.
Answer - LD_LIBRARY_PATH
is an environment variable. It is used for debugging a new library or a non
standard library.......
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.
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.
How secured is Linux? 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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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. SYNTAXcrontab [ -u user ] filecrontab [ -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.
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. Syntaxumask [-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).
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