Linux System Calls Interview Questions And Answers

The systemcall "gettimeofday" can be used to get the time in microseconds. The call takes two arguments.
1- struct timeval
2- struct timezone ( for timezone information). you can have second argument null.
timeval structure has two fields:
tv_sec ( represents time in seconds)
tv_usec ( represents time in microseconds )

the code snapshot is :
////
struct timeval t_time;
gettimeofday(&t_time,NULL);

Raid is Redundant Array of Independent Disks/Device.It is
Technology to improve Disk read & write Performence and
FaultTolerance., By adding new disk u can recover data if
one of disk goes down / fails. parity is a calculated
technique to rebuild data from disk fails.
levels in Raid
0 - stripping
1 - Mirroring
3 - Striping with Parity
5 - Striping with Parity with more fault tolerant widley used.

(1)Suppose any problem in initrd file then kernel panic error showing
(2)Any file system problem or any change or remove file system then showing the kernel panic error

vi /etc/samba/smb.conf
Add these lines

[fedora 9]
comment=publicstuff
path=/share
public=yes
writable=yes
available=yes
create mask=0777
directory mask=0777

firstly create the user.
useradd surendra
passwd surendra

then opne the FTP file:-
vi /etc/vsftpd/vsftpd.conf
add this line at the end of the file
userlist_deny=NO

then open this file,
vi /etc/vsftpd/user_list

and add the above created user in this file to access FTP
services.

and restart the FTP services
/etc/init.d/vsftpd start
chkconfig vsftpd on ( for permanent on )

CUPS (formerly an acronym for Common Unix Printing System,
but now with no official expansion) is a modular printing
system for Unix-like computer operating systems which allows
a computer to act as a print server. A computer running CUPS
is a host that can accept print jobs from client computers,
process them, and send them to the appropriate printer.

If you are using a client with CUPS and a CUPS server has
already been configured, installing the printers on your
client can not get much easier than this: do nothing.
Through broadcasting, the client should find the CUPS server
and automatically configure the printers that are installed
on that print server. This is one of the features of CUPS
that will be really appreciated on large networks.

Manually configuring printers with CUPS, also is a peace of
cake. If you are new to CUPS and/or Unix printing, the way
to go is probably the web interface. If you have to
configure lots of printers, using the command-line will
probably be faster.

The URL to access the CUPS web interface is
http://hostname:631/admin by default. The port can be
changed in cupsd.conf if necessary.

To add a printer from the command-line the general syntax is
lpadmin -p printer -E -v device -m ppd Lpadmin with the -p
option adds or modifies a printer. The printers are saved in
the file The -x option deletes the named printer. Read the
lpadmin man page for available options.

Example 3. command-line examples

/usr/sbin/lpadmin -p testpr1 -E -v socket://192.168.1.9 -m
deskjet.ppd
/usr/sbin/lpadmin -p testpr2 -E -v parallel:/dev/lp0 -m
laserjet.ppd
/usr/sbin/lpadmin -x testpr1

More information about configuring printers and options can
be found in the CUPS documentation. The Software
Administrators Manual will teach you all you need to know
about configuring printers with CUPS.

cpio with find command ,tar with gun/bunzip ,dump with only
ext2 & ext3 Fs used to take backups
Also you have amanda software in RHEL 4.0 version to take a
backup

Simple configuration
--------------------
Install apache: (get the rpm for apache)

# rpm -ivh httpd.rpm

# vi /etc/httpd/conf/httpd.conf

and change
ServerName your-domain.com

Start apache : service httpd start

Verify by pointing the browser to http://localhost/

The sudo command stands for "superuser do". If a server
needs to be administered by a number of people it is
normally not a good idea for them all to use the root
account. This is because it becomes difficult to determine
exactly who did what, when and where if everyone logs in
with the same credentials. The sudo utility was designed to
overcome this difficulty.

The sudo utility allows users defined in the /etc/sudoers
configuration file to have temporary access to run commands
they would not normally be able to due to file permission
restrictions. The commands can be run as user "root" or as
any other user defined in the /etc/sudoers configuration file.

The privileged command you want to run must first begin with
the word sudo followed by the command's regular syntax. When
running the command with the sudo prefix, you will be
prompted for your regular password before it is executed.
You may run other privileged commands using sudo within a
five-minute period without being re-prompted for a password.
All commands run as sudo are logged in the log file
/var/log/messages.

In order to use sudo we first need to configure the sudoers
file.

Do not edit directly the file:

To edit it, use the command

# visudo

******Output***************

# /etc/sudoers
#
# This file MUST be edited with the 'visudo' command as root.
#
# See the man page for details on how to write a sudoers file.
#

Defaults env_reset

# Host alias specification

# User alias specification

# Cmnd alias specification

# User privilege specification
root ALL=(ALL) ALL
**********************************************

You will see the line

root ALL=(ALL) ALL

This lines means that the user root can execute from ALL
terminals, acting as ALL (any) users, and run ALL (any) command.
The first part is the user, the second is the terminal from
where the user can use sudo, the third is as which user he
may act, and the last one, is which commands he may run.

Example:

Granting Access To Specific Users To Specific Files
---------------------------------------------------
amsin21, %operator ALL= /sbin/, /usr/sbin,
/usr/local/apps/check.pl

This entry allows user amsin21 and all the members of the
group operator to gain access to all the program files in
the /sbin and /usr/sbin directories, plus the privilege of
running the command /usr/local/apps/check.pl. Notice how the
trailing slash (/) is required to specify a directory location:

Granting Access to Specific Files as Another User
-------------------------------------------------
The sudo -u entry allows allows you to execute a command as
if you were another user, but first you have to be granted
this privilege in the sudoers file.

This feature can be convenient for programmers who sometimes
need to kill processes related to projects they are working
on. For example, programmer amsin21 is on the team
developing a financial package that runs a program called
monthend as user accounts. From time to time the application
fails, requiring "amsin21" to stop it with the /bin/kill,
/usr/bin/kill or /usr/bin/pkill commands but only as user
"accounts". The sudoers entry would look like this:

amsin21 ALL=(accounts) /bin/kill, /usr/bin/kill, /usr/bin/pkill

User amsin21 is allowed to stop the monthend process with
this command:

# sudo -u accounts pkill monthend

Granting Access Without Needing Passwords
-----------------------------------------

This example allows all users in the group operator to
execute all the commands in the /sbin directory without the
need for entering a password. This has the added advantage
of being more convenient to the user:

%operator ALL= NOPASSWD: /sbin/

Using Aliases in the sudoers File
---------------------------------

Sometimes you'll need to assign random groupings of users
from various departments very similar sets of privileges.
The sudoers file allows users to be grouped according to
function with the group and then being assigned a nickname
or alias which is used throughout the rest of the file.
Groupings of commands can also be assigned aliases too.

In the next example, users amsin21, amsin211 and amsin212
and all the users in the operator group are made part of the
user alias ADMINS. All the command shell programs are then
assigned to the command alias SHELLS. Users ADMINS are then
denied the option of running any SHELLS commands and su:

Cmnd_Alias SHELLS = /usr/bin/sh, /usr/bin/csh, \
/usr/bin/ksh, /usr/local/bin/tcsh, \
/usr/bin/rsh, /usr/local/bin/zsh


User_Alias ADMINS = amsin21, amsin211, amsin212, %operator
ADMINS ALL = !/usr/bin/su, !SHELLS

This attempts to ensure that users don't permanently su to
become root, or enter command shells that bypass sudo's
command logging. It doesn't prevent them from copying the
files to other locations to be run. The advantage of this is
that it helps to create an audit trail, but the restrictions
can be enforced only as part of the company's overall
security policy.

Iptables is the userspace command line program used to
configure the Linux 2.4.x and 2.6.x IPv4 packet filtering
ruleset. Iptables allows administrators to configure the
operating system so that it allows applications and clients
to connect through the network and stop unwanted
applications and clients from communicating and corrupting
the operating system.
It is not specific to Redhat. It is available in all linux
2.4.x and 2.6.x kernels.

Samba provides file and print services to all manner of
SMB/CIFS clients, including the numerous versions of
Microsoft Windows operating systems. Samba configuration
file is smb.conf:

Sample smb.conf
---------------

[global]
# Domain name ..
workgroup = DOMAIN.NAME
# Server name - as seen by Windows PCs ..
netbios name = SERVER1
# Be a PDC ..
domain logons = Yes
domain master = Yes
# Be a WINS server ..
wins support = true

# allow user privileges
#enable privileges = yes

obey pam restrictions = Yes
dns proxy = No
os level = 35
log file = /var/log/samba/log.%m
max log size = 1000
syslog = 0
panic action = /usr/share/samba/panic-action %d
pam password change = Yes

# Allows users on WinXP PCs to change their password
when they press Ctrl-Alt-Del
unix password sync = no
ldap passwd sync = yes

# Printing from PCs will go via CUPS ..
load printers = yes
printing = cups
printcap name = cups

# Use LDAP for Samba user accounts and groups ..
passdb backend = ldapsam:ldap://localhost

# This must match init.ldif ..
ldap suffix = dc=domain,dc=name
# The password for cn=admin MUST be stored in
/etc/samba/secrets.tdb
# This is done by running 'sudo smbpasswd -w'.
ldap admin dn = cn=admin,dc=domain,dc=name

# 4 OUs that Samba uses when creating user accounts,
computer accounts, etc.
# (Because we are using smbldap-tools, call them
'Users', 'Computers', etc.)
ldap machine suffix = ou=Computers
ldap user suffix = ou=Users
ldap idmap suffix = ou=Idmap
# Samba and LDAP server are on the same server in
this example.
ldap ssl = no

# Scripts for Samba to use if it creates users,
groups, etc.
add user script = /usr/sbin/smbldap-useradd -m '%u'
delete user script = /usr/sbin/smbldap-userdel %u
add group script = /usr/sbin/smbldap-groupadd -p '%g'
delete group script = /usr/sbin/smbldap-groupdel '%g'
add user to group script =
/usr/sbin/smbldap-groupmod -m '%u' '%g'
delete user from group script =
/usr/sbin/smbldap-groupmod -x '%u' '%g'
set primary group script = /usr/sbin/smbldap-usermod
-g '%g' '%u'

# Script that Samba users when a PC joins the domain ..
# (when changing 'Computer Properties' on the PC)

#add machine script = /usr/sbin/smbldap-useradd -w '%u'
add machine script = /usr/sbin/useradd -s /bin/false
-d /home/nobody %u


# Values used when a new user is created ..
# (Note: '%L' does not work properly with
smbldap-tools 0.9.4-1)
logon drive = H:
logon home = \\server\%U
logon path = \\server\Profiles\%U
logon script = logon.bat

# This is required for Windows XP client ..
server signing = auto
server schannel = Auto

[homes]
comment = Home Directories
path = /home/users/%U
valid users = %S
read only = No
browseable = No
[netlogon]
comment = Network Logon Service
path = /var/lib/samba/netlogon
admin users = root
guest ok = Yes
browseable = No

[Profiles]
comment = Roaming Profile Share
# would probably change this to elsewhere in a
production system ..
path = /var/lib/samba/profiles
read only = No
profile acls = Yes
browsable = No
hide files = /desktop.ini/ntuser.ini/NTUSER.*/

[printers]
comment = All Printers
path = /var/spool/samba
use client driver = Yes
create mask = 0600
guest ok = Yes
printable = Yes
browseable = No
public = yes
writable = yes
admin users = root
write list = root

[print$]
comment = Printer Drivers Share
path = /var/lib/samba/printers
write list = root
create mask = 0664
directory mask = 0775
admin users = root

Test it with :

# testparm /etc/samba/smb.conf

Load smb config files from /etc/samba/smb.conf
Processing section "[homes]"
Processing section "[netlogon]"
Processing section "[Profiles]"
Processing section "[printers]"
Processing section "[print$]"
Loaded services file OK.
Server role: ROLE_DOMAIN_PDC
Press enter to see a dump of your service definitions

NIS is a service that provides any user on that network
with the same working environment irrespective of the
system on that network which has been used for login
purpose.
For example if NIS server is set up in a single system and
configured to hold user accounts and their passwords and
access information. Then any user on that network can login
to his/her account that is set up in the NIS server from
any system (with nis client running) on that configured
network. This gives a look and feel that the user is logged
into his/her own system. But actually its the account on
the NIS server that is mounted on the local sytem on user
login .

Sendmail should be installed by default when you install Red
Hat Linux. If it is not then you need to install the
Sendmail RPM's with the Red Hat distribution.

Configuring Sendmail
--------------------

1) Edit file "/etc/mail/sendmail.mc" - Look for the line:

DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')

Change this line to:

dnl DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')

Save the file.

2)Make the sendmail configuration file:

# m4 /etc/mail/sendmail.mc > /etc/sendmail.cf

3)Restart Sendmail:

# /etc/rc.d/init.d/sendmail restart

.bash_history

A Server-- it is weather in testing or production-- are primary targets for
the attackers. By taking the proper steps, you can turn a vulnerable box into
a hardened server.
How to secure SSH sessions, configure firewall rules, minimize software, listed below,
1. Encrypt Data communication
-- use scp, ssh avoid FTP, Telnet and Rlogin /rsh
2. Minimize Software to minimize vulnerability
-- use RPM pkg management / YUM utility to remove unwanted packages installed
3. One Network Service per System or Vm Instance
-- Run different network services on separate servers or vm instance.
For example, if an attacker able to successfully exploit software called
Apache flow, he/she get an access to entire server including other services
such as MYSQL, email server and so on.
4. Keep linux software and Kernel up to date.
-- Use yum update or up2date
some distros apt-get update
5. Security essentials like selinux
6. password authentication like password aging, restricting to user previous
passphrases, and locking user accounts after login failures.
7. Disable unwanted services using chkconfig --list | grep "3:on"

RAID 1 is disk striping. no mirroring no parity. Minimum 2 disks required. If any
One disk fails all the data get lost.
RAID 5 is disk striping with parity. Minimum 3 disks required. if anyone disk fails
Data is safe, if two fails data get lost.

Don't know exactly, think of 2TB or 8TB

by using pvremove command.

ext3 is also same as the ext2, but journaling concept is introduced in ext3.
Compared to ext2, ext3 is slow. ext2 less secure compared to ext3. ext2 is less
Performance where as ext3 is very good performance.

resize2fs is only for ext2 filesystem but not ext3.
first unmount the partition
#umount /dev/sda1

#tune2fs -O ^has_journal /dev/sda1 #to remove journal from /dev/sda1

#e2fsck -f /dev/sda1

#resize2fs /dev/sda1 600M #resize the partition

LVM is a mechanism use for providing specality of extending (or) reducing
the sizes of an existing partition.

swap can be created in two ways after the installation,
1. fdisk command
2. create a swap file using dd command

after creating swap file or file system

#mkswap /dev/sda10
#swapon /dev/sda10
#swapon -s #To see the swap devices

by using dd command

#dd if=/dev/zero of=/swap bs=1024 count=1

Which will creates the file size 1024(1GB).

#mkswap /swap
#swapon /swap
#swapon -s #to see the swap devices

ssh is secured shell, allows the user to login remotely with more secured.
whereas telnet also same but authentications like passwords, transfers over a network as text mode. so it is not good to use.

#rpm -qpR filename.rpm

Lists the dependency list of packages.

mainly spinlock used in threads to avoid synchronization,where as semaphore and mutex used to avoid process synchronization.
1.spinlock is something like polling.it spins for resouce until aloocated resouce releases.
2.binary semaphore and mutex are similar.