Linux Admin Interview Questions & Answers
Linux Questions
Q) Q) What is Linux and
why is it so popular?
Answer - Linux is an
operating system that uses UNIX like Operating system.......
Q) Q) What is LILO?
Answer - LILO is Linux
Loader is a boot loader for Linux. It is used to load Linux into the memory and
start the Operating system.......
Q) Q) What is the
difference between home directory and working directory?
Answer - Home directory
is the default working directory when a user logs in. On the other hand,
working directory is the user’s current directory.......
Q) Q) What is the
difference between internal and external commands?
Answer - Internal
commands are commands that are already loaded in the system. They can be
executed any time and are independent.......
Q) Explain the
difference between a static library and a dynamic library.
Answer - Static
libraries are loaded when the program is compiled and dynamically-linked
libraries are loaded in while......
Q) What is
LD_LIBRARY_PATH?
Answer - LD_LIBRARY_PATH
is an environment variable. It is used for debugging a new library or a non
standard library.......
Q) What is the file
server in Linux server?
Answer - File server is
used for file sharing. It enables the processes required fro sharing.......
Q) What is NFS? Q) What
is its purpose?
Answer - NFS is Network
File system. It is a file system used for sharing of files over a
network.......
How do I send email with
linux?
Answer - Email can be sent
in Linux using the mail command. ......
Q) Explain RPM (Red Hat
Package Manager) features.
Answer - RPM is a
package managing system (collection of tools to manage software
packages).......
Q) What is Kernel? Q)
Explain the task it performs.
Answer - Kernel is used
in UNIX like systems and is considered to be the heart of the operating
system.......
Q) What is Linux Shell?
Q) What is Shell Script?
Answer - Linux shell is
a user interface used for executing the commands. Shell is a program the user......
Q) What are Pipes? Q)
Explain use of pipes.
Answer - A pipe is a
chain of processes so that output of one process (stdout) is fed an input
(stdin) to another.......
Q) Explain trap command;
shift Command, getopts command of linux.
Answer - Trap command:
controls the action to be taken by the shell when a signal is received. ......
Q) What Stateless Linux
server? Q) What feature it offers?
Answer - A stateless
Linux server is a centralized server in which no state exists on the single
workstations. ......
Q) What does nslookup
do? Q) Explain its two modes.
Answer - Nslookup is
used to find details related to a Domain name server. Details like IP addresses
of a machine, MX records,......
Q) What is Bash Shell?
Answer - Bash is a free
shell for UNIX. It is the default shell for most UNIX systems. It has a
combination of the C and Korn shell features. ......
Q) Explain some
Network-Monitoring Tools in Linux: ping, traceroute, tcpdump, ntop
Answer - Network
monitoring tools are used to monitor the network, systems present on the
network, traffic etc.......
How does the linux file
system work?
Answer - Linux file
structure is a tree like structure. It starts from the root directory,
represented by '/', and then expands into sub-directories.......
Q) What are the process
states in Linux?
Answer - Process states
in Linux.......
Q) What is a zombie?
Answer - Zombie is a
process state when the child dies before the parent process. In this case the
structural information of the process is still in the process table.......
Q) Explain each system
calls used for process management in linux.
Answer - System calls
used for Process management......
Q) Which command is used
to check the number of files and disk space used and the each user’s defined
quota?
repquota command is used
to check the status of the user’s quota along with the disk space and number of
files used. This command gives a summary of the user’s quota that how much
space and files are left for the user. Every user has a defined quota in Linux.
This is done mainly for the security, as some users have only limited access to
files. This provides a security to the files from unwanted access. The quota
can be given to a single user or to a group of users.
Q) What is the name and path
of the main system log?
By default the main
system log is /var/log/messages. This file contains all the messages and the
script written by the user. By default all scripts are saved in this file. This
is the standard system log file, which contains messages from all system
software, non-kernel boot issues, and messages that go to 'dmesg'. dmesg is a
system file that is written upon system boot.
Q) How secured is Linux?
Q) Explain.
Security is the most
important aspect of an operating system. Due to its unique authentication
module, Linux is considered as more secured than other operating systems. Linux
consists of PAM. PAM is Pluggable Authentication Modules. It provides a layer
between applications and actual authentication mechanism. It is a library of
loadable modules which are called by the application for authentication. It
also allows the administrator to control when a user can log in. All PAM
applications are configured in the directory "/etc/pam.d" or in a
file "/etc/pam.conf". PAM is controlled using the configuration file
or the configuration directory.
Q) Can Linux computer be
made a router so that several machines may share a single Internet connection?
How?
Yes a Linux machine can
be made a router. This is called "IP Masquerade." IP Masquerade is a
networking function in Linux similar to the one-to-many (1: Many) NAT (Network
Address Translation) servers found in many commercial firewalls and network
routers. The IP Masquerade feature allows other "internal" computers
connected to this Linux box (via PPP, Ethernet, etc.) to also reach the
Internet as well. Linux IP Masquerading allows this functionality even if the
internal computers do not have IP addresses.
The IP masquerading can
be done by the following steps:
1. The Linux PC must have
an internet connection and a connection to LAN. Typically, the Linux PC has two
network interfaces-an Ethernet card for the LAN and a dial-up PPP connection to
the Internet (through an ISP).
2. All other systems on
your LAN use the Linux PC as the default gateway for TCP/IP networking. Use the
same ISP-provided DNS addresses on all systems.
3. Enable IP forwarding
in the kernel. By default the IP forwarding is not enabled. To ensure that IP
forwarding is enabled when you reboot your system, place this command in the
/etc/rc.d/rc.local file.
4. Run
/sbin/iptables-the IP packet filter administration program-to set up the rules
that enable the Linux PC to masquerade for your LAN.
Q) What is the minimum
number of partitions you need to install Linux?
Minimum 2 partitions are
needed for installing Linux. The one is / or root which contains all the files
and the other is swap. Linux file system is function specific which means that
files and folders are organized according to their functionality. For example,
all executables are in one folder, all devices in another, all libraries in
another and so on. / or ‘root’ is the base of this file system. All the other
folders are under this one. / can be consider as C: .Swap is a partition that
will be used as virtual memory. If there is no more available RAM a Linux
computer will use an area of the hard disk, called swap, to temporarily store
data. In other words it is a way of expanding your computers RAM.
Which command is used to
review boot messages?
dmesg command is used to
review boot messages. This command will display system messages contained in
the kernel ring buffer. We can use this command immediately after booting to
see boot messages. A ring buffer is a buffer of fixed size for which any new
data added to it overwrites the oldest data in it. Its basic syntax is
dmesg [options]
Invoking dmesg without
any of its options causes it to write all the kernel messages to standard
output. This usually produces far too many lines to fit into the display screen
all at once, and thus only the final messages are visible. However, the output
can be redirected to the less command through the use of a pipe, thereby
allowing the startup messages to be viewed on one screen at a time
dmesg | less
Which utility is used to
make automate rotation of a log?
logrotate command is
used to make automate rotation of log.
Syntax of the command
is:
logrotate [-dv] [-f|]
[-s|] config_file+
It allows automatic
rotation, compression, removal, and mailing of log files. This command is
mainly used for rotating and compressing log files. This job is done every day
when a log file becomes too large. This command can also be run by giving on
command line. We can done force rotation by giving –f option with this command
in command line. This command is also used for mailing. We can give –m option
for mailing with this command. This option takes two arguments one is subject
and other is recipient name.
Q) What are the
partitions created on the mail server hard drive?
The main partitions are
done firstly which are root, swap and boot partition. But for the mail server
three different partitions are also done which are as follows:
1. /var/spool- This is
done so that if something goes wrong with the mail server or spool than the
output cannot overrun the file system.
2. /tmp- putting this on
its own partition prevents any user item or software from overrunning the
system files.
3. /home- putting this
on its own is useful for system upgrades or reinstalls. It allow not to wipe
off the /home hierarchy along with other areas.
Q) What are the fields
in the/etc/passwd file?
It contains all the
information of the users who log into the system. It contains a list of the
system's accounts, giving for each account some useful information like user
ID, group ID, home directory, shell, etc. It should have general read
permission as many utilities, like ls use it to map user IDs to user names, but
write access only for the superuser (root). The main fields of /etc/passwd file
are:
1. Username: It is used when
user logs in. It should be between 1 and 32 characters in length.
2. Password: An x
character indicates that encrypted password is stored in /etc/shadow file.
3. User ID (UID): Each
user must be assigned a user ID (UID). UID 0 (zero) is reserved for root and
UIDs 1-99 are reserved for other predefined accounts. Further UID 100-999 are
reserved by system for administrative and system accounts/groups.
4. Group ID (GID): The
primary group ID (stored in /etc/group file)
5. User ID Info: The
comment field. It allow you to add extra information about the users such as
user's full name, phone number etc. This field use by finger command.
6. Home directory: The
absolute path to the directory the user will be in when they log in. If this
directory does not exists then users directory becomes /
7. Command/shell: The
absolute path of a command or shell (/bin/bash). Typically, this is a shell.
Which commands are used
to set a processor-intensive job to use less CPU time?
nice command is used for
changing priority of the jobs.
Syntax: nice [OPTION]
[COMMAND [ARG]...]
Range of priority goes
from -20 (highest priority) to 19 (lowest).Priority is given to a job so that
the most important job is executed first by the kernel and then the other least
important jobs. This takes less CPU times as the jobs are scheduled and are
given priorities so the CPU executes fast. The priority is given by numbers
like -20 describe the highest priority and 19 describe the least priority.
How to change window
manager by editing your home directory?
/.xinitrc file allows
changing the window manager we want to use when logging into X from that
account. The dot in the file name shows you that the file is a hidden file and
doesn't show when you do a normal directory listing. For setting a window
manager we have to save a command in this file. The syntax of command is: exec
windowmanager.After this, save the file. Next time when you run a startx a new
window manager will open and become default. The commands for starting some
popular window managers and desktop environments are:
-KDE = startkde
-Gnome = gnome-session
-Blackbox = blackbox
-FVWM = fvwm
-Window Maker = wmaker
-IceWM = icewm
Q) How documentation of
an application is stored?
When a new application
is installed its documentation is also installed. This documentation is stored
under the directory named for application. For example if my application name
is App1 then the path of the documentation will be /user/doc/App1. It contains
all the information about the application. It contains date of creating
application, name of application and other important module of the application.
We can get the basic information of application from the documentation.
Q) How shadow passwords
are given?
pwconv command is used
for giving shadow passwords. Shadow passwords are given for better system
security. The pwconv command creates the file /etc/shadow and changes all
passwords to ‘x’ in the /etc/passwd file. First, entries in the shadowed file
which don't exist in the main file are removed. Then, shadowed entries which
don't have `x' as the password in the main file are updated. Any missing
shadowed entries are added. Finally, passwords in the main file are replaced
with `x'. These programs can be used for initial conversion as well to update
the shadowed file if the main file is edited by hand.
Q) How do you create a
new user account?
useradd command is used
for creating a new user account. When invoked without the
-D option, the useradd
command creates a new user account using the values specified on the command
line and the default values from the system. The new user account will be
entered into the system files as needed, and initial files copied, depending on
the command line options. This command uses the system default as home
directory. If –m option is given then the home directory is made.
Q) Which password
package is installed for the security of central password?
Shadow password packages
are used for security of central passwords. Security is the most important
aspect of every operating system. When this package is not installed the user
information including passwords is stored in the /etc/passwd file. The password
is stored in an encoded format. These encoded forms can be easily identified by
the System crackers by randomly encoding the passwords from dictionaries. The
Shadow Package solves the problem by relocating the passwords to another file
(usually /etc/shadow). The /etc/shadow file is set so that it cannot be read by
just anyone. Only root will be able to read and write to the /etc/shadow file.
Q) Which shell do you
assign to a POP3 mail-only account?
POP3 mail only account
is assigned to the /bin/false shell. However, assigning bash shell to a POP3
mail only gives user login access, which is avoided. /bin/nologin can also be used.
This shell is provided to the user when we don’t want to give shell access to
the user. The user cannot access the shell and it reject shell login on the
server like on telnet. It is mainly for the security of the shells. POP3 is
basically used for downloading mail to mail program. So for illegal downloading
of emails on the shell this account is assigned to the /bin/false shell or
/bin/nologin. These both shells are same they both do the same work of
rejecting the user login to the shell. The main difference between these two
shells is that false shell shows the incorrect code and any unusual coding when
user login with it. But the nologin shell simply tells that no such account is
available. So nologin shell is used mostly in Linux.
Q) Which daemon is
responsible for tracking events on Linux system?
syslogd is responsible
for tracking system information and save it to the desired log files. It
provides two system utilities which provide system logging and kernel message
trapping. Internet and UNIX domain sockets support enable this utility package
to support both local and remote logging. Every logged message contains at
least a time and a hostname field, normally a program name field, too. So to
track these information this daemon is used. syslogd mainly reacts to the set
of signals given by the user. These are the signals given to syslogd: SIGHUP:
This lets syslogd perform a re-initialization. All open files are closed, the
configuration file (default is /etc/syslog.conf) will be reread and the syslog
facility is started again. SIGTERM: The syslogd will die. SIGINT, SIGQUIT: If
debugging is enabled these are ignored, otherwise syslogd will die. SIGUSR1:
Switch debugging on/off. This option can only be used if syslogd is started
with the - d debug option. SIGCHLD: Wait for Childs if some were born, because
of waiting messages.
Q) Which daemon is used
for scheduling of the commands?
The crontab command is
used for scheduling of the commands to run at a later time. SYNTAX
crontab [ -u user ] file
crontab [ -u user ] { -l
| -r | -e }
Options
-l List - display the
current crontab entries.
-r Remove the current
crontab.
-e Edit the current
crontab using the editor specified by the VISUAL or EDITOR environment
variables.
When user exits from the
editor, the modified crontab will be installed automatically. Each user can
have their own crontab, and though these are files in /var, they are not
intended to be edited directly. If the –u option is given than the crontab
gives the name of the user whose crontab is to be tweaked. If it is given
without this then it will display the crontab of the user who is executing the
command.
Q) How environment
variable is set so that the file permission can be automatically set to the
newly created files?
umask command is used to
set file permission on newly created files automatically.
Syntax
umask [-p] [-S] [mode]
It is represented in
octal numbers. We can simply use this command without arguments to see the
current file permissions. To change the permissions, mode is given in the
arguments. The default umask used for normal user is 0002. The default umask
for the root user is 0022. For calculating the original values, the values
shown by the umask must be subtracted by the default values. It is mainly used
for masking of the file and directory permission. The /etc/profile script is
where the umask command is usually set for all users. The –S option can be used
to see the current default permissions displayed in the alpha symbolic format.
For example, umask 022
ensures that new files will have at most 755 permissions (777 NAND 022).
The permissions can be
calculated by taking the NAND of original value with the default values of
files and directories.
Update V1.1.
1.When do you need a virtual hosting ?
The term Virtual
Host refers to the practice of maintaining more than one server on one
machine, as differentiated by their apparent hostname. For example, it is often
desirable for companies sharing a web server to have their own domains, with
web servers accessible as www.company1.com and www.company2.com,
without requiring the user to know any extra path information.
2.In which port telnet is listening?
23
3.How to get the listening ports which is
greater than 6000 using netstat ?
4.How to block and openrelay ?
Open relays are e-mail servers that are
configured to accept and transfer e-mail on behalf of any user anywhere,
including unrelated third parties.
The qmail-smtpd daemon will consult the
rcpthosts control file to determine valid destination addresses, and reject
anything else.
5.Q) What is sandwitch configuration in qmail
?
Qmail + Clam + Spamassassin- This is normally
called Sandwitch configuration in qmail.
6.Advantages of Qmail ?
More secure, better
designed, modular, faster, more reliable, easier to configure, don't have to
upgrade it every few months or worry about being vulnerable to something due to
some obscure feature being enabled
qmail supports host
and user masquerading, full host hiding, virtual domains, null clients,
list-owner rewriting, relay control, double-bounce recording, arbitrary RFC 822
address lists, cross-host mailing list loop detection, per-recipient
checkpointing, downed host backoffs, independent message retry schedules, etc.
qmail also includes a drop-in ``sendmail'' wrapper so that it will be used
transparently by your current UAs.
7.Q) What is the
difference between POP3 and IMAP ?
The Difference
POP3 works by
reviewing the inbox on the mail server, and downloading the new messages to
your computer. IMAP downloads the headers of the new messages on the server,
then retrieves the message you want to read when you click on it.
When using POP3, your
mail is stored on your PC. When using IMAP, the mail is stored on the mail
server. Unless you copy a message to a "Local Folder" the messages
are never copied to your PC.
POP3
· You only check e-mail from one computer.
· You want to remove your e-mail from the mail server.
IMAP
· You check e-mail from multiple locations.
· You use Webmail.
8.How to drop packets using iptables ?
Iptables -A INPUT -s xx.xx.xx.xx -d
xx.xx.xx.xx -j DROP
9.Daily routines of Linux Administrators ?
*.Check the health of servers
*.Check for updates
*.Check the Backup
*.Check with the trouble ticketing system for
any unread ticket.
*.Troubleshoot if there any problem
*.Installation of new servers, if needed.
*.Report to the Boss
10.How to take the Dump of a MySQL Database ?
Mysqldump databasename > dumpname
11.How to know the CPU usage of each process ?
Top, uptime
12.How to bind another IP in a NIC ?
Copy the contents eth0 to eth1, and change the
ipaddress. Restart the network. .
13.Transparently proxy all web-surfing through
Squid box
iptables -t nat -A PREROUTING -i eth1 -tcp
--dport 80 -j DNAT --to
iptables -t nat -A PREROUTING -i eth1 -tcp --dport
80 -j DNAT --to
14.Transparently redirect web connections from
outside to the DMZ web server.
iptables -t nat -A PREROUTING -i eth0 -d
192.168.1.1 -dport 80 -j DNAT –to
15 Howto Activate the forwarding
echo 1 >/proc/sys/net/ipv4/ip_forward
16.Kill spoofed packets
for f in /proc/sys/net/ipv4/conf/*/rp_filter;
do
echo 1 > $f
done.
$iptables -A LDROP --proto tcp -j LOG
--log-level info \ --log-prefix “TCP Drop”
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