Virtual Box Interview Preparation Guide

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.