernel version number.

If you use an initial ramdisk then you will need to add an initrd= line in your /etc/lilo.conf

mkinitrd /boot/initrd-full-version.img full-version

3.6 Optional

It is recommended to copy the /usr/src/linux/.config file to /boot/config-<fiull-kernel-version>, just to keep track of the capabilities for the different kernels that have been compiled.

3.7 Rerunning LILO

Finally lilo needs to be run in order to update the boot loader . First lilo can be run in test mode to see if there are any errors in the configuration file:

NOTICE
The LILO bootloader needs to be updated using lilo every time a changed is made in /etc/lilo.conf

4. Exercises

Before starting with the exercises make sure you don’t have an existing kernel tree in /usr/src/. If you do, pay attention to the /usr/src/linux symbolic link.

1. Manually recompile the kernel following the compilation steps.

- Get the kernel-version.src.rpm package from rpmfind or a CD. Installing this package will also give you a list of dependencies, such as the gcc compiler or binutils package if they haven't yet been met.

- Install the package with –i (this will put all the code in /usr/src/ )

- Go into the /usr/src/linux-version directory and list the configs directory

- Copy the kernel config file that matches your architecture into the current directory and call it .config

- Run

make oldconfig

at the command line to take into account this new .config file.

- Edit the Makefile and make sure the version is not the same as your existing kernel. You can get information on your current kernel by running uname –a at the command line or list the /lib/modules directory.

- Run

make menuconfig (or menu or xconfig)

and remove ISDN support from the kernel.

- When you exit the above program the .config file is altered but the changes have not yet taken place in the rest of the source tree. You next need to run

make dep

- Finally to force new object files (.o) to be compiled with these changes you delete all previously compiled code with

make clean

- You can now build the kernel the modules and install the modules with:

make bzImage modules modules_install

- The modules are now installed in the /lib/modules/version directory. The kernel is called bzImage and is in the following directory:

/usr/src/linux/arch/i386/boot/

We need to manually install this kernel (2 steps):

(i)

cp /usr/src/linux/arch/i386/boot/bzImage /boot/vmlinuz-<full-kernel-version>

(ii) That was easy! now edit /etc/lilo.conf and add an ‘image’ paragraph that will tell LILO where to find this kernel and the root filesystem.

- Run /sbin/lilo and reboot

2. Since we downloaded the kernel-version.src.rpm package we can now use this package to recompile a ‘RedHat preconfigured’ kernel. Notice that although no intervention is needed you won’t be able to change the .config menu.

- First rebuild the compiled binary package with

rpm --rebuild kernel-version.src.rpm (...wait!)

- This will eventually generate the kernel-version.i368.rpm in /usr/src/redhat/RPMS/i386/.

- Next, upgrade you kernel with the RPM manager using the –U option.

Booting Linux

Overview

Taking a closer look at the booting process helps troubleshooting when dealing with both hardware and administrative tasks.

We first focus on the role of the init program and its' associated configuration file /etc/inittab. The role of LILO at boot time is investigated in greater depth. Finally we summarize the booting process. The document "From Power to Bash Prompt" written by Greg O'Keefe as well as the boot(7) manpage are both good references for this module.

1. Understanding Runlevels

Unlike most non-UNIX operating systems which only have 2 modes of functionality (on and off), UNIX operating systems, including Linux, have different runlevels such as "maintenance" runlevel or "multi-user" runlevel, etc. Runlevels are numbered from 0 to 6.

Listing 1: Linux runlevels

Runlevel 0 shuts down the machine safely, Runlevel 6 restarts the machine safely Runlevel 1 is single user mode Runlevel 2 is multi-user mode, but does not start NFS Runlevel 3 is full multi-user mode Runlevel 4 is not defined and generally unused Runlevel 5 is like runlevel 3 but runs a Display Manager as well

Both init and telinit are used to switch from one runlevel to another. Remember that init is the first program launched after the kernel has been initialised at boot time. The PID for init is always 1.

Listing 2: The PID for init is always 1

[root@nasaspc /proc]# ps uax |grep init USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND root 1 0.2 0.0 1368 52 ? S 20:17 0:04 init [3]

At each runlevel the system will stop or start a set of specific services. These programs are kept in /etc/rc.d/init.d. This directory contains all the services that the system may run. Once these programs are launched they will stay active until a new runlevel is called. The following services are also called daemons.

Listing 3: List of typical services (or daemons) in /etc/rc.d/init.d/

ls /etc/rc.d/init.d/
anacron	cups	identd	kadmin	krb5kdc	mcserv	nscd	random	smb	xfs
apmd	dhcpd	innd	kdcrotate	kudzu	named	ntpd	rawdevices	snmpd	xinetd
arpwatch	functions	ipchains	keytable	ldap	netfs	pcmcia	rhnsd	squid	
atd	gpm	iptables	killall	linuxconf	network	portmp	rwhod	sshd	
autofs	halt	irda	kprop	lpd	nfs	pgsql	sendmail	syslog	
crond	httpd	isdn	krb524	marsrv	nfslock	pppoe	single	tux	

Note: It is possible to stop or start manually a given daemon in /etc/rc.d/init.d by giving the appropriate argument. For example if you want to restart the apache server you would type:

/etc/rc.d/init.d/httpd restart

When working with runlevels you will instruct a specific predefined set of programs to run and another predefined set of programs to stop running. Say you want to be in runlevel 2, you would type

/sbin/init 2

This in turn forces init to read its configuration file /etc/inittab to find out what should happen at this runlevel.

In particular (assuming we are switching to runlevel 2) the following line in inittab is executed:

l2:wait:/etc/rc.d/rc 2

If you look in /etc/inittab the “/etc/rc.d/rc N” command starts all services in the /etc/rc.d/rcN.d starting with an S and will stop of services starting with a K. These services are symbolic links pointing to the rc-scripts in /etc/rc.d/init.d.

If you don't want a process to run in a given runlevel N you can delete the corresponding symlink in /etc/rc.d/rN.d beginning with a K.

2. The joys of inittab

As promised let's take a look at /etc/inttab.

The file has the following structure:

id : runlevel : action : command

Figure 3: the /etc/inittab file:

id:3:initdefault: # System initialization. si::sysinit:/etc/rc.d/rc.sysinit l0:0:wait:/etc/rc.d/rc 0 l1:1:wait:/etc/rc.d/rc 1 l2:2:wait:/etc/rc.d/rc 2 l3:3:wait:/etc/rc.d/rc 3 l4:4:wait:/etc/rc.d/rc 4 l5:5:wait:/etc/rc.d/rc 5 l6:6:wait:/etc/rc.d/rc 6 -----------------------snip---------------------------------- # Trap CTRL-ALT-DELETE ca::ctrlaltdel:/sbin/shutdown -t3 -r now -----------------------snip---------------------------------- # Run gettys in standard runlevels 1:2345:respawn:/sbin/mingetty tty1 2:2345:respawn:/sbin/mingetty tty2 3:2345:respawn:/sbin/mingetty tty3 4:2345:respawn:/sbin/mingetty tty4 5:2345:respawn:/sbin/mingetty tty5 6:2345:respawn:/sbin/mingetty tty6 # Run xdm in runlevel 5 x:5:respawn:/etc/X11/prefdm –nodaemon

The id field can be anything. If a runlevel is specified then the command and the required action will be performed only at that specific runlevel. If no number is specified then the line is executed at any run level.

Recognisable features in the /etc/inittab file:

The default runlevel: this is set at the beginning of the file with the id id and the action initdefault. Notice that no command is given. This line simply tells init what the default runlevel is.

First program called by init: /etc/rc.d/rc.sysinit. This script sets system defaults such as the PATH variable, determines if networking is allowed, the hostname, etc ...

Default runlevel services: If the default runlevel is 3 then only the line "l3" will be executed. The action is "wait", no other program is launched until all services in run level 3 are running.

The getty terminals: The lines with id's 1-to-6 launch the virtual terminals. This is where you can alter the number of virtual terminals.

Runlevel 5: The final line in inittab launches the Xwindow manager if runlevel 5 is reached.

Remarks:

1. You can set a modem to listen for connections in inittab. If your modem is linked to /dev/ttyS1 then the following line will allow data connections (no fax) after 2 rings:

S1:12345:respawn:/sbin/mgetty -D -x 2 /dev/ttyS1

2. When making changes to /etc/inittab you need to force init to reread this configuration file. This is most easily done using:

/sbin/init q

3. LILO the Linux boot Loader

Information needed by the loader is updated by /sbin/lilo (the bootloader installer) which in turn reads its' configuration file /etc/lilo.conf.

During bootup LILO needs to know essential information such as where the kernel is kept (usually in /boot) and where the filesystem root partition is.

LILO has no understanding of filesystem layout or of where things are. Only offsets on the physical disks. If you are installing a second Linux distribution B that is not running while setting up lilo.conf, you will need to mount partitions such as the /boot partition of B. You must also keep track of where B's root partition is.

init parameters:

Likewise, LILO can also parse the runlevel parameters to init. Once the kernel is loaded, init takes over the booting process. If no parameters are given, init will launch the default runlevel specified in /etc/inittab.

Passing runlevel instructions to init at the LILO prompt

Boot: linux s

Passing Kernel parameters:

Parameters for the kernel can be parsed at the LILO prompt or specified in /etc/lilo.conf with the append option.

Examples

append= "pci=bisoirq" append=”ram=16M” append=”/dev/hdc=ide-scsi” (for CD writers)

Parameters parsed to the kernel at boot time are intended for modules that have been compiled into the kernel, and often help detecting hardware.

During bootup all kernel messages are logged to /var/log/dmesg by default. This file can either be read or flushed to stdout with the /bin/dmesg utility.

4. From boot to bash

We can now attempt to go through the steps a Linux system goes through while booting.

If an initial ram disk is specified it is loaded here. Modules are inserted from the initial ram disk.

The kernel is loaded from the medium, specified in LILO's configuration. As it loads it is decompressed.

The kernel then mounts the root (/) filesystem in accordance with the configuration it receives from LILO (usually read-only).

Hence essential programs in /bin and /sbin are made available.

The kernel then loads init - the first 'userspace' process.

Init reads /etc/inittab and follows its' instructions. In particular rc.sysinit is run. A filesystem integrity check (fsck) is done on the filesystems in accordance with entries in /etc/fstab.

Next init goes into the default runlevel, the gettys start and the boot process is over.

The prompt to login is now managed by the gettys on the ttys. After the user has typed in their username and pressed return;

/bin/login is started.

The user is prompted by /bin/login for the password. The user enters a password and presses return.

The password the user is compared to the password in /etc/passwd or /etc/shadow.

5. Exercises

Take a look at the boot(7) manpage, it covers most of what we did in this module.

1. Use init to change you current runlevel (e.g switch between runlevel 3 and 5).

How do you know what your current runlevel is?

2. Enable the Ctrl+Alt+Del in runlevel 3 only.

How can you force init to read its’ configuration file?

3. Add a new login prompt on tty7.

4. Use dmesg to read the chipset of your ethernet card.

5. Investigate differences between shutdown, halt and reboot.

Which option to shutdown will force an fsck at the next boot?

6. Use the tools chkconfig or ntsysv to disable the sshd daemon in runlevel 2,3,4, and 5

Verify that the symbolic links in the rc2.d, rc3.d, rc4.d and rc5.d directories have changed.

7. Reboot the system. At the boot prompt give the appropriate init= parameter to skip /sbin/init and start a simple bash session.

Managing Groups and Users

1. Creating new users

Step 1: Create an account

The /usr/sbin/useradd command adds new users to the system and the symbolic link adduser points to it.

Syntax:

useradd [options] login-name

Example: add a user with login-name rufus

useradd rufus

Default values will be used when no options are specified. You can list these values with useradd –D.

Default options listed with useradd –D

GROUP=100

HOME=/home

INACTIVE=-1

EXPIRE=

SHELL=/bin/bash

SKEL=/etc/skel

Notice that this information is also available in the file /etc/default/useradd

Step 2: Activate the account with a new password

To allow a user to access his or her account the administrator must allocate a password to the user using the passwd tool.

Syntax:

passwd login-name

These steps create a new user. This has also defined the user’s environment such as a home directory and a default shell. The user has also been assigned to a group, his primary group.

2. Working with groups

Every new user is assigned to an initial (or primary) group. Two conventions exist.

Traditionally this primary group is the same for all users and is called users with a group id (GID) of 100. Many Linux distributions adhere to this convention such as Suse and Debian.

The User Private Group scheme (UPG) was introduced by RedHat and changes this convention without changing the way in which UNIX groups work. With UPG each new user belongs to their own primary group. The group has the same name as the login-name (default), and the GID is in the 500 to 60000 range (same as UIDs).

As a consequence, when using the traditional scheme for groups the user’s umask (see LPI 101) is set to 022, whereas in the UPG scheme the umask is set to 002.

Belonging to groups

A user can belong to any number of groups. However at any one time (when creating a file for example) only one group is the effective group.

The list of all groups a user belongs to is obtained with either the groups or id commands.

Example for user root:

List all ID's:

id
► uid=0(root) gid=0(root) groups=0(root), 1(bin), 2(daemon), 3(sys), 4(adm), 6(disk), 10(wheel), 600(sales)

List all groups:

groups
► root bin daemon sys adm disk wheel sales

Joining a group

Joining a group changes the user’s effective group and starts a new session from which the user can then logout. This is done with the newgrp command.

Example: joining the sales group

	newgrp sales

If the groups command is issued, the first group on the list would no longer be root but sales.

Creating a new group

The groupadd tool is used to administer groups. This will add an entry in the /etc/group file.

Example: Create the group devel

groupadd devel

Adding a user to a group

Administration tasks can be carried out with the gpasswd tool. One can add (-a) or remove (-d) users from a group and assign an administrator (-A). The tool was originally designed to set a single password on a group, allowing members of the same group to login with the same password. For security reasons this feature no longer works.

Example: Add rufus to the group devel

gpasswd -a rufus devel

3. Configuration files

The /etc/passwd and /etc/shadow files:

The names of all the users on the system are kept in /etc/passwd. This file has the following stucture:

Login name Password (or x if using a shadow file) The UID The GID Text description for the user The user's home directory 7.The user's shell

These 7 fields are separated by colons. As in the example below.

/etc/passwd entry with encrypted passwd:

george:$1$K05gMbOv$b7ryoKGTd2hDrW2sT.h:Dr G Micheal:/home/georges:/bin/bash

In order to hide the encrypted passwords from ordinary users you should use a shadow file. The /etc/shadow file then holds the user names and encrypted passwords and is readable only by root.

If you don't have a shadow file in /etc then you should issue the following command:

/usr/sbin/pwconv (passwd -> shadow)

This will leave an 'x' in the 2nd field of /etc/passwd and create the /etc/shadow file. If you don't wish to use shadow passwords you can do so using

/usr/sbin/pwunconv (shadow -> passwd)

Caution: When using a shadow password file the /etc/passwd file may be world readable (644) and the /etc/shadow file must be more restricted (600 or even 400). However when using pwunconv make sure to change the permissions on /etc/password (600 or 400).

The /etc/group and gshadow files:

In the same way, information about groups is kept in /etc/group. This file has 4 fields separated by colons.

Group name The group password (or x if gshadow file exists) The GID A comma separated list of members

Example /etc/group entry:

java:x:550:jade, eric, rufus

As for users there is a /etc/gshadow file that is created when using shadow group passwords. The utilities used to switch backwards and forward from shadow to non-shadow files are as follow

/usr/sbin/grpconv creates the /etc/gshadow file

/usr/sbin/grpunconv deletes the gshadow file

The /etc/login.defs and /etc/skel/ files

The /etc/login.defs file contains the following information:

the mail spool directory:

MAIL_DIR

password aging controls:

PASS_MAX_DAYS, PASS_MIN_DAYS, PASS_MAX_LEN, PASS_WARN_AGE

max/min values for automatic UID selection in useradd:

UID_MIN, UID_MAX

max/min values for automatic GID selection in groupadd:

GID_MIN, GID_MAX

automatically create a home directory with useradd:

CREATE_HOME

The /etc/skel directory contains default files that will be copied to the home directory of newly created users: .bashrc, .bash_profiles, ...

4. Command options

useradd (options)

-c	comment (Full Name)
-d	path to home directory
-g	initial group (GID). The GID must already exist
-G	comma separated list of supplementary groups
-u	user’s UID
-s	user’s default shell
-p	password (md5 encrypted, use quotes!)
-e	account expiry date
-k	the skel directory
-n	switch off the UPG group scheme

groupadd (options)

-g	assign a GID

5. Modifying accounts and default settings

All available options while creating a user or a group can be modified. The usermod utility has the following main options:

usermod (options)

-d	the users directory
-g	the users initial GID
-l	the user's login name
-u	the user's UID
-s	the default shell.

Notice these options are the same as for useradd.

Likewise, you can change details about a group with the groupmod utility. There are mainly two options:

groupmod (options)

-g	the GID
-n	the group name.

Locking an account

A user’s account can be locked by prefixing an exclamation mark to the user’s password. This can also be done with the following command line tools:

Lock	Unlock
passwd -l	passwd -u
usermod -L	usermod -U

When using shadow passwords, replace the x with a *

A less useful option is to remove the password entirely with passwd -d.

Finally, one can also assign /bin/false to the user’s default shell in /etc/passwd.

Changing the password expiry dates:

By default a user’s password is valid for 99999 days, that is 273,9 years (default PASS_MAX_DAYS). The user is warned for 7 days that his password will expire (default PASS_WARN_AGE) with the following message as he logs in:

Warning: your password will expire in 6 days

There is another password aging policy number that is called PASS_MIN_DAYS. This is the minimum number of days before a user can change his password; it is set to zero by default.

The chage tool allows an administrator to change all these options.

Usage: chage [ -l ] [ -m min_days ] [ -M max_days ] [ -W warn ]

[ -I inactive ] [ -E expire ] [ -d last_day ] user

The first option –l lists the current policy values for a user. We will only discuss the –E option. This locks an account at a given date. The date is either in UNIX days or in YYYY/MM/DD format.

Notice that all these values are stored in the /etc/shadow file, and can be edited directly.

Removing an account:

A user’s account may be removed with the userdel command line. To make sure that the user’s home directory is also deleted use the -r option.

userdel -r jade

6. Exercises

1. Creating users

Use adduser to create a user called tux with user ID 600 and group ID 550

Use usermod to change this user’s home directory.

Does the new directory need to be created?

Is the content of /etc/skel copied to the new directory?

Can the contents of the old home directory still be accessed by user tux?

Use usermod to add tux to the group wheel.

2. Working with groups

Create a group called sales using groupadd.

Add tux to this group using gpasswd.

Login as tux and join the group sales using newgrp.

3. Conifiguration files

Add a user to the system by editing /etc/passwd and /etc/group

Create a group called share and add user tux to this group by manually editing /etc/group

4. Modifying an Account

Change the expiry date for user tux’s account using usermod.

Lock the user’s account. (Use tools or edit /etc/shadow ...)

Prevent the user from login in by changing the user’s default shell to /bin/false

Change the PASS_MAX_DAYS for user tux to 1 in /etc/shadow

5. Changing default settings

Use useradd -D to change the system's default settings such that every new user will be

assigned /bin/sh instead of /bin/bash. (Notice that this will change the file in /etc/defaults/)

Edit /etc/login.defs and change the default PASS_MAX_DAYS so that new users need to change their password every 5 days

Network Configuration

1. The Network Interface

The network interface card (NIC) must be supported by the kernel. To determine which card you are using you can get information from dmesg, /proc/interrupts, /sbin/lsmod. or /etc/modules.conf

Example:

dmesg
► Linux Tulip driver version 0.9.14 (February 20, 2001)
PCI: Enabling device 00:0f.0 (0004 -> 0007)
PCI: Found IRQ 10 for device 00:0f.0
eth0: Lite-On 82c168 PNIC rev 32 at 0xf800, 00:A0:CC:D3:6E:0F, IRQ 10.
eth0: MII transceiver #1 config 3000 status 7829 advertising 01e1.

cat /proc/interrupts
► 0: 8729602 XT-PIC timer
1: 4 XT-PIC keyboard
2: 0 XT-PIC cascade
7: 0 XT-PIC parport0
8: 1 XT-PIC rtc
10: 622417 XT-PIC eth0
11: 0 XT-PIC usb-uhci
14: 143040 XT-PIC ide0
15: 180 XT-PIC ide1

/sbin/lsmod
► Module Size Used by
tulip 37360 1 (autoclean)

From the example above we see that the Ethernet card’s chipset is Tulip, the i/o address is 0xf800 and the IRQ is 10. This information can be used either if the wrong module is being used or if the resources (i/o or IRQ) are not available.

This information can either be used to insert a module with a different i/o address (using the modprobe or insmod utilities) or can be saved in /etc/modules.conf (this will save the settings for the next bootup).

2. Host Information

The following files are used to store networking information.

/etc/resolv.conf contains a list of DNS servers

nameserver 192.168.1.108
nameserver 192.168.1.1
search linuxit.org

/etc/HOSTNAME is used to give a name to the PC

One can also associate a name to a network interface. This is done in differently across distributions.

/etc/hosts contains your machine's IP number as well as a list of known hosts

# Do not remove the following line, or various programs
# that require network functionality will fail.
127.0.0.1 localhost localhost.localdomain
# other hosts
192.168.1.108 mesa mesa.domain.org
192.168.1.119 pico

/etc/sysconfig/network defines if networking must be started. (can also contain the HOSTNAME variable)

NETWORKING=yes
HOSTNAME=mesa.domain.org
GATEWAY=192.168.1.1
GATEWAYDEV=

/etc/sysconfig/network-scripts/ifcfg-eth0 The configuration parameters for eth0

DEVICE=eth0
BOOTPROTO=none
BROADCAST=192.168.1.255
IPADDR=192.168.1.108
NETWORK=192.168.1.0
ONBOOT=yes
USERCTL=no

3. Stop and Start Networking

● From the command line

The main tool used to bring up the network interface is /sbin/ifconfig. Once initialised the kernel module aliased to eth0 in /etc/modules.conf (e.g tulip.o) is loaded and assigned an IP and netmask value.

As a result the interface can be switched on and off without loosing this information as long as the kernel module is inserted.

Examples: Using ifconfig.

/sbin/ifconfig eth0 192.168.10.1 netmask 255.255.128.0
/sbin/ifconfig eth0 down
/sbin/ifconfig eth0 up

Another tool is /sbin/ifup. This utility reads the system’s configuration files in /etc/sysconfig/ and assigns the stored values for a given interface. The script for eth0 is called ifcfg-eth0 and has to be configured. If a boot protocol such as DHCP is defined then ifup will start the interface with that protocol.

Examples: Using ifup.

/sbin/ifup eth0
/sbin/ifup ppp0
/sbin/ifdown eth0

●. Using the network script

At boot time the ethernet card is initialised with the /etc/rc.d/init.d/network script. All the relevant networking files are sourced in the /etc/sysconfig/ directory.

In addition the script also reads the sysctl options in /etc/sysctl.conf, this is where you can configure the system as a router (allow IP forwarding in the kernel). For example the line:

net.ipv4.ip_forward = 1

will enable ip forwarding and the file /proc/sys/net/ipv4/ip_forward will contain a one.

The network script is started with the following command

/etc/rc.d/init.d/network restart

●. Renewing a DHCP lease

The following tools can query the DHCP server for a new IP:

pump

dhcpclient

A client daemon exists called dhcpcd (do not confuse this with the DHCP server daemon dhcpd)

4. Routing

A noticeable difference when using ifup is the system’s routing table. This is because either the /etc/sysconfig/network file is read, where a default gateway is stored, or the DHCP server has sent this information together with the IP number. The routing tables are configured, checked and changed with the /sbin/route tool.

Routing examples:

Add a static route to the network 10.0.0.0 through the device eth1 and use 192.168.1.108 as the gateway for that network:

/sbin/route add -net 10.0.0.0 gw 192.168.1.108 dev eth1

Add a default gateway:

/sbin/route add default gw 192.168.1.1 eth0

Listing the kernel routing table:

/sbin/route -n
► Kernel IP routing table
Destination Gateway Genmask Iface
192.168.1.0 0.0.0.0 255.255.255.0 eth0
10.1.8.0 192.168.1.108 255.0.0.0 eth1
127.0.0.0 0.0.0.0 255.0.0.0 lo
0.0.0.0 192.168.1.1 0.0.0.0 eth0

Default Gateway:

In the last listing, the Destination field is a list of networks. In particular, 0.0.0.0 means ‘anywhere’. With this in mind, there are two IP’s in the Gateway field. Which one is the default gateway ?

 To avoid having to enter static routes by hand special daemons

gated or routed

are run to dynamically update routing tables across a network

If you belong to the 192.168.10.0 network and you add a route to the 192.168.1.0 network you may find that machines in the latter network are not responding. This is because no route has been set from the 192.168.1.0 network back to your host!! This problem is solved using dynamic routing.

Permanent Static Routes

If you have several networks with more than one gateway you can use the /etc/sysconfig/static-routes (instead of routing daemons). These routes will be added at boot time by the network script.

A routing scenario:

5. Common Network Tools

Here is a short list of tools helpful when trouble shouting network connections.

ping host:

This tool sends an ICMP ECHO_REQUEST datagram to a host and expects an ICMP ECHO_RESPONSE.

Options for ping:

-b	ping a broadcast address

-c N	send N packets

-q	quiet mode: display only start and end messages

netstat:

You may get information on current network connections, the routing table or interface statistics depending on the options used.

Options for netstat:

-r	same as /sbin/route

-I	display list of interfaces

-n	don’t resolve IP addresses

-p	returns the PID and names of programs (only for root)

-v	verbose

-c	continuous update

Example: Output of netstat –-inet –n :

►	Active Internet connections (w/o servers)

Proto Recv-Q Send-Q Local Address Foreign Address State

tcp 0 0 192.168.1.10:139 192.168.1.153:1992 ESTABLISHED

tcp 0 0 192.168.1.10:22 192.168.1.138:1114 ESTABLISHED

tcp 0 0 192.168.1.10:80 192.168.1.71:18858 TIME_WAIT

In the above listing you can see that the local host has established connections on ports 139, 22 and 80.

arp:

Display the kernel address resolution cache.

Example:

arp
► Address HWtype HWaddress Iface
192.168.1.71 ether 00:04:C1:D7:CA:2D eth0

traceroute:

Displays the route taken from the local host to the destination host. Traceroute forces intermediate routers to send back error messages (ICMP TIME_EXCEEDED) by deliberately setting the tty (time to live) value too low. After each TIME_EXEEDED notification traceroute increments the tty value, forcing the next packet to travel further, until it reaches its’ destination.

Example:

CMD:	/usr/sbin/traceroute -n www.redhat.com

►	traceroute: Warning: www.redhat.com has multiple addresses; using 216.148.218.197

traceroute to www.redhat.com (216.148.218.197), 30 hops max, 38 byte packets

1 192.168.1.1 0.440 ms 0.347 ms 0.341 ms

---- snip ---

14 12.122.2.145 112.116 ms 110.908 ms 112.002 ms

15 12.122.2.74 156.629 ms 157.028 ms 156.857 ms

16 12.122.255.222 156.867 ms 156.641 ms 156.623 ms

17 216.148.209.66 159.982 ms 157.462 ms 158.537 ms

18 216.148.218.197 157.395 ms 156.789 ms 156.080 ms

Options for traceroute:

-f ttl	change the initial time to live value to ttl instead of 1

-n	do not resolve IP numbers

-v	verbose

-w sec	set the timeout on returned packets to sec

6. Exercises

1. In the Routing Scenario section of this chapter give the routing table for the LAN’s gateway.

2. Start your network interface manually

ifconfig eth0 192.168.0.x

List the kernel modules. Make sure that the eth0 module is loaded (check /etc/modules.conf).

3. Stop the network interface with:

(i) ifconfig eth0 down

Verify that you can bring the interface back up without entering new information:

(ii) ifconfig eth0 up

4. Stop the interface and remove the kernel module (rmmod module). What happens if you repeat step 3(ii)?

5. Divide the class into two networks A (192.168.1.0) and B (10.0.0.0).

Try accessing machines across networks

Choose an existing machine to be the gateway (on either network)

On the gateway machine only! do the following:

-- allow IP forwarding:

echo 1 > /proc/sys/net/ipv4/ip_forward

-- bring up an aliased interface (this will work as a second interface).

If you are on the 192.168.1.0 network then do the following

ifup eth0:1 10.0.0.x (where x is a an available IP).

add a route to the new network forcing it to use the eth0:1 device

-- add a route to the other network using the gateway machine (you will need to know either the eth0 or eth0:1 setting of this gw depending on which network you are on)

TCP/IP Networks

1. Binary Numbers and the Dotted Quad

Binary numbers

10 = 21	100 = 22	101 = 22 + 1	111 = 100 + 010 + 001

This means that a binary number can easily be converted into a decimal as follows:

10000000	=	27	=	128

01000000	=	26	=	64

00100000	=	25	=	32

00010000	=	24	=	16

00001000	=	23	=	8

00000100	=	22	=	4

00000010	=	21	=	2

00000001	=	20	=	1

The Dotted Quad:

The familiar IP address assigned to an interface is called a dotted quad. In the case of an ipv.4 address this is 4 bytes (4 times 8 bits) separated by dots.

Decimal	Binary
192.168.1.1	11000000.10101000.00000001.00000001

2. Broadcast Address, Network Address and Netmask

An IP number contains information about both the host address (or interface) and network address.

The Netmask

A netmask is used to define which part of the IP address is used for the network, it is also called a subnet mask.

A 16 bit and 17 bit netmask:

255.255.0.0	16-bit	1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 0 0 0 0 0 0 0 0 . 0
255.255.128.0	17-bit	1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 0 0 0 0 0 0 0 . 0

The broadcast is usually given in decimal.

Example: with a 16-bit netmask the following IPs are on the same networks:

00100000 .	10000000 .	00000001 .	00000001
00100000 .	10000000 .	00000000 .	00000011

This means that any bits that are changed inside the box (8+8=16 bits) will change the network address and the interfaces will need a gateway to connect to each other.

In the same way, any bits that are changed ouside the box will change the interface address without changing networks.

For example with a 24-bit netmask the above two IPs would be on different neworks:

00100000 .	10000000 .	00000001 .	00000001
00100000 .	10000000 .	00000000 .	00000011

The Network Address

Every network has a number which is needed when setting up routing. The network number is a portion of the dotted quad. The host address portion is replaced by zero’s.

Typical network address: 192.168.1.0

The Broadcast Address

A machine’s broadcast address is a range of hosts/interfaces that can be accessed on the same network.

For example a host with the broadcast address 10.1.255.255 will access any machine with an IP address of the form 10.1.x.x. Typical broadcast: 192.168.1.255

The dotted quad revisited

Simple logical operations can be applied to the broadcast, netmask and network numbers.

To retrieve the network address from an IP number simply AND the IP with the netmask..

Network Address = IP AND Netmask

Similarly the broadcast address is found with the network address OR ‘not MASK’.

Broadcast Address = Network OR not[Netmask]

Here AND and OR are logical operations on the binary form of these addresses

Example:

Take the IP 192.168.3.5 with a net mask 255.255.255.0. We can do the following operations:

Network address	=	IP	AND	MASK

11000000. 10101000.00000011.00000101	(192.168.3.5)

AND

11111111.11111111.11111111.00000000	(255.255.255.000)

_____________________________________________

11000000.10101000.00000011.00000000	(192.168.3.0)

Broadcast Address	=	IP	OR	NOT-MASK

11000000. 10101000.00000011.00000101	(192.168.3.5)

OR

00000000.00000000.00000000.11111111	(000.000.000.255)

_____________________________________________

11000000.10101000.00000011.11111111	(192.168.3.255)

It is clear from the above example that an IP number together with a netmask is enough to retrieve all the information relative to the network and the host.

3. Network Classes

● Reserved IP addresses

For private networks a certain number of IP addresses are allocated which are never used on the Internet. These reserved IP’s are typically used for LAN’s.

The following table displays the various private/reserved classes.

Table1: Reserved addresses

1	Class A	10.x.x.x
16	Class B	172.16.x.x -- 172.31.x.x
255	Class C	192.168.o.x

● IP classes

Class A: 8-bit network address and 24-bit host address

The first byte of the IP number is reserved for the network address. So the default subnet mask would be 255.0.0.0. The 3 remaining bytes are available to set host interfaces.

Since 255.255.255 and 0.0.0 are invalid host numbers there are 224 – 2 = 16 777 214 possible hosts.

IP numbers have the first byte ranging from 1 to 127. This corresponds to a binary range of 00000001 to 01111111. The first two bits of a class A address can be set to “00” or “01”.

Class B: 16-bit network address and 16-bit host address

The two first bytes of the IP number are reserved for the network address. The default subnet mask is 255.255.0.0. There are 216 – 2 = 65 534 possible hosts.

The first byte ranges from 128 to 191. Notice that the binary range of the first byte is 10000000 to 10111111. That is the first two bits of a class B address are always set to “10”.

Class C: 24-bit network address and 8-bit host address

The three first bytes are reserved for the network address. The default subnet mask is 255.255.255.0. There are 28 – 2 = 254 possible hosts.

The first byte ranges from 192 to 223. This corresponds to a binary range from 11000000 to 11011111. From this we conclude that the first two bits of a class C address is always set to “11”.

4. Subnets

Subneting occurs when bits reserved for hosts are used for the network. This is determined by the netmask and results in networks being split.

For example a regular class A netmask 255.0.0.0 can be altered to allow the first 1-bit of the second byte to be part of the network. This results in a 9-bit network address and a 23-bit host address IP.

The binary netmask looks like

11111111.10000000.00000000.00000000 or 255.128.0.0

Another way to indicate that a 9-bit network address is in use is to give the IP number 10.1.8.1 as 10.1.8.1/9

We will take the example of a class C address 192.168.1.0. We investigate a 25-bit then a 26-bit network.

25-bit network

Netmask: 11111111.11111111.11111111.10000000 or 255.255.255.128

Since Network = IP AND Netmask, we see from the netmask that two network addresses can be formed depending on the hosts range:

1. Host addresses in the 192.168.1.0xxxxxxx range result in a 192.168.1.0 network. We say the network number is 0

2. Host addresses in the 192.168.1.1xxxxxxx range result in a 192.168.1.128 network. We say the network number is 128

Table2: In both cases substitution of the x’s bye zeros or ones have a special meaning

Network address	Substitute with 1’s	Substitute with 0’s
0	Broadcast: 127	Network: 0
128	Broadcast: 255	Network: 128

We are left with the task of counting the number of hosts on each network. Since the host address is 7-bit long and we exclude 2 values (all 1’s and all 0’s) we have 27 – 2 = 126 hosts on each network or a total of 252 hosts.

Notice that if the default subnet mask 255.255.255.0 is used we have 254 available host addresses. In the above example 192.168.1.127 and 192.168.1.128 have a special meaning and that is why only 252 host addresses can be used.

26-bit network

Netmask: 11111111.11111111.11111111.11000000 or 255.255.255.192

Here again depending on the host’s address 4 different network addresses can be determined with the AND rule.

1. Host addresses in the 192.168.1.00xxxxxx range result in a 192.168.1.0 network.

2. Host addresses in the 192.168.1.01xxxxxx range result in a 192.168.1.64 network.

3. Host addresses in the 192.168.1.10xxxxxx range result in a 192.168.1.128 network.

4. Host addresses in the 192.168.1.11xxxxxx range result in a 192.168.1.192 network.

Substituting the x’s with 1’s in the numbers above give us the corresponding broadcast addresses: 192.168.1.63, 192.168.1.127, 192.168.1.191, 192.168.1.255

Each subnet has 26 – 2 = 62 possible hosts or a total of 248.

5. The TCP/IP Suite

TCP/IP is a suite of protocols used on the Internet. The name is meant to describe that several protocols are needed in order to carry data and programs accross a network. The main two protocols are TCP Transmission Control Protocol and IP Internet Protocol.

To simplify, IP handles packets or datagrams only (destination address, size...) whereas TCP handles the connection between two hosts. The idea is that protocols relay each other, each one doing its’ specialised task. In this context one speaks of the TCP/IP stack.

The protocols intervene therefore at various layers of the networking process.

Table1:The 4 layer TCP/IP model:

Application	application level (FTP, SMTP, SNMP)
Transport	handles hosts (TCP, UDP)
Internet	routing (IP, ICMP, IGMP, ARP)
Network Access	network cards, e.g Ethernet, token ring …

● Protocol Overview

IP	The Internet Protocol (IP) is the transport for TCP, UDP, and ICMP data. IP Provides an unreliable connectionless service, allowing all integrity to be handled by one of the upper layer protocols, I.e. TCP, or some application-specific devices. There is no guarantee that a datagram will reach the host using IP alone. The IP protocol handles the addressing and the routing between networks. IP is the datagram delivery service.
TCP	Transmission Control Protocol (TCP) provides a reliable connection orientated service to applications that use it. TCP is connection orientated and checks on each host the order in which the packets are sent/received and also verifies that all the packets are transmitted. Applications such as telnet or ftp use the TCP protocol and don’t need to handle issues over data loss etc …
UDP	The User Datagram Protocol provides direct access to IP for application programs but unlike TCP, is connectionless and unreliable. This provides less overhead for applications concentrated on speed. If some form of packet accounting is needed this has to be provided by the application.
ICMP	The Internet Control Message Protocol is used by routers and hosts to report on the status of the network. It uses IP datagrams and is itself connectionless
PPP	The Point to Point Protocol establishes a TCP/IP connection over phone lines. It can also be used inside encrypted connections such as pptp.

6. TCP/IP Services and Ports

The list of known services and their relative ports is generally found in /etc/services. The official list of services and associated ports is managed by the IANA (Internet Assigned Numbers Authority).

Since the port field is a 16-bit digit there are 65535 available numbers. Numbers from 1 to 1023 are privileged ports and are reserved for services run by root. Most known applications will listen on one of these ports.

We will look at the output of portscans. Beware that unauthorised portscanning is illegal although many people use them.

Here is the output of a portscan:

Port State Service

21/tcp open ftp

22/tcp open ssh

23/tcp open telnet

25/tcp open smtp

70/tcp open gopher

79/tcp open finger

80/tcp open http

This shows open ports, these are ports being used by an application.

The /etc/services main ports:

ftp-data 20/tcp

ftp 21/tcp

telnet 23/tcp

smtp 25/tcp mail

domain 53/tcp

domain 53/udp

http 80/tcp # www is used by some broken

www 80/tcp # progs, http is more correct

pop-2 109/tcp # PostOffice V.2

pop-3 110/tcp # PostOffice V.3

sunrpc 111/tcp

sftp 115/tcp

uucp-path 117/tcp

nntp 119/tcp usenet # Network News Transfer

ntp 123/tcp # Network Time Protocol

netbios-ns 137/tcp nbns

netbios-ns 137/udp nbns

netbios-dgm 138/tcp nbdgm

netbios-dgm 138/udp nbdgm

netbios-ssn 139/tcp nbssn

imap 143/tcp # imap network mail protocol

NeWS 144/tcp news # Window System

snmp 161/udp

snmp-trap 162/udp

7. Exercices

Registering a service with xinetd

1. Write a bash script that echo’s “Welcome” to stdout. Save it in /usr/sbin/hi

#!/bin/bash

echo Welcome

Change the permission on the script to make it executable.

2. In /etc/xinetd.d create a new file called fudge with the following:

service fudge

{

socket_type = stream

server =/usr/sbin/hi

user = root

wait = no

disable = no

}

3. Add a service called fudge in /etc/services that will use port 60000.

4. Restart xinetd and telnet to port 60000

5. You have been assigned a range of IPs on the 83.10.11.0/27 network.

How many networks have the same first 3 bytes as yours?

How many hosts are on your network?

What is the broadcast address for this first network?

Network Services

Network services can either continuously run as standalone applications which listen for connections and handle clients directly or they can be called by the network daemon inetd (old) or xinetd.

1. The inetd daemon (old)

This daemon is started at boot time and listens for connections on specific ports. This allows the server to run a specific network daemon only when needed.

For example, the telnet service has a daemon /usr/sbin/in.telnetd which handles telnet sessions. Instead of running this daemon all the time inetd is instructed to listen on port 23. These instructions are set in /etc/inetd.conf.

Fig1: The inetd daemon

The fields of /etc/inetd.conf contain the following:

service-name	valid name from /etc/services
socket type	stream for TCP and dgram for UDP
protocol	valid protocol from /etc/protocols
flag	nowait if multithreaded and wait if single-threaded
user/group	run application as user or group.
program	usually tcpd
argument	the name of the program to be run for this service

Example:

pop-3 stream tcp nowait root /usr/sbin/tcpd ipop3d

Notice: The /etc/services file is used to make the correspondence between service names and socket port numbers. The fields in services are as follows:

service-name port/protocol [aliases]

2. The xinetd Daemon

This is the most recent version of inetd. The tcpd daemon is no longer used, instead xinetd does everything. Configuration is done either through a single file /etc/xinetd.conf or by editing individual files in /etc/xinetd.d/ corresponding to the services being monitored by xinetd. It is possible to migrate from the old inetd configuration file to the configuration files for the modern xinetd. Nothing else needs to be done.

Structure of service file in xinetd.d
Service-name {
disable = yes/no
socket_type = stream for TCP and dgram for UDP
protocol = valid protocol from /etc/protocols
wait = <yes or no>
user= the user the application runs as
group= the group the application runs as
server= the name of the program to be run for this service
}

3. TCP wrappers

If programs have been compiled with libwrap then they can be listed in /etc/hosts.allow and /etc/hosts.deny. The libwrap library will verify these files for matching hosts.

Default format for /etc/hosts.{allow,deny} :

DAEMON : hosts [EXCEPT hosts ] [: spawn command]

One can also use these files to log unauthorised services. This can help as an early warning system. Here are a few examples.

Getting information about a host:

/etc/hosts.allow

in.telnetd: LOCAL, .my.domain

/etc/hosts.deny

in.telnetd: ALL : spawn (/usr/sbin/safe_finger –l @%h | mail root) &

Redirect to a bogus service or “honey pot” :

/etc/hosts.allow

in.telnetd:	ALL : twist	/dtk/Telnetd.pl

The last example comes from the dtk (Deception Tool Kit) that can be downloaded from http://all.net/dtk/download.html

4. Setting up NFS

Client settings

For a Linux client to mount remote file systems

1. the nfs file system must be supported by the kernel

2. the portmapper daemon must be running.

The portmapper is started by the /etc/rc.d/init.d/portmap script. The mount utility will mount the filesystem. A typical entry in /etc/fstab would be:

nfs-server:/shared/dir	/mnt/nfs	nfs defaults 0 0

Server settings

A NFS server needs portmap to be running before starting the nfs server. The nfs server should be started or stopped with the /etc/rc.d/init.d/nfs script.

The main configuration file is /etc/exports.

Sample /etc/exports file:

/usr/local/docs *.local.org(rw, no_root_squash) *(ro)

The /usr/local/docs directory is exported to all hosts as read-only, and read-write to all hosts in the .local.org domain.

The default root_squash option which avoids the root user (uid = 0) on the client to access the share on the server can be changed with the no_root_squash option.

 The /etc/exports file matches hosts such as *.machine.com where as /etc/hosts.allow/deny match hosts such as .machine.com

If the /etc/exports file has been changed then the exportfs utility should be run. If existing directories in /etc/exportfs are modified then it may be necessary to unmount all nfs shares before remounting them all. Individual directories can be mount or unmounted with exportfs.

Unexporting and exporting all directories in /etc/exports:

exportfs -ua ; exportfs -a

5. SMB and NMB

Linux machines can access and provide Windows shared resources (directories and printers). The protocol used for this is the MS Windows Server Message Block SMB. Samba is the most common Linux tool which provides client and server software.

From the Command Line

The smbclient utility is used to list shared resources. Remote directories are typically mounted with smbmount although ‘mount –t smbfs’ can also be used.

Examples:

Send a pop up message to the win98desk computer

smbclient -M win98desk

Mount the shared directory of the winserv computer

smbmount //winserver/shared /mnt/winserver/shared

The Samba server is configured with the /etc/smb.conf file. The server is stopped and started with the /etc/rc.d/init.d/smb script. Notice that smb will also starts the NMB services. This is the NetBIOS Message Block which enables name resolution in the Windows realm.

Figure1: Nautilus Browsing SMB shares:

Main entries in /etc/smb.conf:

[global]
workgroup = LINUXIT
os level = 2
kernel oplocks = No
security = user
encrypt passwords = Yes
guest account = nobody
map to guest = Bad User
[homes]
comment = Home Directories
read only = No
create mask = 0640
directory mask = 0750
browseable = No
[printers]
comment = All Printers
path = /var/tmp
create mask = 0600
printable = Yes
browseable = No

SWAT and Webmin GUI Configuration

If you install the swat package then you can administrate a samba server via a web-based GUI on port 901.

Another popular general administration tool is webmin. It can be downloaded at www.webmin.com

NOTICE
The configuration file /etc/samba/smb.conf is a good source of documentation. All options are explained and can be switch on by deleting the comment character `;` Also read the smb.conf(5) manpage

6. DNS services

The Resolvers

When a program needs to resolve a host name it uses a mechanism called a resolver. The resolver will first consult the /etc/nsswitch file (previously /etc/host.conf) and determine which method should be used to resolve host names (local files, name server, NIS, or ldap server)

The /etc/host.conf (or /etc/nsswitch.conf) file

These files are scanned by the resolver. They indicate whether files, dns servers, ldap databases or nis servers should be consulted.

Example (/etc/nsswitch):

hosts:	files dns nis

networks:	files

The first line indicates that files (here /etc/hosts) should be queried first and then a DNS server if this fails. The second line instructs to use the /etc/network file for network information.

The /etc/hosts file

With a small number of networked computers it is possible to convert decimal IP numbers into names using the /etc/hosts file. The fields are as follows:

IP machine machine.domain alias

Example /etct/hosts file:

192.168.1.233 io io.my.domain
61.20.187.42 callisto callisto.physics.edu

The /etc/resolv.conf file

If the resolver needs to use a domain name server (DNS) then it will consult the /etc/resolv.conf file for a list of available servers to query from.

Hierarchical structure

Name servers have a hierarchical structure. Depending on the location in the fully qualified domain name (FQDM) a domain is called top-level, second-level or third-level.

Example top-level domains

com	Commercial organisations

edu	US educational institutions

gov	US government institutions

mil	US military institutions

net	Gateways and network providers

org	Non commercial sites

uk	UK sites

Types of DNS servers

Domains can be further divided into sbdomains. This limits the amount of information needed to administer a domain. Zones have a master domain name server (previously called a primary DNS) and one or several slave domain name servers (previously called secondary). Administration of a name server consists of updating the information about a particular zone. The master servers are said to be authoritative.

DNS Configuration Files

In old versions of BIND (prior to BIND version 8) the configuration file was /etc/named.boot. With BIND version 8 the /etc/named.conf file is used instead. One can use the named-bootconf.pl utility to convert old configuration files.

The /etc/named.boot file:

directory	/var/named

cache	.	named.ca

primary	myco.org	named.myco

primary	0.0.127.in-addr.arp	named.local

primary	1.168.192.in-addr.arp	named.rev

The first line defines the base directory to be used. The name.ca file will contain a list of DNS IP addresses for querying external addresses. The third line is optional and contains records for the local LAN. The two next entries are for reverse lookups.

In /etc/named.conf:

cache	is replaced by hint

secondary	is replaced by slave

primary	is replaced by master.

Applying these changes to BIND4 configuration files will generate BIND8 and BIND9 files such as the following.

The /etc/named.conf file:

options	{

directory “/var/named”;

};

zone	“.”	{

type hint;

file “named.ca”;

};

zone “myco.org”	{

type master;

file “named.myco”;

};

zone “1.168.192.in-addr.arp” {

type master;

file “named.rev”;

};

zone “0.0.127.in-addr.arpa” {

type master;

file “named.local”;

};

DNS zone files

In this example the server is set as a caching-only server. All the zone files contain resource records.

Sample named.local zone file:

@ IN SOA localhost. root.localhost. (

2001022700 ; Serial

28800 ; Refresh

14400 ; Retry

3600000 ; Expire

86400 ) ; Minimum

IN NS localhost.

1 IN PTR localhost.

This is a very simple zone file but it gives us enough information to understand the basic mechanism of a name server.

The @ sign will resolve to the related zone declared in /etc/named.conf. This allows any zone file to be used as a template for further zones (see the exercises).

Table1:Common Record Types

NS	Specify the zones primary name server
PTR	Reverse mapping of IP numbers to hostnames
MX	Mail exchange record
A	Associate an IP address with a hostname
CNAME	Associate an alias with the host’s main name

Table2: Zone parameters

@ IN SOA	Start Of Authority. Identifies the zone followed by options enclosed in brackets.
serial	Is manually incremented when data is changed. Secondary servers query the master server’s serial number. If it has changed, the entire zone file is downloaded
refresh	Time in seconds before the secondary server should query the SOA record of the primary domain. This should be at least a day.
retry	Time interval in seconds before attempting a new zone transfer if the previous download failed
expire	Time after which the secondary server discards all zone data if it contact the primary server. Should be a week at least
minimum	This is the ttl for the cached data. The default is one day (86400 seconds) but should be longer on stable LANs

7. Sendmail main Configuration

Sendmail is the most popular mail transfer agent (MTA) on the Internet. It uses the Simple Mail Transfer Protocol (SMTP) and runs as a daemon listening for connections on port 25.

The sendmail script which stops or starts the sendmail daemon is usually located in the /etc/rc.d/init.d/ directory.

The main configuration file is /etc/mail/sendmail.cf (or /etc/sendmail.cf). Here you can specify the name of the server as well as the names of the hosts from which and to which mail relay is allowed.

The /etc/aliases file contains two fields as follows:

alias: user

When changes to /etc/aliases have been made the newaliases command must be run to rebuild the database /etc/aliases.db.

When mail is accepted by the server it is concatenated in a single file with the name of the user. These files are stored in /var/spool/mail/. Depending on the Mail User Agent used, a user can either store these messages in his home directory or download them on another machine.

If the server is relaying, or if the network is slow and many messages are being sent, mail is stored in the mail queue /var/spool/mqueue. You can query the queue with the mailq utility or sendmail –bp. An administrator can flush the server’s queue with sendmail –q.

Finally in order to register a domain name as a valid email address an MX record needs to be added to the DNS database.

For example if mail.company.com is a mail server, then in order for it to accept mail such as joe@company.com you should have the following configuration:

1. Add company.com to /etc/mail/local-host-names

2. company.com	MX 10 mail.company.com	in a DNS zone file

8. The Apache server

Configuration Files

The /etc/httpd/conf/httpd.conf file contains all the configuration settings

Older releases of apache had two extra files, one called access.conf where restricted directories were declared, and another file called srm.conf specifying the server’s root directory.

Configuration Highlights:

ServerType standalone/inetd

ServerRoot	“/etc/httpd”

DocumentRoot	“/var/www/html”

<Directory “/var/www/cgi-bin”>

AllowOverride None

Options ExecCGI

Order allow,deny

Allow from all

</Directory>

<VirtualHost 122.234.32.12>

DocumentRoot	“/www/docs/server1”

ServerName	virtual.mydomain.org

</VirtualHost>

Running Apache

To stop and start the server one can use the /etc/rc.d/init.d/httpd script. On a busy server it is preferable to use apachectl especially with the graceful option which will restart the server only when current connections have been dealt with.

The main log files are in /var/log/httpd/. It may be useful for security reasons to regularly check the error_log and access_log files.

9. Exercises

Setting up a DNS master server

As an exercise we will install the BIND9 rpm package bind9-9.1.3-252.i386.rpm and configure a domain called gogo.com.

1. Carry out the following alterations in /etc/named.conf:

Copy/Paste the following paragraphs and alter as follows:

zone "localhost" in { becomes

type master;

file "localhost.zone";

}

zone "gogo.com" in {

type master;

file "gogo.zone";

}

zone "0.0.127.in-addr.arpa" in { becomes type master; file "127.0.0.zone"; };	zone "2.168.192.in-addr.arpa" in { type master; file "192.168.2.zone"; };

2. In /var/named:

cp 127.0.0.zone 192.168.2.zone

cp local.zone gogo.zone

3. Change the appropriate fields in the new zone files. Add a host called harissa.

4. Add the line “nameserver 127.0.0.1” to /etc/resolv.conf.

5. Use host to resolve harissa.gogo.com

Apache administration

Basic configurations in /etc/httpd/conf/httpd.conf

1. Change the port directive Port from 80 to 8080. (If you are using http2 then change the Listen directive).

2. Check that apache is responding with telnet localhost 8080. You should get:

Trying 127.0.0.1...

Connected to localhost.linuxit.org.

Escape character is '^]'.

Next type ‘GET /’ to download the index file.

3. Set “StartServer” to 15. Restart the httpd and check that 15 processes are started (instead of the default 8)

IP based virtual server

Your ethernet card must be aliased to a new IP (say new-IP)

ifconfig eth0:0 new-IP

Add the following paragraph to /etc/httpd/conf/httpd.conf:

<VirtualHost new-IP>

DocumentRoot /var/www/html/virtual

ServerName www1

</VirtualHost>

Setting up a shared SMB directory

In most cases you won’t need to add smbusers to the system to do this. Simply edit smb.conf and add the following:

[public]

comment = Example Shared Directory

path = /home/samba

guest ok = yes

writeable = yes

Setting up a shared printer:

[global]

--- snip ---

printcap name = /etc/printcap

load printers = yes

[printers]

comment = All Printers

path = /var/spool/samba

browseable = no

# Set public = yes to allow user 'guest account' to print

guest ok = yes

writable = no

printable = yes

Bash Scripting

1. The bash environment

Variables

When you type a command at the prompt the bash shell will use the PATH variable to find which executable on the system you want to run. You can check the value of path using the echo command:

echo $PATH

/usr/bin:/bin:/usr/sbin:/usr/X11R6/bin:/usr/local/bin:/sbin/:/usr/local/sbin/

In fact many variables are needed by the shell to accommodate for each user’s environment. For example PWD, HOME, TERM and DISPLAY are such variables.

To initialise and declare a variable the syntax is as follows:

VARIABLE=VALUE

Remember not to put any spaces around the ‘=’ sign. Once a variable is declared and initialised it can be referenced by using the dollar symbol in front as here:

echo $VARIABLE

When a shell session is started a number of configuration files are read and most of the variables are set.

To free a variable from its current value use unset.

Configuration files

One can distinguish configuration files which are read at login time and configuration files which are read for each new bash session.

Login configuration files:

The files which are read at login are /etc/profile and ~/.bash_profile (bash will look for alternative files too such as ~/.profile).

Next bash will read it’s runtime control files ~/.bashrc and (if it exists) /etc/bashrc.

The bashrc files:

These files are read each time a new shell session is launched (such as a new xterm). The files are /etc/bashrc and ~/.bashrc.

Alias and functions can be saved in the ~/.bashrc

Function syntax:

function-name ()

{

command1;

command2;

}

You can test which files are being read by adding an echo Profile line in /etc/profile, the type:

bash	No profile is read, you shouldn’t see anything

bash -login	This forces bash to act as a login bash, the word

Profile should show up.

The following commands control the way bash starts:

bash -norc

bash -noprofile

Notice that any new bash session will inherit the parent’s global variables defined in /etc/profile and ~/.bash_profile.

2. Scripting Essentials

The script file

A shell script is a list of instructions saved in a flat file. Only two things are necessary.

1. The script’s first line must be #!/bin/bash (for a bash script)

2. The file must be readable and executable (with 755 permission for example)

If these lines are not present it is possible to run the script program by typing

bash program-name

Passing variables to the script

Variables entered at the command line are referenced inside the script as $1 for the first argument, $2 for the second, etc …

Example script, mycat:

#!/bin/bash

cat $1

This script is expecting one argument, a file, and will display the content of the file using cat. To run this script on the lilo.conf file, you would run:

./mycat /etc/lilo.conf

Another way of passing variables to a script is by letting the script prompt the user for input interactively. This is achieved using the read command. The default name of the read variable is REPLY. Here is the modified script:

Interactively passing:

#!/bin/bash

echo -n "Which file shall I display ?"

read

cat $REPLY

or

read -p “File to display: “ FILENAME

cat $FILENAME

Special Variables

Special variables can only be referenced and are automatically set by bash. These are the most common special variables you will encounter:

$*	List of all variables entered at the command line

$#	Number of arguments entered at the command line

$0	The name of the script

$!	PID of the most recent background command

$$	PID of the current shell

$?	Exit code of the last command

For the positional parameters $1, $2 etc … there is a shift operator which renames each parameter in a cyclic way as follows.

$2 becomes $1

$3 becomes $2 … etc

This can be summarised as $(n+1)  $n

3. Logical evaluations

Logical statements are evaluated with the test command or the brackets [ ]. In both case the result is stored in the $? variable such that:

if the statement is true then	$? is 0

if the statement is false then	$? is not 0

Here are some examples to illustrate:

using test	using [ ]	meaning
test –f /bin/bash	[ -f /bin/bash ]	test if /bin/bash is a file
test -x /etc/passwd	[ -x /bin/passwd ]	test if /etc/passwd is executable

One can evaluate more than one statement at a time using the || (OR) and && (AND) logical operators on the command line. For example we could test if /bin/bash is executable and in /etc/inittab exists:

test -x /bin/bash && test /etc/inittab [ -e /bin/kbash ] || [ -f /etc/passwd ]

This is the same as using the flags -o and -a within the test operator for example

test -x /bin/bash -a -f /etc/inittab
[ -e /bin/kbash -o -f /etc/passwd ]

4. Flow Control and Loops

if then

Syntax:	if	CONDITION ; then

command1

command2

fi

#!/bin/bash
if [ -x /bin/bash ] ; then
echo “The file /bin/bash is executable”

while loop

Syntax:	while CONDITION is true; do

command

done

Example: Aligne 10 hashes (#) then exit

#!/bin/bash
COUNTER=0
while [ $COUNTER -lt 100 ]; do
echo -n "#"
sleep 1
let COUNTER=COUNTER+1
done

Until loop

Syntax:	until CONDITION is false; do

command

done

Example: Same as above, notice the C style increment for COUNTER

#!/bin/bash
COUNTER=20
until [ $COUNTER -lt 10 ]; do
echo -n "#"
sleep 1
let COUNTER-=1
done

for loop

Syntax	for VARIABLE in SET; do

command

done

Example: For example the set 'SET' can be the lines of a file

#!/bin/bash

for line in `cat /etc/lilo.conf`; do

IMAGE=$(echo $line | grep image)

if [ “$IMAGE” != “” ]; then

echo Kernel configured to boot: $line

fi

done

5. Expecting user input

We assume that the script is waiting for user input, depending on the answer, the rest of the program will execute something accordingly. There are two possible ways to achieve this: select and case.

Using case

Syntax:	case $VARIABLE in

CHOICE command ;;

CHOICE command ;;

esac

Using select

Syntax:	select VARIABLE in SET; do

if [ $VARIABLE = CHOICE ]; then

command

fi

if [ $VARIABLE = CHOICE ]; then

command

fi

done

6. Working with Numbers

While shell scripts seamlessly handle character strings, a little effort is needed to perform very basic arithmetic operations.

Binary operations

Adding or multiplying numbers together can be achieved using either expr or the $(( )) construct.

Example:

expr 7 + 3; expr 2 \* 10; expr 40 / 4; expr 30 – 11

$((7+3)); $((2*10)); $((40/4)); $((30-11))

Comparing values

Test operators:

Numbers	Strings
-lt	<
-gt	>
-le	<=
-ge	>=
-eq	=
-ne	!=

7. Exercises

1. On the command line export the variable TEST

export TEST=old

2. Write the script

#!/bin/bash
echo old variable: $TEST
export TEST=new
echo exported variable: $TEST

3. What is the value of $TEST once the script has run?

4. The following script called test_shell will print the PID of the shell that is interpreting it

test_shell
#!/bin/bash
if [ -n $(echo $0 |grep test) ]; then
echo The PID of the interpreter is: $$
else
echo The PID of the interpreter is: $$
fi

5) Set the permissions to 755 and test the following commands

test_shell
./test_shell
bash test_shell
. test_shell
source test_shell exec ./test_shell

Basic Security

1. Local Security

The BIOS

If anyone has access to a rescue disks or a linux disk that boots from a floppy or a CDROM it is extremely easy to gain read access to any files on the system. To prevent this the BIOS should be set to boot only off the hard drive. Once this is done set a password on the BIOS.

LILO

LILO can be given options at boot time. In particular some Linux distributions will not ask for a password when starting the system in single user mode or runlevel 1.

There are two options that should be added to the /etc/lilo.conf:

the restricted option prompts the user for a password

the password="" option, set the password string.

Restricted means that LILO cannot be given any parameters without the "password" specified in lilo.conf.

boot=/dev/hda
install=/boot/boot.b
prompt
timeout=50
password="password"
restricted

File permissions

To prevent attackers causing too much damage it is recommended to take the following steps.

1) Make vital system tools immutable, or logfiles append-only:

chattr +i /bin/login chattr +i /bin/ps chattr +a /var/log/messages

2) Make directories /tmp and /home nosuid or noexec:

Lines to be changed in /etc/fstab /tmp /tmp ext2 nosuid 1 2 /home /home ext2 noexec 1 2

3) Find all files on the system that don't belong to a user or a group:

find / -nouser –o –nogroup find / -perm +4000

Log Files

The main logs are

/var/log/messages : contains information logged by the syslogd daemon

/var/log/secure. : contains information on failed logins, added users, etc.

The last tool lists all successful logins and reboots. The information is read from the /var/log/wtmp file.

The who and w tools list all users currently logged onto the system using the /var/run/utmp file.

User Limits

When the /etc/nologin file is present (can be empty) it will prevent all users from login in to the system (except user root). If the nologin file contains a message this will be displayed after a successful authentication.

In the /etc/security/ directory are a collection of files that allow administrators to limit user CPU time, maximum file size, maximum number of connections, etc

/etc/security/access.conf : dissallow logins for groups and users from specific locations.

/etc/security/limits.conf

The format of this file is

<domain> <type> <item> <value>

domain	a user name, a group name (with @group)

type	hard or soft

item	core	- limits the core file size (KB)

data	- max data size (KB)

fsize	- maximum filesize (KB)

memlock	- max locked-in-memory address space (KB)

nofile	- max number of open files

cpu	- max CPU time (MIN)

proc	- max number of processes

as	- address space limit

maxlogins	- max number of simultaneous logins for this user

priority	-	the priority to run user process with

locks	- max number of file locks the user can hold

2. Network Security

Network security can be separated into two main categories:

Host Based Security

Access to resources can be granted based on the host requesting the service. This is handled by tcp_wrappers. The libwrap library also known as tcp_wrappers provides host based access control lists for a variety of network services. Many services, such as xinetd, sshd, and portmap, are compiled against the libwrap library thereby enabling tcp_wrapper support for these services.

When a client connects to a service with tcp_wrapper support, the /etc/hosts.allow and /etc/hosts.deny files are parsed to challenge the host requesting the service. Based on the outcome the service will either be granted or denied.

The hosts_access files have 2, optionally 3 colon separated fields. The first field is the name of the process, the second is the fully qualified host name or domain name with a "leading dot", IP address or subnet with a "trailing dot". Wildcards like ALL and EXCEPT are also accepted.

The syntax for the /etc/hosts.{allow | deny} file is as follows:

service : hosts [EXCEPT] hosts

Example:

/etc/hosts.deny ALL: ALL EXCEPT .example.com /etc/hosts.allow ALL: LOCAL 192.168.0. in.ftpd: ALL sshd: .example.com

Tcp_wrappers can run a command locally upon a host match in the host_access files.

This is accomplished with the spawn command. With the use of the % character, substitutions can be made for the host name and the service.

Example:

/etc/hosts.deny ALL: ALL : spawn (/bin/echo `date` from %c for %d >> /var/log/tcpwrap.log)

For more information on the use of % substitutions see the hosts_access (5) man page.

Port Based Security

With packet filtering functionality built into the Linux kernel, it is possible to limit access to resources by creating rulesets with utilities such as ipchains and iptables, which are able to evaluate a packet entering any of its network interfaces. The rules determine what happens to each packet.

We will cover ipchains and iptables separately. However Ipchains and iptables share the following options

-A Append rule to a chain -D Delete a rule -P Change the default Policy for a chain -I Insert -F Flush the rules(s) in a chain -N Create a user defined chain -X Delete a user defined chain -L List

-- ipchains

There are three built in chains in ipchains:

input, forward and output

These chains, respectively are evaluated when the packets

1) enter the network interface

2) transit to another interface or host

3) exit the network interface and have been either generated by the local host or forwarded

TARGETS:

The possible actions (or TARGETS) are ACCEPT,DENY,REJECT,MASQ, REDIRECT and RETURN, or can possibly point to another user defined chain. Targets are specified with the -j flag.

Example: All packets from 192.168.0.254 will be logged and denied

ipchains -A input -s 192.168.0.254 –-log -j DENY

POLICY: If a packet has gone through all the rules in a specific chain unaltered then it will be dealt with by the default policy rule for that chain. Valid policy targets are DENY (silently drop the packets) or ACCEPT.

Example: Set the policy for all chains to DENY

ipchains -P input DENY
ipchains -P forward DENY
ipchains -P output DENY

-- iptables

One of the main differences with ipchains is that the filtering rules (decisions to allow or deny a packet, etc..) have been separated from packet alteration operations (network address translation (NAT), etc). This has been achieved by introducing independent tables, each table is assigned a specific role and each table contains its own built-in chains.

Iptables has three tables each containing the following built-in chains:

filter:	this table is the default and deals with filtering rules using its built-in chains INPUT, OUTPUT and FORWARD

nat:	only network address translation (NAT) operations are defined in this table. The built-in chains are PREROUTING,

POSTROUTING and INPUT

mangle:	this table handles packet alterations other than natting. There are two built-in chains PREROUTING and OUTPUT.

NOTICE: the built-in chains are all in UPPERCASE!!

TARGETS: Different targets are valid depending on the table. Valid targets for the filter table are DROP, REJECT, ACCEPT or MIRROR. Valid targets for the nat table are REDIRECT (in the PREROUTING and OUTPUT chains), MASQUERADE (in the POSTROUTING chain), DNAT (in the PREROUTING and OUTPUT chains) and SNAT (in the POSTROUTING and OUTPUT chains).

Example: All packets from 192.168.0.254 will be logged and denied

iptables -A input -s 192.168.0.254 -j LOG
iptables -A input -s 192.168.0.254 -j DROP

POLICY: The chain policy can be set to either DROP or ACCEPT

Example: The default policy is set to drop all packets

iptables -P INPUT DROP iptables -P FORWARD DROP iptables -P OUTPUT DROP

-- more background

With the development of the 2.4 Linux kernel came the development of the Netfilter project, which uses the iptables utility to manage firewall rules. Another major difference between iptables and ipchains is that iptables has support for evaluating the packets based on their state in terms of other packets that have passed through the kernel. It is this stateful packet evaluation that makes iptables far superior.

Example:	1)Deny all packets on the INPUT chain:

ipchains -P INPUT DENY

2) Accept established connections that have been initiated by the host:

ipchains -A INPUT -m state –-state ESTABLISHED -j ACCEPT

Example:

A Basic script that will work as a gateway. Here are the highlights:

- allow IP forwarding:

echo "1" > /proc/sys/net/ipv4/ip_forward

- masquerade:

$IPTABLES -t nat -A POSTROUTING -o $INET_IFACE -j MASQUERADE

- allow connections to port 80 ONLY:

$IPTABLES -A INPUT -p TCP -i $INET_IFACE -m state --state NEW --dport http -j ACCEPT

#!/bin/sh # Variables IPTABLES="/sbin/iptables" LAN_IFACE="eth0" INET_IFACE="eth1" INET_IP="1.2.3.4" LOCALHOST_IP="127.0.0.1/32" LAN_IP="192.168.0.1/32" LAN_BCAST="192.168.0.0/24" # Setup IP Masquerading echo "1" > /proc/sys/net/ipv4/ip_forward $IPTABLES -t nat -A POSTROUTING -o $INET_IFACE -j MASQUERADE # Specify the default policy for the built in chains $IPTABLES -P INPUT DROP $IPTABLES -P FORWARD DROP $IPTABLES -P OUTPUT DROP # Specify INPUT Rules $IPTABLES -A INPUT -i !$INET_IFACE -j ACCEPT $IPTABLES -A INPUT -p TCP -i $INET_IFACE -m state --state NEW --dport http -j ACCEPT $IPTABLES -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT # Specify FORWARD Rules $IPTABLES -A FORWARD -i $LAN_IFACE -j ACCEPT $IPTABLES -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT # Specify OUTPUT RULES $IPTABLES -A OUTPUT -p ALL -s $LOCALHOST_IP -j ACCEPT $IPTABLES -A OUTPUT -p ALL -s $LAN_IP -j ACCEPT

3. The Secure Shell

● Host Authentication

With ssh both the host and the user authenticate. The host authentication is done by swapping keys. The host’s public and private keys are usually kept in /etc/ssh if you are using OpenSSH. Depending on the protocol used the host key file will be called ssh_host_key for Protocol 1 and ssh_host_rsa_key or ssh_host_dsa_key for Protocol 2. Each of these keys have their corresponding public key, for example ssh_host_key.pub.

When an ssh client connects to a server, the server will give the hosts public key. At this stage the user will be prompted with something like this:

The authenticity of host 'neptune (10.0.0.8)' can't be established.

RSA key fingerprint is 8f:29:c2:b8:b5:b2:e3:e7:ec:89:80:b3:db:42:07:f4.

Are you sure you want to continue connecting (yes/no)?

If you accept to continue the connection the server’s public key will be added to the local $HOME/.ssh/known_hosts file.

● User Authentication (using passwords)

Then the user is prompted for the password for his account on the remote server and logs in.

● User Authentication (using keys)

The user authentication can also involve swapping keys. For this the user will need to generate a pair of private/public keys. For example:

ssh-keygen -t dsa -b 1024

will generate a 1024 bit DSA key. By default these keys will be saved in $HOME/.ssh and in this example are called id_dsa and id_dsa.pub.

If we assume we have a id_dsa.pub we can ‘plant’ this key on a remote account and avoid typing passwords for further connections. To do this we need to copy the content of the file id_dsa.pub into a file called authorized_keys2 kept in the remote $HOME/.ssh directory.

WARNING
All private keys in /etc/ssh and ~/.ssh should have a permission of 600

● sshd configuration file

Sample /etc/ssh/sshd_config file:

#Port 22

#Protocol 2,1

#ListenAddress 0.0.0.0

#ListenAddress ::

# HostKey for protocol version 1

#HostKey /etc/ssh/ssh_host_key

# HostKeys for protocol version 2

#HostKey /etc/ssh/ssh_host_rsa_key

#HostKey /etc/ssh/ssh_host_dsa_key

●ssh configuration file

Sample /etc/ssh/ssh_config or $HOME/.ssh/config file:

# Host *

# ForwardX11 no

# RhostsAuthentication no

# RhostsRSAAuthentication no

# RSAAuthentication yes

# PasswordAuthentication yes

# HostbasedAuthentication no

# CheckHostIP yes

# IdentityFile ~/.ssh/identity

# IdentityFile ~/.ssh/id_rsa

# IdentityFile ~/.ssh/id_dsa

# Port 22

# Protocol 2,1

# Cipher 3des

4. Time Configuration

The System date

The system date can be changed with the date command.The syntax is:

date MMDDhhmmCCYY[.ss]

The Harware Clock

The hardware clock can be directly changed with the hwclock utility. The main options are:

-r or –-show	prints the current times

-w or –-systohc	set the hardware clock to the current system time

-s or –-hctosys	set the system time to the current hardware clock time

Using NTP

The Coordinated Universal Time (UTC) is a standard used to keep track of time based on the Earth's rotation about it's axis. However because of the slight irregularities of the rotation leap seconds need to be inserted into the UTC scale using atomic clocks.

Since computers are not equiped with atomic clocks the idea is to use a protocol to synchronize computer clocks across the Internet. NTP stands for Network Time Protocol and is one such protocol.

Computers that are directly updated by an atomic clock are called primary time servers and are used to update a larger number of secondary time servers. This forms a tree structure similar to the DNS structure. The root servers are on the first level or stratum, the secondary server on the second and so on.

Configuring a client to query an NTP server:

An NTP daemon called ntpd is used to regulary query a remote time server.All that is needed is a server entry in /etc/ntp.conf pointing to a public or corporate NTP server. Public NTP servers can be found online.

The NTP protocol can also estimate the frequecy errors of the hardware clock from a sequence of queries, this estimate is written to a file refered to with the driftfile tag.

Mininal /etc/ntp.conf file
server ntp2.somewhere.com
driftfile /var/lib/ntp/drift

Once ntpd is started it will itself be an NTP server providing services on port 123 using UDP.

One off queries:

The ntp package also provides the ntpdate tool which can be use to set the time on the command line:

ntpdate ntp2.somewhere.com

5. Kernel security

There are several security options available in the Linux kernel. These include mainly the syn_cookie mechanism. Stack overflow is handled by a security patch called openwall or OWL.

● tcp_syncookies

To enable this option you simply do the following:

[root@nasaspc /proc]#echo "1" > /proc/sys/net/ipv4/tcp_syncookies

This will instruct the kernel to send a cookie to the client in it's SYN+ACK response. In this mode the server then closes the socket and waits for the client's ACK with the appropriate cookie.

If the tcp_syncookies file is not present in the /proc directory then you need to recompile the kernel with syncookies support.

Notice: By default, even if syncookies are supported by the kernel, you need to activate the support by adding a "1" to /proc/sys/net/ipv4/tcp_syncookies. This is usually done in /etc/rc.d/rc.local. However a more efficient solution would be to add an entry to /etc/sysctl.conf

● The owl security patch (this section is not an LPI objective)

This patch takes care of most stack related issues and is beyond the scope of this course. It is however easy to test weather or not your system is vulnerable with binaries provided with the downloaded patch.

Resources for the owl patch and the Linux kernel: http://www.openwall.com http://www.kernel.org/pub/linux/kernel/v2.2/

There is only support for kernel 2.2-19 so far.

After downloading linux-2.2.19.tar.gz and linux-2.2.19-ow1.tar.gz in the /usr/src/ directory, make sure you delete the linux symbolic link.

[root@nasaspc src]#pwd /usr/src/ [root@nasaspc src]#rm -rf linux

You next unbundle the packages.

[root@nasaspc src]#tar xvzf linux-2.2.19.tar.gz [root@nasaspc src]#tar xvzf linux-2.2.19-owl.tar.gz

To test your system go into the linux-2.2-19-owl directory. There is a directory called optional that contains a file called stacktest.c.

[root@nasaspc optional]#pwd /usr/src/linux-2.2.19-ow1/optional [root@nasaspc optional]#gcc stacktest.c -o stacktest

[root@nasaspc optional]#stacktest Usage: ./stacktest OPTION Non-executable user stack area tests -t call a GCC trampoline -e simulate a buffer overflow exploit -b simulate an exploit after a trampoline call [root@nasaspc optional]#stacktest -e Attempting to simulate a buffer overflow exploit... Succeeded.

To apply the patch you need to go into the linux directory. Here are the commands.

Applying the openwall patch:

[root@nasaspc linux]#pwd /usr/src/linux [root@nasaspc linux]#patch -p1 < /usr/src/linux-2.2-19-owl/linux-2.2.19-ow1.diff

Now if you do make menuconfig you should see a new entry called Security options. The default selections are fine. From here you proceed with the compilation and installation of the kernel as usual.

Linux System Administration

Overview

We will concentrate on the main tasks of system administration such as monitoring log files, scheduling jobs using at and cron. This also includes an overview of the documentation available (manpages and online resources) as well as some backup concepts.

1. Logfiles and configuration files

The /var/log/ directory

This is the directory where most logfiles are kept. Some applications generate their own log files (such as squid or samba). Most of the system logs are managed by the syslogd daemon. Common system files are :

cron	keeps track of messages generated when cron executes

mail	messages relating to mail

messages	logs all messages except private authentication authpriv, cron, mail and news

secure	logs all failed authentications, users added/deleted etc

The most important log file is messages where most activities are logged.

The /etc/syslog.conf file

When syslogd is started it reads the /etc/syslog.conf configuration file by default. One can also start syslogd with -f and the path to an alternative config file. This file must contain a list of items followed by a priority, followed by the path to the log-file:

item1.priority1 ; item2.priority2 /path-to-log-file

Valid items are :

auth and authpriv	user general and private authentication

cron	cron daemon messages

kern	kernel messages

mail

news

user	user processes

uucp

Valid priorities are: (from highest to lowest)

emerg

alert

crit

err

warning

notice

info

debug

*

none

Priorities are minimal! All higher priorities will be logged too. To force a priority to be info only you need to use an '=' sign as in:

user.=info /var/log/user_activity

Listing of /etc/syslog.conf

# Log all kernel messages to the console. # Logging much else clutters up the screen. #kern.* /dev/console # Log anything (except mail) of level info or higher. # Don't log private authentication messages! *.info;mail.none;news.none;authpriv.none /var/log/messages # The authpriv file has restricted access. authpriv.* /var/log/secure # Log all the mail messages in one place. mail.* /var/log/maillog # Log cron stuff cron.* /var/log/cron # Everybody gets emergency messages, plus log them on another # machine. *.emerg * *.emerg @10.1.1.254 # Save boot messages also to boot.log local7.* /var/log/boot.log # news.=crit /var/log/news/news.crit news.=err /var/log/news/news.err news.notice /var/log/news/news.notice

2. Log Utilities

The logger command

The first utility logger conveniently logs messages to the /var/log/messages file:

If you type the following:

logger program myscipt ERR

The end of /var/log/messages should now have a message similar to this:

Jul 17 19:31:00 localhost penguin: program myscript ERR

local settings

The logger utility logs messages to /var/log/messages by default. There are local items defined that can help you create your own logfiles as follows. local0 to local7 are available items for administrators to use. The availability depends on the system (RedHat local7 logs boot-time information in /var/log/boot.log). Add the following line to /etc/syslog.conf:

local4.*	/dev/tty9

Restart the syslogd

killall -HUP syslogd

The next command will be logged on the /dev/tty9

logger -p local4.notice "This script is writing to /dev/tty9"

An interesting device is the /dev/speech this is installed with the Festival tools.

logrotate

The log files are updated using logrotate. Usually logrotate is run daily as a cron job. The configuration file /etc/logrotate.conf contains commands to create or compress files.

Listing of logrotate.conf

# rotate log files weekly
weekly
# keep 4 weeks worth of backlogs
rotate 4
# send errors to root
errors root
# create new (empty) log files after rotating old ones
create
# uncomment this if you want your log files compressed
compress
# RPM packages drop log rotation information into this directory
include /etc/logrotate.d
# no packages own lastlog or wtmp -- we'll rotate them here
/var/log/wtmp {
monthly
create 0664 root utmp
rotate 1
}

3. Automatic Tasks

Using cron

The program responsible for running crons is called crond. Every minute the crond will read specific files containing command to be executed. These files are called crontabs.

User crontabs are in /var/spool/cron/<username>. These files should not be edited directly by non-root users and need to be edited using the crontab tool (see below).

The system crontab is /etc/crontab. This file will periodically exectute all the scripts in /etc/cron.* this includes any symbolic link pointing to scripts or binaries on the system.

To manipulate cron entries one uses the crontab utility. Scheduled tasks are view with the -l option as seen below:

crontab -l
# DO NOT EDIT THIS FILE - edit the master and reinstall
# (/tmp/crontab.1391 installed on Tue Jul 17 17:56:48 2001)
# (Cron version -- $Id: crontab.c,v 2.13 1994/01/17 03:20:37 vixie Exp $)
0 * * 07 2 /usr/bin/find /home/penguin -name core -exec rm {} \;

Does the user root have any crontabs?

Similarly the -e option will open your default editor and lets you enter a cron entry.

User root can use the -u to view and change any user's cron entries

To delete your crontab file use crontab -r.

This is the format for crontabs :

Minutes(0-59) Hours(0-23) Day of Month(1-31) Month(1-12) Day of Week(0-6) command

Permissions:

By default any user can use crontab. However you can control the accessibility with /etc/cron.deny and /etc/cron.allow.

Scheduling with “at”

The at jobs are run by the atd daemon. At jobs are spooled in /var/spool/at/

The at command is used to schedule a one off task with the syntax

at [time]

Where time can be expressed as:

now

3am + 2days

midnight

10:15 Apr 12

teatime

For a complete list of valid time formats see /usr/share/doc/at-xxx/timespec.

You can list commands that are scheduled with atq or at -l. The at jobs are saved in /var/spool/at/:

ls /var/spool/at/
a0000100fd244d spool

When using atq you should have a list of jobs proceeded by a number. You can use this number to dequeue it:

atq
1 2001-07-17 18:21 a root

From the atq listing we see that the job number is 1, so we can remove the job from the spool as follows:

at -d 1

Permissions:

By default at is restricted to the root user. To override this you must either have an empty /etc/at.deny

or have a /etc/at.allow with the appropriate names.

4. Backups and Compressions

Backup strategies

There are three main strategies to back up a system:

Full: copy all files

Incremental: The first incremental copies all files added or changed since the last full backup, and subsequently copies all the files added or changed since the last incremental backup

Differential: Copies all files added or changed since the last full backup

Example: If you made a full backup and 3 differential backups before a crash, how many tapes would you need to restore ?

Creating archives with tar

The main option to create an archive with tar is -c. You can also specify the name of the archive as the first argument if you use the -f flag.

tar -cf home.tar /home/

If you don't specify the file as an argument tar -c will simply output the archive as standard output:

tar -c /home/ > home.tar

Extracting archives with tar

Extracting is straight forward. Replace the -c flag with an -x. This will cause the archive file to create directories if necessary and copy the archived files in your current directory. To redirect the output of the extracted archive into the directory /usr/share/doc, for example, you can do:

tar xf backeddocs.tar -C /usr/share/doc

Compressions

All archives can be compressed using different compression utilities. These flags are available when creating, testing or extracting an archive:

tar option	compression type
Z	compress
z	gzip
j	bzip2.

The cpio utility

The cpio utility is used to copy files to and from archives. List of files must be given to cpio either through a pipe (as when used with find) or via a file redirection such as with;

- Extract an archive on a tape:

cpio -i < /dev/tape

- Create an archive for the /etc directory:

find /etc | cpio -o > etc.cpio

The dump and restore utilities

Finally, it is also possible to perform backups per device using dump. Remember that the field after the options in /etc/fstab is used to specify if a device should be backed up or not.

Information about the previous full or incremental backups is stored in /etc/dumpdates. Dump can automatically do upto 9 incremental backups.

By default dump will save the archive to /dev/st0.

What to backup

The following table extracted from the FHS document is used to determine how often specific directories need to be backed up:

	shareable	unshareable
static	/usr, /opt	/etc, /boot
variable	/var/mail	/var/run, /var/spool/mail

5. Documentation

Manpages and the whatis database

The manpages are organised in sections
NAME	the name of the item followed by a short one line description.
SYNOPSYS	the syntax for the command
DESCRIPTION	a longer description
OPTIONS	a review of all possible options and their function
FILES	files that are related to the current item (configuration files etc)
SEE ALSO	other manpages related to the current topic

These are the main sections one would expect within a manpage.

The whatis database stores the NAME section of all the manpages on the system. This is done through a daily cron. The whatis database has the following two entries:

name(key) – one line description

The syntax for whatis is:

whatis <string>

The output is the full NAME section of the manpages where string matched named(key)

One can also use the man command to query the whatis database. The syntax is

man -k <string>

Unlike whatis this will query both the “name” and the “one line description” entries of the database. If the string matches a word in any of these fields the above query will return the full NAME section.

Example: (the matching string has been highlighted)

whatis lilo
lilo (8) - install boot loader
lilo.conf [lilo] (5) - configuration file for lilo

man -k lilo
grubby (8) - command line tool for configuring grub, lilo, and elilo
lilo (8) - install boot loader
lilo.conf [lilo] (5) - configuration file for lilo

The FHS recommends manpages to be kept in /usr/share/man

Manpage Sections
Section 1	Information on executables
Section 2	System calls, e.g mkdir(2)
Section 3	Library calls, e.g stdio(3)
Section 4	Devices (files in /dev)
Section 5	Configuration files and formats
Section 6	Games
Section 7	Macro packages
Section 8	Administration commands
Section 9	Kernel routines

To access a specific section N one has to enter:

man N command

Examples:

man mkdir
man 2 mkdir

man crontab
man 5 crontab

Info pages

The FHS recommends info pages be kept in /usr/share/info. These pages are compressed files that can be read with the info tool.

The original GNU tools used info pages rather than manpages. Since then most info pages have been rewritten as manpages. However information about GNU projects such as gcc or glibc is still more extensive in the info pages compared to the manpages.

Online documents

GNU projects include documents such as a FAQ, README, CHANGELOG and sometimes user/admin guides. The formats can either be ASCII text, HTML, LateX or postscript.

These documents are kept in the /usr/share/doc/ directory.

HOWTOs and The Linux Documentation Project

The Linux Documentation Project provides many detailed documents on specific topics. These are structured guides explaining concepts and implementations. The website URL is www.tldp.org.

The LDP documents are freely redistributable and can be contributed too using a GPL type licence.

Usenet News Groups

The main newsgroups for Linux are the comp.os.linux.* groups (e.g comp.os.linux.networking, comp.os.linux.security ...). Once you have setup a news reader to connect to a news server (usually available through an ISP or a University campus) one downloads a list of all existing discussion groups and subscribes/unsubscribes to a given group.

There are many experienced as well as new users which rely on the newsgroups to get information on specific tasks or projects. Take the time to answer some of these questions if you feel you have the relevant experience.

NOTICE
The man -k option queries both fields in the whatis database. This will find everything about a given item. There is a tool called apropos (meaning about) which will do the same thing as man -k.

6. Exercises

Logging

Change /etc/syslog.conf to output some of the logs to /dev/tty9 (make sure you restart syslogd and that the output is properly redirected)

Add a custom local5 item with critical priority to /ect/syslog.conf and direct the output to /dev/tty10. Restart syslogd and use logger to write information via local5.

3. Read the /etc/rc.d/init.d/syslog script and change /etc/sysconfig/syslog to allow remote hosts to send log outputs.

Scheduling

4.Create a cron entry which starts xclock every 2 minutes. Remember that cron is unaware of system variables such as PATH and DISPLAY.

5. Use at.to start xclock in the next five minutes.

Archiving

6. Use find to list all files that have been modified during the past 24 hours.

(hint: Redirect the output of find -mtime –1 to a file)

7.Use cpio to create an archive called Incremental.cpio.

(ans: Use the file created above and do cat FILE | cpio –ov > Incremental.cpio)

8 Use tar to create an archive of all files last accessed or changed 5 mins ago. (HINT: use find to create a list of files, then save the list to a file. The tar tool has a switch to take input from a file.

9. Test the archive before extracting it.

10. Extract the archive you have just created.

Setting up PPP

1. Serial Modems

Linux assumes in general that serial modems are connected to a serial port (one of the /dev/ttySN devices). So you first need to find out which serial port the modem is connected to.

The setserial –g command will query the serial ports. If the resource is not available then the UART value will be unknown.

Sample output for setserial:

setserial -g /dev/ttyS[0-3]

/dev/ttyS0, UART: 16550A, Port: 0x03f8, IRQ: 4

/dev/ttyS1, UART: 16550A, Port: 0x02f8, IRQ: 3

/dev/ttyS2, UART: unknown, Port: 0x03e8, IRQ: 4

/dev/ttyS3, UART: unknown, Port: 0x02e8, IRQ: 3

For non-serial modems it is possible to get information about available resources in /proc/pci. Here the i/o and IRQ settings can be transferred to a free /dev/ttyS? device. This is achieved with the following 2 lines:

setserial /dev/ttyS2 port 0x2000 irq 3

setserial /dev/ttyS2 autoconfig

The last line simply deals with setting up the proper UART settings.

These settings will be lost at the next boot and can be saved in /etc/rc.serial. This script is one of the last scripts executed by rc.sysinit at boot time.

The rc.serial script:

#!/bin/bash

TTY=/dev/ttyS2

PORT=0x2000

IRQ=3

echo "Setting up Serial Card ..."

/bin/setserial $TTY port $PORT irq $IRQ 2>/dev/null

/bin/setserial $TTY autoconfig 2>/dev/null

2. Dialup Configuration

Once the modem is known to be connected to a serial device it is possible to send modem specific instruction such as ATZ or ATDT. One tool that will act as a terminal interface is minicom.

minicom screenshot:

Another common tool is wvdialconf. This tool will automatically scan for modems on the ttyS’s and create a configuration file. This file is used to handle password authentication and initialise the pppd daemon once the connection is established.

3. pppd and chat

First of all the chat script is used to communicate with a remote host’s modem. It is a series of expect/send strings. The format is:

‘expected query’ ‘answer’

Expected queries from the modem are:

‘ ‘	‘OK’	‘CONNECT’	‘login’	‘password’	‘TIMEOUT’	‘>’

The script is read sequentially and starts with the empty query ‘ ‘ which is matched with the command ‘ATZ’. Once the modem is initialised it sends back the query ‘OK’. To this the script will answer with a ‘ATDT’ dialing command. This conversation goes on and on until the ‘>’ prompt is reached at which stage one can run pppd.

Sample chat script:

'ABORT' 'BUSY'

'ABORT' 'ERROR'

'ABORT' 'NO CARRIER'

'ABORT' 'NO DIALTONE'

'ABORT' 'Invalid Login'

'ABORT' 'Login incorrect'

'' 'ATZ'

'OK' 'ATDT01172341212'

'CONNECT' ''

'ogin:' 'adrian'

'ord:' 'adrianpasswd'

'TIMEOUT' '5'

'>'	pppd

Of course this is one way of doing things. One can also start pppd manually and then invoke the chat script as follows:

pppd /dev/ttyS2 115200 \

nodetach \

lock \

debug \

crtscts \

asyncmap 0000000 \

connect "/usr/sbin/chat -f /etc/sysconfig/network-scripts/chat-ppp0"

The lines below the pppd commands can be saved in /etc/ppp/options. This file contains most of the features which makes the strength and flexibility of pppd. For example require-chap will use the /etc/ppp/chap-secrets for authentication.

4. PPPD peers

There is a directory called peers in /etc/ppp/. In this directory one can create a file that contains all the necessary command line options for pppd. In this way peer connections can be started by all users.

Below is an example of a PPP peer file:

# This optionfile was generated by pppconfig 2.0.10.

hide-password

noauth

connect "/usr/sbin/chat -f /etc/sysconfig/network-scripts/chat-ppp0"

/dev/ttyS0

115200

defaultroute

noipdefault

user uk2

The previous peer file (called uk2) would be used as follows:

# pppd call uk2

This will dial the number specified in the “chat script” and authenticate as the user “uk2”. Please noteNote that this requires a corresponding entry in the /etc/ppp/chap-secrets, and /etc/ppp/pap-secrets. The format for pap and chap secrets is as follows:

# Secrets for authentication using CHAP

# client	server	secret	IP addresses

uk2	*	"uk2"	*

This format allows different passwords to be used if you connect to different servers. It also allows you to specify an IP address. This is probably not going to work when connecting to an ISP, but when making private connections, you can specify IP addresses if there is a need. One example would be where you need to audit your network activity, and want to specify which users get a certain IP address.

5. Wvdial

This is the default method used by Red Hat to connect to a dial up network. To configure wvdial, it is easier to use one of the configuration tools provided with either Gnome or KDE. They configure the

/etc/wvdial.conf file.

Below is a sample wvdial.conf file:

[Modem0]

Modem = /dev/ttyS0

Baud = 115200

Dial Command = ATDT

Init1 = ATZ

FlowControl = Hardware (CRTSCTS)

[Dialer UK2]

Username = uk2

Password = uk2

Phone = 08456091370

Inherits = Modem0

To use wvdial from the command line, you would execute it with the following syntax:

# wvdial <dialer-name>

In the example configuration file the following command would dial the connection called “uk2”

# wvdial uk2

Printing

The two objectives of this chapter are firstly to introduce the GNU printing tools available on Linux machines and secondly to understand the configuration files for a print server.

1. Filters and gs

For non-text formats Linux and UNIX systems generally use filters. These filters translate JPEG or troff file formats into a postscript type format. This could directly be sent to a postscript printer, but since not all generic printers can handle postscript, an intermediate ‘virtual postscript printer’ is used called gs (ghostscript), finally translating the postscript into PCL.

The commercial version of ghostscript is Aladdin Ghostscript and the GNU version is an older version.

The gs utility has a database of printer drivers it can handle (this list is usually up to date, for example many USB printers are supported) and converts the postscript directly into PCL for these known models. The gs utility plays a central role in Linux printing.

2. Printers and print queues

As seen above simple ascii text printing is not handled in the same way as image or postscript files. If you only have one printer and you would like to printout your mail for example, it may not be necessary to use a filter. You may want to define a queue without filters, which would print mail faster. You could also define a queue on the same printer, which would only handle postscript files.

All queues and printers are defined in /etc/printcap. Here is the full configuration of a remote printer 192.168.1.20 using the remote queue named ‘lp’:

lp:\

:sd=/var/spool/lpd/lp:\

:mx#0:\

:sh:\

:rm=192.168.1.20:\

:rp=lp:

The essential options here are rm the remote host, sd the spool directory and rp the name of the remote queue. Notice that no filters are specified (you would use if for input filter). All the filtering is done on the remote host.

3. Printing Tools

lpr:

The lpr utility is used to submit jobs to a printer. This is a modern version of lp (line print). From a user’s point of view it is helpful to understand that a printer can be associated with more than one queue. Here are two examples to print a file called LETTER.

Send job to default printer:

lpr LETTER

Send job to the’ ljet’ queue:

lpr -Pljet LETTER

Table1: Main Options for lpr

-#num	Print num copies
-Ppq	Specify the print queue pq
-s	Make a symbolic link in the spool directory rather than copy the file in

lpq:

A user can monitor the status of print queues with the lpq utility. Here are a few examples.

Show jobs in default queue:

lpq

Show jobs for all queues on the system:

lpq -a

Show jobs in the ‘remote’ queue:

lpq -Premote

lprm:

Depending on the options in /etc/lpd.perms users may be allowed to delete queued jobs using lprm.

Remove last job submitted:

lprm

Remove jobs submitted by user dhill:

lprm dhill

Remove all submitted jobs:

lprm -a (or simply lprm -)

It is possible to remove a specific spooled job by referencing the job number; this number is given by lpq.

lpc:

The Line Printer Control utility is used to control the print queues and the printers. The print queues can be disabled or enabled. Notice that lprm on the other hand can remove jobs from the queue but doesn’t stop the queue.

One can either use lpc interactively (lpc has its own prompt), or on the command line.

Here is an output of lpc –help:

CMD: /usr/sbin/lpc help

► Commands may be abbreviated. Commands are:

abort enable disable help restart status topq ?

clean exit down quit start stop up

The enable/disable/topq/up options relate to queues

The start/stop/down options relate to printers

4. The configuration files

/etc/printcap

As seen earlier in the chapter, this file defines all printers and queues that the system can use (remote and local).

The default printer can be specified with either variables LPDEST or PRINTER: PRINTER=lp

If no environmental variable is set the default printer is the first printer defined in /etc/printcap.

The main definitions are:

lp	device name, usually /dev/lp0 for the parallel port

mx	maximum file size (zero=nolimit)

sd	spool directory (/var/spool/lpd/<queuename>/)

if	input filter

rm	remote host address or IP

rp	remote queue name

If this file is modified you will need to restart the lpd daemon.

/etc/lpd.conf

This is a very lengthy file and by default all options are commented out. This file is used if an administrator wishes to have more control (i.e remote access authentication, user permissions …) over the printing.

/etc/lpd.perms

This file controls permission for the lpc, lpq and lprm utilities. In particular you can grant users the right to dequeue their current job (using the lprm tool) with the line :

ACCEPT	SERVICE=M	SAMEHOST SAMEUSER

LPRng uses a system of keys to shorten the entries in lpd.perms. This is however not very to understand. For example the service ‘M’ corresponds to lprm in the above line.

Sample /etc/lpd.perms file:

## Permissions are checked by the use of 'keys' and matches. For each of

## the following LPR activities, the following keys have a value.

##

## Key Match Connect Job Job LPQ LPRM LPC

## Spool Print

## SERVICE S 'X' 'R' 'P' 'Q' 'M' 'C'

## USER S - JUSR JUSR JUSR JUSR JUSR

## HOST S RH JH JH JH JH JH

## GROUP S - JUSR JUSR JUSR JUSR JUSR

## IP IP RIP JIP JIP RIP JIP JIP

## PORT N PORT PORT - PORT PORT PORT

## REMOTEUSER S - JUSR JUSR JUSR CUSR CUSR

## REMOTEHOST S RH RH JH RH RH RH

## REMOTEGROUP S - JUSR JUSR JUSR CUSR CUSR

## REMOTEIP IP RIP RIP JIP RIP RIP RIP

## CONTROLLINE S - CL CL CL CL CL

## PRINTER S - PR PR PR PR PR

## FORWARD V - SA - - SA SA

## SAMEHOST V - SA - SA SA SA

## SAMEUSER V - - - SU SU SU

## SERVER V - SV - SV SV SV

## LPC S - - - - - LPC

## AUTH V - AU AU AU AU AU

## AUTHTYPE S - AU AU AU AU AU

## AUTHUSER S - AU AU AU AU AU

## AUTHFROM S - AU AU AU AU AU

## AUTHSAMEUSER S - AU AU AU AU AU

##

## KEY:

## JH = HOST host in control file

## RH = REMOTEHOST connecting host name

## JUSR = USER user in control file

## AUTH will match (true) if authenticated transfer

## AUTHTYPE will match authentication type

## AUTHUSER will match client authentication type

## AUTHFROM will match server authentication type and is NULL if not from server

## AUTHSAMEUSER will match client authentication to save authentication in job

##

## Example Permissions

##

## # All operations allowed except those specifically forbidden

## DEFAULT ACCEPT

##

## #Reject connections from hosts not on subnet 130.191.0.0

## # or Engineering pc's

## REJECT SERVICE=X NOT REMOTEIP=130.191.0.0/255.255.0.0

## REJECT SERVICE=X NOT REMOTEHOST=engpc*

##

## #Do not allow anybody but root or papowell on

## #astart1.astart.com or the server to use control

## #facilities.

## ACCEPT SERVICE=C SERVER REMOTEUSER=root

## ACCEPT SERVICE=C REMOTEHOST=astart1.astart.com REMOTEUSER=papowell

##

## #Allow root on talker.astart.com to control printer hpjet

## ACCEPT SERVICE=C HOST=talker.astart.com PRINTER=hpjet REMOTEUSER=root

## #Reject all others

## REJECT SERVICE=C

##

## #Do not allow forwarded jobs or requests

## REJECT SERVICE=R,C,M FORWARD

##

#

# allow root on server to control jobs

ACCEPT SERVICE=C SERVER REMOTEUSER=root

# allow anybody to get server, status, and printcap

ACCEPT SERVICE=C LPC=lpd,status,printcap

# reject all others

REJECT SERVICE=C

#

# allow same user on originating host to remove a job

ACCEPT SERVICE=M SAMEHOST SAMEUSER

# allow root on server to remove a job

ACCEPT SERVICE=M SERVER REMOTEUSER=root

REJECT SERVICE=M

# all other operations allowed

DEFAULT ACCEPT

/etc/hosts.{lpd,equiv}

These files were used by the LPR printing suite and presented a security risk. When running a print server you needed to specify which hosts could access the printer in /etc/hosts.lpd. You also needed to add the hosts to /etc/hosts.equiv.

These files are now replaced in LPRng by the /etc/lpd.perms file

5. Exercises

1.	Start printtool and create a new local queue called lp.

2.	Customise the device /dev/tty10 as the printer device (remember to

do chmod 666 /dev/tty10 to allow printing on this device). You now have

a virtual printer on your system!

3.	Send jobs to the print queue using lpr and pr (pre-formatting tool)

4.	With your system’s print tool, define different remote queues:

- a UNIX queue

- a SMB queue

If you are the server, make sure the appropriate rules are defined in /etc/lpd.perms

In each case

- check the /etc/printcap file. Which filter is used? How is the remote host defined?

- check the /var/spool/lpd/ directory.

5.	Stop the various printer queues and printers with lpc.

6.	Check the contents of each queue with lpq

7.	De-queue selected jobs with lprm

LPI 102 Objectives

1. Kernel

Manage/Query kernel and kernel modules at runtime

Manage a kernel and kernel loadable modules. Use command-line utilities to get information about the kernel modules and the running kernel. Load modules with correct parameters and unload them. Load modules using aliases.

Keywords: /lib/modules/kernel-version/modules.dep, /etc/modules.conf, /etc/conf.modules
depmod, insmod, lsmod, rmmod, modinfo, modprobe, uname

Reconfigure, build, and install a custom kernel and kernel modules
Customise, build, and install a kernel and kernel loadable modules from source Customise the current kernel. Build a new kernel or new kernel modules as needed. Install the new kernel and reconfigure the boot loader.

Keywords: /usr/src/linux/*, /usr/src/linux/.config, /lib/modules/kernel-version/*, /boot/*
make, config, menuconfig, xconfig, oldconfig, modules, install, modules_install, depmod

2. Boot, Initialisation, Shutdown and Runlevels

Boot the system

Follow the system through the booting process. Parse parameters to the boot loader (runlevel and kernel options). Check events in the log files.
Keywords: dmesg, /var/log/messages, /etc/modules.conf, LILO, GRUB

Change runlevels and shutdown or reboot system

Manage the system’s runlevels. The default runlevel. The single user mode. Shutdown and reboot. Alert users before switching runlevel.
Keywords: shutdown, init, /etc/inittab

3. Printing

Manage printers and print queues

Manage print queues and print jobs. Monitor print server and user print queues. Troubleshoot general printing problems.
Keywords: lpc, lpq, lprm, lpr, /etc/printcap

Print files
Manage print queues and manipulate print jobs. Add and remove jobs from printer queues. Convert text files to postscript for printing.
Keywords: lpr, lpq, mpage

Install and configure local and remote printers
Install a printer daemon. Install and configure a print filter (e.g.: apsfilter, magicfilter). Make local and remote printers accessible for a Linux system. SMB shared printers.

Keywords: lpd, /etc/printcap, /etc/apsfilter/*, /var/lib/apsfilter/*,
/etc/magicfilter/*, /var/spool/lpd/*

4. Documentation

Use and manage local system documentation

Use and administer the manpages and the material in /usr/share/doc. Find relevant man pages. Search man page sections. Find a command and all the documentation related to it. Configure access to man sources and the man system.
Keywords: man, apropos, whatis, MANPATH

Find Linux documentation on the Internet
Find and use Linux documentation. Use Linux documentation from sources such as the Linux Documentation Project (LDP), vendors and third-party websites. Linux specific newsgroups. Newsgroup archives. Mailing lists.

Notify users on system-related issues

Notify users about current issues related to the system. Logon messages.
Keywords: /etc/issue, /etc/issue.net, /etc/motd

5. Shells, Scripting, Programming and Compiling

Customise and use the shell environment
Customise shell environments to meet users' needs. Set environment variables at login, or when spawning a new shell. Write bash functions for frequently used sequences of commands.

Keywords: ~/.bash_profile, ~/.bash_login, ~/.profile, ~/.bashrc, ~/.bash_logout, ~/.inputrc
function, export, env, set, unset

Customise or write simple scripts
Customise existing scripts. Write simple new shell scripts. Use standard sh syntax (loops, tests). Use command substitution. Test command return-values and file status. Conditionally mailing the superuser. The she-bang (#!) sign. Manage location, ownership, execution and suid rights of scripts.

Keywords: while, for, test, chmod

6. Administrative Tasks

Manage users and group accounts and related system files

Add, remove, suspend and change user accounts. Manage groups. Change user/group info in passwd/group databases. Create special purpose and limited accounts.
Keywords: chage, gpasswd, groupadd, groupdel, groupmod, grpconv, grpunconv,
passwd, pwconv, pwunconv, useradd, userdel, usermod
/etc/passwd, /etc/shadow, /etc/group, /etc/gshadow

Tune the user environment and system environment variables
Modify global and user profiles. Set up environment variables. Maintain the skel directory. Set command search path.
Keywords: env, export, set, unset, /etc/profile, /etc/skel

Configure and use system log files to meet administrative and security needs

Configure system logs. Manage type and level of information logged. Manually scan log files for notable activity. Monitoring log files: automatic rotation and archiving. Track down problems noted in logs.
Keywords: logrotate, tail –f, /etc/syslog.conf, /var/log/*

Automate system administration tasks by scheduling jobs to run in the future

Use cron or anacron to run jobs at regular intervals. Use at to run jobs once. Manage cron and at jobs. Configure user access to cron and at services.
Keywords: at, atq, atrm, crontab
/etc/anacrontab, /etc/at.deny, /etc/at.allow, /etc/crontab, /etc/cron.allow, /etc/cron.deny, /var/spool/cron/*

Maintain an effective data backup strategy
Plan a backup strategy. Automatically backup filesystems to various media. Dump a raw device to a file and vice versa. Perform partial and manual backups. Verify the integrity of backup files. Partially or fully restor backups.
Keywords: cpio, dd, dump, restore, tar

Maintain system time

Maintain the system time and synchronize the clock over NTP. Set the system date and time. Set the BIOS clock to the correct time in UTC, configuring the correct timezone for the system and configuring the system to correct clock drift to match NTP clock.
Keywords: date, hwclock, ntpd, ntpdate
/usr/share/zoneinfo, /etc/timezone, /etc/localtime, /etc/ntp.conf, /etc/ntp.drift

7. Networking Fundamentals

Fundamentals of TCP/IP

Understand IP-addresses, network masks and broadcast address. Determine the network address, broadcast address and netmask when given an IP-address and the number of bits. Network classes and classless subnets (CIDR) and the reserved addresses for private network use. It includes the understanding of the function and application of a default route. It also includes the understanding of basic internet protocols (IP, ICMP, TCP, UDP) and the more common TCP and UDP ports (20, 21, 23, 25, 53, 80, 110, 119, 139, 143, 161).
Keywords: /etc/services, ftp, telnet, host, ping, dig, traceroute, whois

TCP/IP configuration and troubleshooting
View, change and verify configuration settings for various network interfaces. Manual and onboot configuration for interfaces and routing tables. Configure and correct routing tables. Configure Linux as a DHCP client.

Keywords: /etc/HOSTNAME or /etc/hostname, /etc/hosts, /etc/networks, /etc/host.conf, /etc/resolv.conf, /etc/nsswitch.conf
ifconfig, route, dhcpcd, dhcpclient, pump, host, hostname (domainname, dnsdomainname), netstat, ping, traceroute, tcpdump

Configure Linux as a PPP client

Understand the basics of the PPP protocol. Configure PPP for outbound connections. Define the chat sequence when connecting. Initialisation and termination of a PPP connection with a modem, ISDN or ADSL. Set up PPP to automatically reconnect if disconnected.

Keywords: /etc/ppp/options.*, /etc/ppp/peers/*, /etc/wvdial.conf
/etc/ppp/ip-up, /etc/ppp/ip-down, wvdial, pppd

8. Networking Services

Configure and manage inetd, xinetd, and related services

Configure services available through inetd. Use tcpwrappers. Start, stop, and restart internet services. Configure basic network services including telnet and ftp. Set a service to run as another user instead of the default in inetd.conf.
Keywords: /etc/inetd.conf, /etc/hosts.allow, /etc/hosts.deny, /etc/services, /etc/xinetd.conf, /etc/xinetd.log

Operate and perform basic configuration of sendmail

Modify simple parameters in sendmail configuration files. Create mail aliases. Manage the mail queue. Start and stop sendmail. Configure mail forwarding and perform basic troubleshooting of sendmail. The objective includes checking for and closing open relay on the mailserver. It does not include advanced custom configuration of Sendmail.
Keywords: /etc/sendmail.cf, /etc/aliases, /etc/mail/*, ~/.forward
mailq, sendmail, newaliases

Operate and perform basic configuration of Apache

Modify simple parameters in Apache configuration files. Start, stop, and restart httpd. Does not include advanced custom configuration of Apache.
Keywords: apachectl, httpd, httpd.conf

Properly manage the NFS, smb, and nmb daemons

Mount remote filesystems using NFS. Configure NFS for exporting local filesystems. Start, stop, and restart the NFS services. Install and configure Samba using GUI tools or direct edit of the /etc/smb.conf file. Sharing of home directories and printers, as well as correctly setting the nmbd as a WINS client.
Keywords: /etc/exports, /etc/fstab, /etc/smb.conf, mount, umount

Setup and configure basic DNS services

Configure hostname lookups and troubleshoot problems with local caching-only name server. Understand the domain registration and DNS translation process. Differences between bind 4 and bind 8 configuration files.
Keywords: /etc/hosts, /etc/resolv.conf, /etc/nsswitch.conf, /etc/named.boot (v.4) or /etc/named.conf (v.8), named

Set up secure shell (OpenSSH)

Obtain and configure OpenSSH. Basic OpenSSH installation and troubleshooting. Configure sshd to start at system boot.
Keywords: /etc/hosts.allow, /etc/hosts.deny, /etc/nologin, /etc/ssh/sshd_config, /etc/ssh_known_hosts, /etc/sshrc
sshd, ssh-keygen

9. Security

Perform security administration tasks

Ensure local security policies. Configure TCP wrappers. Find files with SUID/SGID bit set. Verify packages. Set or change user passwords and password aging information. Update binaries as recommended by CERT, BUGTRAQ or distribution's security alerts. Basic knowledge of ipchains and iptables.
Keywords: /proc/net/ip_fwchains, /proc/net/ip_fwnames, /proc/net/ip_masquerade, find, ipchains, passwd, socket, iptables

Setup host security

Set up a basic level of host security. Configure syslog , shadowed passwords. Set up a mail alias for root. Turn off unused network services.
Keywords: /etc/inetd.conf or /etc/inet.d/*, /etc/nologin, /etc/passwd, /etc/shadow, /ets/syslog.conf

Setup user level security

Configure user level security. Limits on user logins, processes, and memory usage.
Keywords: quota, usermod

Index

/etc/shadow 29

chage32

cron79, 81

date93

depmod 12

gpasswd28

groupadd30, 31

groupadd 28

groups 27

id 27

init19, 20, 21, 22

insmod12

LILO17, 22, 23

logrotate81

lpd94

lsmod12

make bzImage 15

make clean15

make config13

make dep14

make menuconfig13

make modules 15

make modules_install15

make oldconfig14

make xconfig13

make zImage 15

man72

modinfo12

modprobe 12

modules.conf12

modules.dep12

passwd26

peers91

rmmod12, 41

route37

sendmail59

shutdown21

socket51

syslog.conf79

tar84

test77

useradd26, 30, 31

usermod31

LinuxIT Technical Education Centre

Contents

_____________________________________________________________________

47

The Linux Kernel _____________________________________________________________________

Booting Linux ____________________________________________________________________

Managing Groups and Users ______________________________________________________________________

Network Configuration ______________________________________________________________________

TCP/IP Networks ______________________________________________________________________

Network Services _____________________________________________________________________

Bash Scripting ____________________________________________________________________

Basic Security _____________________________________________________________________

Linux System Administration

____________________________________________________________________

________________________________________________________________________

Setting up PPP

__________________________________________________________________________

Printing

_________________________________________________________________________

Index

Appendix

All original materials are ©2002 LinuxIT. All Rights Reserved. Kleesh Consultants Ltd T/A LinuxIT registered in England No:04470427. Visit www.linuxit.com for more information.

Licence Agreement

Introduction

_______________________________________________________________________

The Linux Kernel

none

2003-05-02T11:35:00