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Questions Interview A

TFTP protocol port 69 UDP or TCP

TFTP is a very simple file transfer protocol.copy files across a network TFTP is UDP  based port 69 TFTP server and port 69 UDP or TCP or Both TFTP is udp based so there is no need for the deny tcp statement. TFTP is using for PXE boot Due to its simple design, TFTP could  Full Article…

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CentOS init startup scripts

Some services are configured to start in your Xen virtual machine out of the box. The scripts that run on start up are stored in the /etc/init.d/ directory. You can start/stop/restart services with out rebooting your Xen virtual machine by running the script with the argument of start, stop, or restart. For example: /etc/init.d/scriptname restart  Full Article…

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Understanding Linux CPU LOAD Load average

Load averages are the three numbers shown with the uptime and top commands – they look like this: load average: 0.09, 0.05, 0.01 Most people have an inkling of what the load averages mean: the three numbers represent averages over progressively longer periods of time (one, five, and fifteen minute averages), and that lower numbers  Full Article…

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Where Are the Linux Kernel Files?

The kernel file, in Ubuntu, is stored in your /boot folder and is called vmlinuz-version. The name vmlinuz comes from the unix world where they used to call their kernels simply “unix” back in the 60’s so Linux started calling their kernel “linux” when it was first developed in the 90’s. When virtual memory was  Full Article…

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The /proc filesystem

The /proc/ directory — also called the proc file system — contains a hierarchy of special files which represent the current state of the kernel — allowing applications and users to peer into the kernel’s view of the system. Under Linux, all data are stored as files. Most users are familiar with the two primary  Full Article…

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Can describe the Linux filesystem and the directories found in there?

/bin : All the executable binary programs (file) required during booting, repairing, files required to run into single-user-mode, and other important, basic commands viz., cat, du, df, tar, rpm, wc,history, etc. /boot : Holds important files during boot-up process, including Linux Kernel. /dev : Contains device files for all the hardware devices on the machine e.g.,  Full Article…

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I assume you know what redundancy is. Can you tell me can you use it on a server? What components is it referring to?

In computing, failover is switching to a redundant or standby computer server, system, hardware component or network upon the failure or abnormal termination of the previously active application, server, system, hardware component, or network.   Server redundancy refers to the amount and intensity of backup, failover or redundant servers in a computing environment. It defines  Full Article…

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How would you install a software package in a given distribution: RedHat/Debian

To update the package lists type apt-get update To search for a package type apt-cache search followed by a keyword such as spreadsheet. Type apt-get install “program name”. For example, to install the Dillo web browser, you would type apt-get install dillo. To remove software is just as easy. apt-get remove ${packagename} Although the repositories  Full Article…

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full interview

Systems Engineer Systems Engineer –  Operations Engineering Who we are: EC2 OpsEng is at the heart of the leading Infrastructure-as-a-Service cloud computing platform. We manage the thundering herd of servers and systems that comprise EC2, using a service-oriented approach to large-scale systems management. You’ll be part of a world-class team in a fast-paced environment that  Full Article…

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what’s a Buffer?

In the context of computers buffer typically refers to computer memory that is used to temporarily store data while information is transitioning from one location to another. If the source of the data produces them at a lower rate than they are retrieved by the receiving process it will be more efficient to accumulate the data in  Full Article…

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Layer Four Traceroute (LFT)

Layer Four Traceroute (LFT) is a fast, multi-protocol traceroute engine, that also implements numerous other features including AS number lookups through Regional Internet Registries and other reliable sources, Loose Source Routing, firewall and load balancer detection, etc. LFT is best known for its use by network security practitioners to trace a route to a destination  Full Article…

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Does traceroute use UDP

he type of packet that is sent differs depending on the implementation. By default Windows tracert uses ICMP and both Mac OS X and Linux traceroute use UDP. I don’t have BSD or Solaris machines or any other OS on hand to check but the man page for the Mac OS X version mentions its  Full Article…

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What is setuid?

setuid means set user ID upon execution. If setuid bit turned on a file, user executing that executable file gets the permissions of the individual or group that owns the file. You need to use the ls -l or find command to see setuid programs. All setuid programs displays S or s in the permission  Full Article…

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What is swap? Also, “when is it bad?”

In it’s simplest sense, the SWAP partition acts as an overflow to your (RAM) memory. If your memory is filled up completely, any additional applications will be run off of the SWAP partition rather than memory This sounds like an easy way to increase the amount of usable memory without actually getting more RAM, but  Full Article…

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Comparison between TCP/IP and OSI

Like OSI network model, TCP/IP also has a network model. TCP/IP was on the path of development when the OSI standard was published and there was interaction between the designers of OSI and TCP/IP standards. The TCP/IP model is not same as OSI model. OSI is a seven-layered standard, but TCP/IP is a four layered  Full Article…

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OSI Model

Layer # Name Mnemonic Encapsulation Units Devices or Components Keywords/Description 7 Application All data PC Network services for application processes, such as file, print, messaging, database services 6 Presentation People data Standard interface to data for the application layer. MIME encoding, data encryption, conversion, formatting, compression 5 Session Seem data Interhost communication. Establishes, manages and  Full Article…

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sticky bit on a directory

The output of the command ls -ld /tmp is: drwxrwxrwt 30 root root 20480 Mar 11 14:17 /tmp So I have two main questions: What is the letter t after the permissions? As far as I know /tmp is used to create temporary files related to different users in the system, so how come it has permission rwxrwxrwx (777)? A sticky  Full Article…

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Learn Sticky Bit with examples

Sticky Bit is mainly used on folders in order to avoid deletion of a folder and its content by other users though they having write permissions on the folder contents. If Sticky bit is enabled on a folder, the folder contents are deleted by only owner who created them and the root user. No one else  Full Article…

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Manage software RAID arrays

1. Create a new RAID array Create (mdadm –create) is used to create a new array: 1 mdadm –create –verbose /dev/md0 –level=1 /dev/sda1 /dev/sdb2 or using the compact notation: 1 mdadm -Cv /dev/md0 -l1 -n2 /dev/sd[ab]1 2. /etc/mdadm.conf /etc/mdadm.conf or /etc/mdadm/mdadm.conf (on debian) is the main configuration file for mdadm. After we create our RAID  Full Article…

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Linux installs using kickstart and PXE

Here is what worked: ——————– Goals: 1) Create a linux build that could be used on -the laptops at work. -the servers at work. 2) No floppy should be required 3) Build should not trash a windows installation that co-exists on the laptop Notes: —— Although Kickstart is an RFC standard, it only seems to  Full Article…

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How to Use MDADM Linux Raid

Create a 6 drive raid 5 array called /dev/md0 with chunk size of 16384: (typically, bigger chunk sizes are better for bigger files, default is 512) mdadm –create –level=5 –chunk=16384–raid-devices=6 /dev/md0 /dev/sdb /dev/sdc /dev/sdd /dev/sde /dev/sdf /dev/sdg Assemble raid array that is not in the config file: mdadm –assemble /dev/md0 /dev/sdb /dev/sdc /dev/sdd /dev/sde /dev/sdf  Full Article…

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mdadm cheat sheet

create a new array mdadm –create /dev/md0 –level=1 –raid-devices=2 /dev/sda1 /dev/sdb1 or… mdadm -C /dev/md0 -l1 -n2 /dev/sd[ab]1   add array to the configuration file: mdadm –detail –scan >> /etc/mdadm.conf or on debian (sigh)… mdadm –detail –scan >> /etc/mdadm/mdadm.conf   can only remove failed disks from an array, so fail a disk: mdadm –fail /dev/md0  Full Article…

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Creating a RAID Device With mdadm

To create a RAID device, edit the /etc/mdadm.conf file to define appropriate DEVICE and ARRAY values: DEVICE /dev/sd[abcd]1 ARRAY /dev/md0 devices=/dev/sda1,/dev/sdb1,/dev/sdc1,/dev/sdd1 In this example, the DEVICE line is using traditional file name globbing (refer to the glob(7) man page for more information) to define the following SCSI devices: /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1 The ARRAY line defines a RAID device (/dev/md0) that is comprised of the SCSI  Full Article…

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Setup Kickstart Server in Linux

Red Hat Linux operating system installations can be done via a network connection using a Kickstart server. It is frequently much faster than using CDs and the process can be automated. Example Kickstart Get the kickstart cfg from http server and start the install boot: linux ks=http://server.com/path/to/kickstart/file Get the kickstart cfg from nfs server and  Full Article…

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Red Hat Kickstart

Red Hat has created the kickstart installation method. With this method, a system administrator can create a single file containing the answers to all the questions that would normally be asked during a typical Red Hat interactive Linux installation. Kickstart files can be kept on single server system, and read by individual computers during the  Full Article…

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HTTP Hypertext Transfer Protocol

The operation of HTTP Hypertext Transfer Protocol involves the communication between a Hypertext Transfer Protocol (HTTP) client application (Usually web browser) and a Hypertext Transfer Protocol (HTTP) server application (Web servers like IIS). Hypertext Transfer Protocol (HTTP) uses Transmission Control protocol TCP as the Transport Layer Protocol at Know port number80. Once the TCP  connection  Full Article…

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STP – Spanning-Tree Protocol

Spanning-Tree Protocol (STP) prevents loops from being formed when switches or bridges are interconnected via multiple paths. The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them. Spanning tree also allows a network design to include spare (redundant) links to provide automatic backup paths if an active  Full Article…

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What is inode?

An inode is an entry created on a section of the disk set aside for a file system. The inode contains nearly all there is to know about a file, which includes the location on the disk where the file starts, the size of the file, when the file was last used, when the file  Full Article…

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What is a subnetting

Subnetting enables the network administrator to further divide the host part of the address into two or more subnets. In this case, a part of the host address is reserved to identify the particular subnet. This is easier to see if we show the IP address in binary format.   A subnet allows the flow of network  Full Article…

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Port Numbers

20 FTP data (File Transfer Protocol) 21 FTP (File Transfer Protocol) 22 SSH (Secure Shell) 23 Telnet 25 SMTP (Send Mail Transfer Protocol) 43 whois 53 DNS (Domain Name Service) 68 DHCP (Dynamic Host Control Protocol) 79 Finger 80 HTTP (HyperText Transfer Protocol) 110 POP3 (Post Office Protocol, version 3) 115 SFTP (Secure File Transfer  Full Article…

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What is the difference between UDP and TCP?

TCP 1) TCP -Transmission control protocol 2) TCP is a connection oriented protocol. 3) Three way handshake happens between client and server. 4) TCP is a reliable data transfer 5) slow transmission of data compare to UDP. 6) TCP is used to send file like database,where reliability play the first role UDP 1) UDP -User  Full Article…

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What are the different RAID levels?

There are at least nine types of RAID plus a non-redundant array (RAID-0): RAID-0: This technique has striping but no redundancy of data. It offers the best performance but no fault-tolerance. RAID-1: This type is also known as disk mirroring and consists of at least two drives that duplicate the storage of data. There is no striping.  Full Article…

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Crossover Cable

Sometimes you will use crossover cable, it’s usually used to connect same type of devices. A crossover cable can be used to: 1) Connect two computers directly. 2) Connect a router’s LAN port to a switch/hub’s normal port. (Normally used for expanding network) 3) Connect two switches/hubs by using normal port in both switches/hubs. In  Full Article…

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Straight Cable

Usually use straight cable to connect different type of devices. This type of cable will be used most of the time and can be used to: 1) Connect a computer to a switch/hub’s normal port. 2) Connect a computer to a cable/DSL modem’s LAN port. 3) Connect a router’s WAN port to a cable/DSL modem’s  Full Article…

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Gateway – network gateway

A gateway is a network point that acts as an entrance to another network. A network gateway is an internetworking system capable of joining together two networks that use different base protocols. A network gateway can be implemented completely in software, completely in hardware, or as a combination of both. Depending on the types of  Full Article…

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DHCP – Dynamic Host Configuration Protocol

The Dynamic Host Configuration Protocol (DHCP) is a standardized networking protocol used by servers on an IP network to allocate IP addresses to computers. The purpose of DHCP is to automate the IP address configuration of a computer without a network administrator. An IP address is assigned to a computer for a set interval, after  Full Article…

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Redundancy of the Internet connection – data center

A data center is a facility used to house computer systems and associated components, such as telecommunications and storage systems. It generally includes redundant or backup power supplies, redundant data communications connections, environmental controls (e.g., air conditioning, fire suppression) and security devices. IT operations are a crucial aspect of most organizational operations around the world.  Full Article…

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IPTables In Linux Explained

IPtables (Netfilter) : IPtables is the default firewall for Linux. Its a vast subject which can not be covered in one post. I will try to give as much info as possible at the same time not to make it complex. Lets start with basics.   What is a firewall? Ans : A firewall is a part of a computer  Full Article…

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SFTP Secure File Transfer Protocol

SFTP is similar to FTP. SFTP stands for Secure File Transfer Protocol and is the encrypted version of FTP. SFTP is more secure and tends to be more reliable than FTP, so it is the recommended option where possible.   The command for starting sftp session is as follows: sftp username@server1 OR sftp username@192.168.1.5 Step  Full Article…

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Linux VFS -Virtual File System

In Linux, Everything Is a File. In Linux, everything is a file, and all files can be found in a single, unified directory hierarchy, also known as the file system. Linux assigns a name to each device too, but this is not how the files on that device are accessed. There are no drive letters  Full Article…

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benefits of upgrading filesystem from ext2?

Wikipedia covers most of this: http://en.wikipedia.org/wiki/Ext3 http://en.wikipedia.org/wiki/Ext4 The big advantage of ext3 is journalling, which means you don’t need to fsck every time you unmount uncleanly (e.g. power failure / kernel panic). ext4 makes fscking much faster, when you do need to. I’d recommend you definitely to upgrade to ext4 and not only to ext3,  Full Article…

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Differences Between IPTables and IPChains

Both ipchains and iptables use chains of rules that operate within the Linux kernel to filter packets based on matches with specified rules or rule sets. However, iptables offers a more extensible way of filtering packets, giving the administrator greater control without building undue complexity into the system. You should be aware of the following significant differences between ipchains and iptables: Using iptables, each filtered  Full Article…

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Basic iptables howto

Iptables is a firewall, installed by default on all official Ubuntu distributions (Ubuntu, Kubuntu, Xubuntu). When you install Ubuntu, iptables is there, but it allows all traffic by default. Ubuntu 8.04 Comes with ufw – a program for managing the iptables firewall easily. There is a wealth of information available about iptables, but much of it is  Full Article…

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IPTables Tables, Chains, Rules Fundamentals

iptables firewall is used to manage packet filtering and NAT rules. IPTables comes with all Linux distributions. Understanding how to setup and configure iptables will help you manage your Linux firewall effectively. iptables tool is used to manage the Linux firewall rules. At a first look, iptables might look complex (or even confusing). But, once  Full Article…

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Linux init Run Levels

SysV init and systemd both support the notion of run levels but manage them in different ways. SysV init uses symbolically linked start/stop init scrips located in directories defining the services to be run (/etc/rc.d/rc[0-6].d/) while systemd manages the services with “cgroups”. Typical run levels include: 0 — Halt 1 — Single-user text mode 2  Full Article…

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STP protocol

spanning tree protocol (STP) Where two bridges are used to interconnect the same two computer network segments, spanning tree is a protocol that allows the bridges to exchange information so that only one of them will handle a given message that is being sent between two computers within the network. The spanning tree protocol prevents  Full Article…

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OSI 7 Layer model

The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy. Easy Way to Remember  Full Article…

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DNS and How DNS Works?

DNS stands for Domain Name System, or Domain Name Server. DNS resolves an IP address to a hostname or vice versa. DNS resolves hostname to IP address (forward lookup), resolves IP address to hostname (reverse lookup), it allows machines to logically grouped by name domain, provides email routing. DNS port: 53 dameon: named DNS port  Full Article…

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Difference between core and processor?

A core is usually the basic computation unit of the CPU – it can run a single program context (or multiple ones if it supports HW threads such as hyperthreading on intel CPUs), maintaining the correct program state, registers, and correct execution order, and performing the operations through ALUs. For optmization purposes, a core can  Full Article…

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Linux File Systems: Ext2 vs Ext3 vs Ext4

ext2, ext3 and ext4 are all filesystems created for Linux. This article explains the following: High level difference between these filesystems. How to create these filesystems. How to convert from one filesystem type to another.   Ext2 Ext2 stands for second extended file system. It was introduced in 1993. Developed by Rémy Card. This was  Full Article…

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Linux Ext2 VS Ext3 File Systems in Linux

The differences between Ext2 and Ext3 file systems are as follows. Sl.No EXT2 EXT3 Journaling No journaling Has journaling Speed of file system(read-write) Bit faster Bit slower then ext2 file-system Data corruption File system may be corrupted due to unplanned reboots Prevents file-system corruption Way to recover data Require fsck to recover data after unplanned reboot Does not require(automatic  Full Article…

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Manage all HP servers, from anywhere – iLO

Precise control and web-based remote management available 24/7 is a necessity, not a luxury. Integrity iLO makes it easy to remotely manage Integrity servers from just about anywhere in the world. Remotely manage system health over a secure LAN interface through either a text-based command-line control or from a browser-based, easy-to-use GUI. HP iLO solutions use a  Full Article…

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iLO – HP Integrated Lights-Out

HP iLO – The preferred IT administrator in every ProLiant server When reliability is essential for your system health, HP Integrated Lights-Out (iLO) provides the automated intelligence to maintain complete control. HP iLO functions out-of-the-box without additional software installation and regardless of the servers’ state of operation. Get complete access from any location via a  Full Article…

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What is LILO and its uses?

LILO is an alternative for GRUB. It is the LInux boot LOader. Because GRUB is has more features it is commonly used with most distributions. LILO is smaller, brilliant for booting from floppy disk, but otherwise legacy. (I’m sure some people will get angry with me for saying LILO is legacy, but for current full  Full Article…

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Linux Boot Process – Chart

Boot process chart System Startup BIOS system start BIOS located on flash memory MBR Bootloader MBR contain bootloader . Bootloader (grub) goes thru two stages.   GRUB Stage1 Stage2 Kernel Init After the two stages, kernel and initrd images is loaded in memory Init Kernel sarts first process init. It looks for /etc/inittab file Runlevel  Full Article…

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How to configure IP address in Linux

Every node participating in networking needs a valid IP address. On Linux command prompt IP address is assigned by a network configuration window. This window can be invoked by selecting network configuration sub menu form setup command or directly executing system-config-network commands. Run setup command form root user #setup this will launch a new window  Full Article…

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Linux Filesystem

In Linux, everything is a file.   Directory Tree . Linux Filesystem Hierarchy /bin – binary   ls , cd, pwd /boot /dev /etc /home – home directory /initrd /lib /lost+found /media /mnt /opt – optional files optional /proc /root – root’s home /sbin  – super user root ifup, ifconfig, ifdown /usr – user program files  Full Article…

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