Friday, October 18, 2013

1) What is the difference between TCP and UDP?

The basic difference is that TCP establishes a connection before sending data and this allows it to control the dataflow and guarantee that all packets get delivered. UDP simply chucks datagrams onto the wire and if some get lost or arrive in bad order there’s no way to request a resend. However UDP has low network overhead so some services such as DNS resolution, SNMP, DHCP, RIP and VOIP use UDP for its speed and any errors are usually dealt with on the application layer rather than network layer.

2) What is the TCP hand shake?

TCP requires three packets to set up a socket connection, before any user data can be sent. This is called the tree way TCP handshake. First the requester sends a SYN packet and expects a SYN-ACK packet, to which the initiator replies with ACK packet plus the first chunk of user data. From there on the TCP connection is established and two sides exchange user data using features such as message acknowledgment, retransmission and timeout.

3) How does DNS resolution work?

A client application requests an IP address from the name server usually by connecting to UDP port 53. The name server will attempt to resolve the FQDN based on its resolver library, which may contain authoritative information about the host requested or cached data about that name from an earlier query. If the name server does not already have the answer, it will turn to root name servers to determine the authoritative for the FQDN in question. Then, with that information, it will query the authoritative name servers for that name to determine the IP address.

4) What is an MX record?

MX record numerically ranks the mail servers you would prefer to receive email for a domain. The MX record with the lowest number is preferred over the others, but you can set multiple email servers with the same value for simple load balancing.

5) Describe Linux boot-up sequence

BIOS reads the MBR where Boot Loader sits, Boot Loader reads Kernel into memory, Kernel starts Init process, Init reads inittab, executes rc.sysinit, the rc script than starts services to reach the default run level and once this is done the last thing that gets run is the rc.local script.

6) What is an INODE?

All files have its description stored in a structure called ‘inode’. The inode contains info about the file-size, access and modification time, permission and so on. In addition to descriptions about the file, the inode contains pointers to the data blocks of the file.

7) How do you search for a pattern and than replace it in an entire file?

Using Sed or in Vi editor the search usually involves character ‘s’ slash the pattern to be searched, slash the pattern to replace it with, slash ‘g’ which stands for entire file.

8) How do you list and flush all IPtables?

Using the iptables command with –L switch first to see all the rules currently present in memory and than –F to flush them.

9) How do you list compiled-in Apache modules?

Run the httpd daemon as a command with –l parameter.

10) What is a zombie?

Zombie processes can be identified in the output of ‘ps’ by the presence of ‘Z’ in the STAT column. Zombies are child processes whose parent process died without reaping its children. Zombies can’t be killed with the usual KILL signal.

11) What does /etc/skell directory contains?

     The /etc/skel directory contains files and directories that are automatically copied over to a
new user's home directory when such user is created by the useradd or adduser command.

12) What is RAID?

      RAID, stands for Redundant Array of Inexpensive Disks. RAID is a method by which same data or information is spread across several disks, using techniques such as disk striping (RAID Level 0), disk mirroring (RAID Level 1), and disk striping with parity (RAID Level 5) to achieve redundancy, lower latency, increased bandwidth, and maximized ability to recover from hard disk crashes.

13) What is SWAP Space?

Swap space in Linux is used when the amount of physical memory (RAM) is full. If the system needs more memory resources and the RAM is full, inactive pages in memory are moved to the swap space. While swap space can help machines with a small amount of RAM, it should not be considered a replacement for more RAM. Swap space is located on hard drives, which have a slower access time than physical memory.

14) How you will create swap file of size 4 GB and explain swap file entry in /etc/fstab file?


Use “dd” command to create swap file.
dd if=/dev/zero  of=/SWAPFILE  bs=1024  count=4
mkswap /SWAPFILE
swapon –a
Entry into /etc/fstab file.

15) Tell me the steps to remove the swap file?


Firstly disable the swap file by “swapoff” command.
Remove Swap file entry from /etc/fstab file.
Now remove the swap file by “rm” command.

16) What can we do with “parted” command or utility?


- View the existing partition table
- Add partitions from free space or additional hard drives
- Change the size of existing partitions

17) How we will check free space on drive /dev/sda with parted command?

#parted /dev/sda

18)Can we resize the size of a partition?


Yes, we can resize the size of partition by “parted” command.
#parted /dev/sda


To resize the partition, use the resize command followed by the minor number for the partition, the starting place in megabytes, and the end place in megabytes. For example:

resize 3 1024 2048

After resizing the partition, use the print command to confirm that the partition has been resized correctly, is the correct partition type, and is the correct file system type.

19) What is the fastest way to enter a series of commands from the command-line?

Write the commands, each separated by a semi-colon. Press enter after the last command. 

The semi-colon would inform the shell that multiple commands are being entered at the command line, to be executed serially.

20) What are the process states in Unix?

As a process executes it changes state according to its circumstances. Unix processes have the following states:

Running : The process is either running or it is ready to run .
Waiting : The process is waiting for an event or for a resource.
Stopped : The process has been stopped, usually by receiving a signal.
Zombie : The process is dead but have not been removed from the process table.

21) What command should you use to check the number of files and disk space used and each user's defined quotas?


22) What command is used to remove the password assigned to a group?

gpasswd -r

23) What can you type at a command line to determine which shell you are using?

echo $SHELL 

24) Write a command to find all of the files which have been accessed within the last 30 days.

find / -type f -atime -30 > filename.txt

25) What daemon is responsible for tracking events on your system?


26) What do you mean a File System?

File System is a method to store and organize files and directories on disk. A file system can have different formats called file system types. These formats determine how the information is stored as files and directories.