Virtual Box Interview Questions And Answers

If the VirtualBox kernel module doesn't load then the error will be shown to the user saying that "Error inserting vboxdrv: Invalid argument". In this may the possibility that the kernel of the system might not have uploaded correctly. The problem can be checked by dmesg command that tells the error due to which the failure has occurred. The kernel of the system has its own set of configurations and it checks the configuration of the virtualbox kernel to compile the module.

To provide the http services user need Virtual box guest running on a NAT connection. The connection which is being setup uses HTTP port forwarding method to provide the http services. The http services can be provided on the Virtual box guest using the host computer by using the http server running on the host and using the port 5000. The command that is used:
VboxManage modifyvm <guest_name> --natpf1 "any-name, tcp, , 5000, , 5000".
This command will allow the virtual box guest system to communicate with the host system by forwarding all the requests on localhost of the host using port 5000.

There are 5 types of network modes that is used by Virtual Box. These are as follows:
1. Unattached: it is a mode where there is no cable required to be connected to any system and there is a standalone system running the virtual machines.
2. NAT (Network address Translation): is used to give response where the level IP addresses are low.
3. Network adapter host: it is used when DHCP server is not being provided on VPN. In this manual configuration of IP addresses is being done. For example Tftpd32: this is a network mode that works under windows and used for IP configuration.
4. Internal network: work the same way as bridge mode
5. Bridge: it is a network mode where the virtual machines can communicate with the host machine without any restrictions.

VboxClient is a process that creates files in the format of .vboxclient*.pid in the home directory of the user. This helps process to know that a process is already running and it prevents another service from starting. This doesn't allow the deletion of the files after the service of the process being stopped. The files are always owned by root. Once stopped it prevents the process to start in future.

Virtual box supports 32 bit and 64 bit ELF format for the core files. The file is being managed by VboxManage and the file that is being used is debugvm. The core files of the VM consists of memory and CPU dumps of VM. The example is given about the layout of VM core below:

[ ELF 64 Header]
[ Program Header, type PT_NOTE ]
-> offset to COREDESCRIPTOR
[ Program Header, type PT_LOAD ]
-> Memory offset of range
-> File offset
[ Note Header, type NT_VBOXCPU ] - one for each vCPU
[ vCPU 1 Note Header ]
[ CPUMCTX - vCPU 1 dump ]
[ Additional Notes + Data ] - currently unused
[ Memory dump ]

Virtaul box consists of in built debugger that debugs the virtual machine. This debugger controls and maintains the virtual machine state. It supports symbolic symbols debugging that determines the guest operating system version and locate the kernel symbols that is being used in guest’s memory. Debugging commands are used to check, execute and find the status of a symbol. Debuggers are important as it provides necessary errors that has to be removed, to successfully run the virtual machine instance for debugging.

Release log file is being maintained the moment Virtual box starts up. It consists of information about virtual machine configuration and the events that are being perfomed during runtime. The log file that is being maintained is known as Vbox.log. At the beginning of the startup of the virtual machine the original log file gets renamed to .1-.3. These logs help to troubleshoot the problem if any. It saves for each error a different log which can be seen for the possible solutions. It consists of diagnostics information like Host operating system, version of OS, etc.

Virtual box uses qemu functionalities like virtual hardware devices concepts and the recompiler that is used with the qemu as its fallback mechanism. Quemu allows the use of virtual machine monitor that executes in real time and maintain the application at run time. It provides more efficiency and high performance when used with virtual box.

There are 5 types of network modes that is used by Virtual Box. These are as follows:
1. Unattached: it is a mode where there is no cable required to be connected to any system and there is a standalone system running the virtual machines.
2. NAT (Network address Translation): is used to give response where the level IP addresses are low.
3. Network adapter host: it is used when DHCP server is not being provided on VPN. In this manual configuration of IP addresses is being done. For example Tftpd32: this is a network mode that works under windows and used for IP configuration.
4. Internal network: work the same way as bridge mode
5. Bridge: it is a network mode where the virtual machines can communicate with the host machine without any restrictions

VirtualBox is a type-2 hypervisor and VMWare ESX server is type-1 hypervisor that runs as a bare metal (i.e. directly as an OS). Virtualbox is a standalone software that is used to run on an OS and uses the system kernel to communicate with the applications. The work is in process and Sun microsystem working on the VirtualBox that can run as OS and it is named as xVM server.

To share the virtual machines to different platform the VDI that is virtual device interfaces has to be shared between both the OS so that they can read and write the file. There is a provision to create a new virtual machine on a system and select the VDI from the host on which you are currently working and select it as a hard drive with the same previous settings. The settings like MAC address and virtual NICs cards also have to be same to avoid the problem of activation. To share the command that can be written is as follows:
Machine uuid= "{ some hex value}" name= "name of your OS" OSType= "Type of OS you are using"

NAT is called as Network address translation and it functions the same way. It just translates TCP or UDP packages. To setup NAT on the host system there is a requirement for IP protocol and the ping command that is not translated using this networking method. To set the NAT user need to set the DNS correctly that matches with the host system. DNS relay has to be set to forward all DNS queries to search the correct system as a server. The test can be done by setting all the configuration by going to the browser and entering the IP address of the guest. If it doesn't work then setting of DNS has to be done again. Checking of the firewall has to be seen to see that the connections are not getting blocked and it is getting enough permissions to run its own IP through the virtual machine.

VirtualBox uses the COM (Component object model) which is developed by microsoft. It uses it for inter and intra process communication. This COM object allows easy sharing of common configuration between different virtual machines and its processes. It provide many user interface option to easily customize the graphics according to their requirements. The user interface is based on a common architecture that includes all the configurations and status information that is globally defined by the process VboxSVC. This process is out-of-process COM server and terminates itself after few seconds. This unexpected behavior keep the COM server running and keep the configuration files locked Due to this there are issues to save the configuration and settings for the user.

While using other servers like iSCSI server deadlock can occur on the host system that try to run the iSCSI target in a guest virtual machine. There are many flaws that can come within an OS to response to the virtual machine's request. Some problems are caused by cache manager component that gives some delays in the response related to an error. Till the time the error is coming the guest gets blocked and unstability occurs in the virtual machine itself. To solve this problem environment variables can be set by setting the variable VBOX_DISABLE_HOST_DISK_CACHE to 1.

RDP client is used to connect virtual machines. This RDP client is also called as Remote Desktop connection. By using this client the communication between the virtual machines gets delayed. This delay is occurring due to the fact that RDP client connects input for a set amount of time before sending it to the RDP server. The duration of the time can be lowered down by using the registry key. If smaller values then the default value then the duration of delays can be delayed. The registry files that can be changed are as follows:
HKEY_CURRENT_USER\Software\Microsoft\Terminal Server Client\Min Send Interval
The setting can be done in the local machine also.
HKEY_LOCAL_MACHINE\Software\Microsoft\Terminal Server Client\Min Send Interval

If hardware devices are not recognized by VirtualBox then first thing to do is to set the media change notification that notifies the user when his medium changes. Setting and configuration should be changed such that it doesn't get turned off. Second thing that can be done is tweaking the registry of the system by going to:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\HardwareDevice\Autorun
This registry entry will allow the hardware devices to be recognized. Set the value as 1 to turn on the automatic notification.