What was the first computer virus in the DOS system?

What was the first computer virus in the DOS system?

• I Love You Virus
• Storm Worm Virus
• Melissa Virus
• Brain Virus 

EXPLANATION

 The 1986 Brain virus is considered to be the first computer virus to infect MS-DOS and PC-DOS. It was transmitted through an infected boots sector of a floppy disk, and was originally intended to detect and track pirated copies of the authors’ floppy-based software. Unlike most modern malware, the virus included contact information for the authors. You can read more about the Brain virus here

Of course, other viruses on other platforms predated the Brain virus. The Elk Cloner virus for the Apple II appeared five years earlier, in 1981. It was also a floppy disk boot sector virus, which infected Apple DOS 3.3. And ten years before that, in 1971, the Creeper virus made its way through the ARPANET to infect DEC PDP-10 mainframe computers running the TENEX operating system. There are other examples as well, but these are considered “firsts.”

Recorded experimental work in academia with self-replicating programs dates as far back as 1949. Most of the academic efforts were never released into the wild.

Read More

Which command can be used in PowerShell but not in CMD?

Which command can be used in PowerShell but not in CMD?

• CD
• LS
• TREE
• DIR

EXPLANATION

PowerShell is mostly used by system administrators to manage the network as well as the systems and applications that are part of that network. It can efficiently interpret both Batch command and PowerShell commands, whereas cmd can interpret only Batch commands

PowerShell is a cross-platform solution, which means that now you can run it, of course, on Windows, but also on several different Linux distributions, on MacOSX 10.12 and also on the ARM platform! 

Read More

Which of these applications is included in an Office 365 E3 License?

Which of these applications is included in an Office 365 E3 License?

• OneNote
• Access
• Visio
• Project

EXPLANATION

Office 365 E3

• Includes full desktop versions of Office apps

• Each user can install Office mobile apps on 5 PCs or Macs, 5 tablets, and 5 phones

• Five layers of security and monitoring help keep customer data safe

• Reduce IT costs, and move to the cloud on your terms

• Guaranteed 99.9% uptime, and 24/7 support from Microsoft

Office 365 E3 includes

• Word
• Excel
• PowerPoint
• Outlook
• OneNote
• SharePoint
• OneDrive
• Microsoft Teams

 

 

Read More

A PC has physical connections to two seperate networks/ subnets. One subnet has internet access, the other is an isolated test network. When the test network is connected the working internet connection fails. What is the issue, how do you know?

A PC has physical connections to two seperate networks/ subnets. One subnet has internet access, the other is an isolated test network. When the test network is connected the working internet connection fails. What is the issue, how do you know?

• There is a broadcast storm on the isolated network, rapid activity lights blinking on connection.
• A PC can't be a member of multiple subnets at once, the internet works when test network is offline
• A route for the isolated network has a metric < or = to internet connection, use route print command
• The PC has an invalid default gateway on the isolated network and can't connect to another network. 

EXPLANATION

2 PC's.
PC1 = 192.168.20.1/24
PC2 = 192.168.30.1/24

PC1 wants to ping PC2
The first thing it does is compare the IP address of PC2 with its own IP/subnetmask. It realises that PC2 is on another network.
PC1 checks its routing table to se if it has a route to PC2's subnet. Most likely it does not. But it should have a default route.
So what PC1 does is do an ARP for the default gateways IP and gets the MAC of the default address. (if it doesnt have a default gateway it then drops the ping)

PC1 then encapsulates the ping (icmp) in a ethernet frame with a destination address = MAC address of default gateway.

So when the ethernet frame arrives at the L2 switch it forwards the frame to the default gateway. NOT PC2.

Read More

In Microsoft Azure, What server size/series uses credits and slows down if the credits run out?

In Microsoft Azure, What server size/series uses credits and slows down if the credits run out?

• F series
• D series
• B series
• A series

EXPLANATION

 B-series 

These are known as “burstable” VMs.  They are very useful but the way they work is a bit complicated.  B-series are used for non-CPU intensive workloads (e.g. domain controllers, file servers) and cost about 50% of an equivalently sized Ds-series VM.  The reason they’re cheaper is because Azure imposes a quota on how much of the total CPU cores can be used.  This quota is usually a fraction of the total available CPU. 
For instance, B2m’s quota is 60% of a single CPU, which is 30% of the 2 CPUs visible in the VM.  Every second that the VM is using less than its quota (less than 60% of a single CPU) it is “banking credits”.  These banked credits can be used to burst up to the total available CPUs (100% of 2 CPUs, in this example) when needed.  While bursting, the VM is consuming its banked credits.  Once credits run out, the VM’s CPU utilization is throttled down to its 60% quota.
Why use B-series VMs?  They are cheaper.  For approximately the same price that you would pay for a Ds-series VM, you can get a B-series with double the CPUs and double the RAM.  However, they should only be used for workloads that are either not CPU intensive or “bursty”, meaning they only occasionally need all the CPU but most of the time the CPU is idle. 
For instance, an Active Directory domain controller is not utilizing its CPU very heavily on a regular basis.  However, when Windows Updates run, the VM will use all its available CPU horsepower.  B-series are perfect for Domain Controllers since they bank credits while idle and then consume them when needed to update or do some other CPU intensive task.

• Purpose: General, non-CPU intensive workloads (e.g. AD domain controllers, file servers) 
• CPU clock speed: varies 
• vCPU-to-RAM ratio: varies from 1:1 to 1:4 for VMs larger than B2s 
• Storage supported: Standard and Premium 
• Approximate average list price per CPU: ranges from $13/month to $40/month 

Nerdio Tips:

• Don’t use B-series VMs for CPU intensive workloads 
• When a B-series VM is first provisioned, it doesn’t have any banked credits and is subject to its quota limit on the CPU, which means it’s slow.  Once the VM is running idle for some time, credits get banked and the VM performance improves when it needs to burst. 
• Don’t shut down B-series VMs overnight when they are not in use.  This will not allow the VMs to bank credits for the following day of usage.

Read More

On ARM64 processors, what exception level does the kernel run in?

On ARM64 processors, what exception level does the kernel run in?

• EL3
• EL2
• EL0
• EL1

EXPLANATION

Privilege and Exception levels

Before we explain the details of the Armv8-A exception model, let's start by introducing the concept of privilege. Modern software expects to be split into different modules, each with a different level of access to system and processor resources. An example of this is the split between the operating system kernel, which has a high level of access to system resources, and user applications, which have a more limited ability to configure the system.

Armv8-A enables this split by implementing different levels of privilege. The current level of privilege can only change when the processor takes or returns from an exception. Therefore, these privilege levels are referred to as Exception levels in the Armv8-A architecture. Each Exception level is numbered, and the higher levels of privilege have higher numbers.
As shown in the following diagram, the Exception levels are referred to as EL<x>, with x as a number between 0 and 3. For example, the lowest level of privilege is referred to as EL0.

A common usage model has application code running at EL0, with an operating system running at EL1. EL2 is used by a hypervisor, with EL3 being reserved by low-level firmware and security code.

Read More

Which routing loop avoidance / prevention mechanism should be disabled on a multi-point interface?

Which routing loop avoidance / prevention mechanism should be disabled on a multi-point interface?

• spanning tree
• reverse path forwarding
• route poisoning
• split horizon 

EXPLANATION

A routing loop is a serious network problem which happens when a data packet is continually routed through the same routers over and over. The data packets continue to be routed within the network in an endless circle. A routing loop can have a catastrophic impact on a network, and in some cases, completely disabling the network. Normally Routing Loop is a problem associated with Distance Vector Protocols. How routing loops affect network performance? • A major portion of the precious bandwidth which is available for normal user traffic of the affected routers will be consumed by looping IP datagram packets. • The major portion of the processing power of the affected routers is used to process the looping IP datagram packets. Routing Loop can happen in large internetworks when a second topology change emerges before the network is able to converge on the first change. Convergence is the term used to describe the condition when all routers in an internetwork have agreed on a common topology. Link state protocols tend to converge very quickly, while distance vector protocols tend to converge slowly. The following methods are used to avoid Routing Loops. Maximum hop Count Maximum hop count mechanism can be used to prevent Routing Loops. Distance Vector protocols use the TTL (Time-to-Live) value in the IP datagram header to avoid Routing Loops. When an IP datagram move from router to router, a router keeps track of the hops in the TTL field in the IP datagram header. For each hop a packet goes through, the packet’s TTL field is decremented by one. If this value reaches 0, the packet is dropped by the router that decremented the value from 1 to 0. Split Horizon A split horizon is a routing configuration that stops a route from being advertised back in the direction from which it came. Split Horizon mechanism states that if a neighbouring router sends a route to a router, the receiving router will not propagate this route back to the advertising router on the same interface. Route Poisoning Route Poisoning is another method for avoiding routing loops. When a router detects that one of its connected routes has failed, the router will poison the route by assigning an infinite metric to it. Hold-down Timers Hold-down timer is another mechanism used to prevent bad routes from being restored and propagated by mistake. When a route is placed in a hold-down state, routers will neither advertise the route nor accept advertisements about it for a specific interval called the hold-down period.

Read More