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The FROM statement in a Dockerfile specifies the base image from which the subsequent instructions are executed1. The value of the FROM statement can be either an image name, an image name with a tag, or an image ID1. The image name can be either a repository name or a repository name with a registry prefix2. For example, ubuntu is a repository name, and docker.io/ubuntu is a repository name with a registry prefix2. The tag is an optional identifier that can be used to specify a particular version or variant of an image1. For example, ubuntu:focal refers to the image with the focal tag in the ubuntu repository2. The image ID is a unique identifier that is automatically generated when an image is built or pulled1. For example, sha256:9b0dafaadb1cd1d14e4db51bd0f4c0d56b6b551b2982b2b7c637ca143ad605d2 is an image ID3.

Therefore, the only valid value in the FROM statement among the given options is ubuntu:focal, which is an image name with a tag. The other options are invalid because:

docker://ubuntu:focal is not a valid image name format. The docker:// prefix is used to specify a transport protocol, not a registry prefix4.

registry:ubuntu:focal is not a valid image name format. The registry prefix should be a valid hostname or IP address, not a generic term2.

file:/tmp/ubuntu/Dockerfile is not a valid image name format. The file: prefix is used to specify a local file path, not an image name5.

http://docker.example.com/images/ubuntu-focal.iso is not a valid image name format.  The http:// prefix is used to specify a web URL, not an image name 5.

References:

1: Dockerfile reference | Docker Docs

2: docker - Using FROM statement in dockerfile - Stack Overflow

3: How to get the image id from a docker image - Stack Overflow

4: skopeo - Docker Registry v2 API tool - Linux Man Pages (1)

5: How to build a Docker image from a local Dockerfile? - Stack Overflow

The type attribute of a element in a libvirt domain definition specifies the hypervisor used for running the domain. The allowed values are driver specific, but include “xen”, “kvm”, “hvf” (since 8.1.0 and QEMU 2.12), “qemu” and "lxc"1. Therefore, the valid values among the options are C. kvm and E. lxc. KVM stands for Kernel-based Virtual Machine, which is a full virtualization solution for Linux on x86 hardware containing virtualization extensions (Intel VT or AMD-V)2. LXC stands for Linux Containers, which is an operating system-level virtualization method for running multiple isolated Linux systems (containers) on a single control host3. The other options are not valid values for the type attribute, asthey are either not hypervisors or not supported by libvirt. References:http://libvirt.org/formatdomain.html

https://libvirt.org/formatcaps.html

Sparse images are a type of virtual disk images that grow in size as data is written to them, but do not shrink when data is deleted from them. Sparse images may consume an amount of space different from their nominal size, which is the maximum size that the image can grow to. For example, a sparse image with a nominal size of 100 GB may only take up 20 GB of physical storage if only 20 GB of data is written to it. Sparse images allocate backend storage at the first usage of a block, which means that the physical storage is only used when the virtual machine actually writes data to a block. This can save storage space and improve performance, as the image does not need to be pre-allocated or zeroed out.

Sparse images are not automatically shrunk when files within the image are deleted, because the virtual machine does not inform the host system about the freed blocks. To reclaim the unused space, a special tool such as virt-sparsify1 or qemu-img2 must be used to compact the image. Sparse images can be used with both full virtualization and paravirtualization, as the type of virtualization does not affect the format of the disk image. Sparse images are not automatically resized when their maximum capacity is about to be exceeded, because this would require changing the partition table and the filesystem of the image, which is not a trivial task. To resize a sparse image, a tool such as virt-resize3 or qemu-img2 must be used to increase the nominal size and the filesystem size of the image. References: 1 (search for “virt-sparsify”), 2 (search for “qemu-img”), 3 (search for “virt-resize”).

To attach a container to a network when creating it, the --network flag must be used with the name of the network as the argument. The --network flag specifies the network mode for the container. By default, the network mode is bridge, which means the container is connected to the default bridge network. However, if a custom network is created, such as isolated_nw in this case, the container must be explicitly attached to it using the --network flag. For example, to create a container named web1 and attach it to the isolated_nw network, the command would be:

docker create --name web1 --network isolated_nw nginx

The other options are not valid parameters for docker create. The --eth0, --ethernet, and --attach flags do not exist. The --alias flag is used to specify an additional network alias for the container on a user-defined network, but it does not attach the container to the network. References:

docker network create | Docker Documentation1

docker create | Docker Documentation

Networking overview | Docker Docs2

Libvirt supports creating and managing various types of virtual networks that can be used to connect virtual machines to each other or to the external network. One of the common types of virtual networks is the NAT-based network, which uses network address translation (NAT) to allow virtual machines to access the outside world through the host’s network interface. By default, libvirt creates a NAT-based network called ‘default’ when it is installed and started. This network appears as a standard Linux bridge device on the host system, named virbr0. The bridge device has an IP address of 192.168.122.1/24 and acts as a gateway and a DHCP server for the virtual machines connected to it. The bridge device also has iptables rules to forward and masquerade the traffic from and to the virtual machines. The virtual machines connected to the ‘default’ network have their own IP addresses in the 192.168.122.0/24 range and their own MAC addresses generated by libvirt. The virtual machines can communicate with each other, with the host, and with the external network through the bridge device and the NAT mechanism12.

The other statements in the question are false regarding networking with libvirt. Libvirt’s network functionality is not limited to connecting virtual machines to a physical network interface of the host system. Libvirt can also create isolated networks that do not have any connection to the outside world, or routed networks that use static routes to connect virtual machines to the external network without NAT3. Libvirt does not assign the same MAC address to all virtual machines and isolate their network interfaces at the link layer. Libvirt assigns a unique MAC address to each virtual machine and allows them to communicate with each other at the network layer4. Libvirt does not require a dedicated network interface that may not be used by the host system. Libvirt can share the host’s network interface with the virtual machines using NAT or bridging, or it can pass a physical network interface to a virtual machine exclusively using PCI passthrough5. Libvirt does not support exactly one virtual network and connect all virtual machines to it. Libvirt supports creating and managing multiple virtual networks with different names and configurations, and connecting virtual machines to different networks according to their needs6. References:

libvirt: Virtual Networking

libvirt: NAT forwarding (aka “virtual networks”)

libvirt: Routed network

libvirt: MAC address

libvirt: PCI passthrough of host network devices

[libvirt: Network XML format]

The command virsh pool-create-as vms dir --target /vms creates and starts a transient storage pool named vms of type dir with the target directory /vms12. This command resolves the issue of the storage pool not found error, as it makes the existing directory /vms visible to libvirt as a storage pool. The other commands are invalid because:

dd if=/dev/zero of=/vms bs=1 count=0 flags=name:vms is not a valid command syntax. The dd command does not take a flags argument, and the output file /vms should be a regular file, not a directory3.

libvirt-poolctl new --name=/vms --type=dir --path=/vms is not a valid command name. There is no such command as libvirt-poolctl in the libvirt package4.

qemu-img pool vms:/vms is not a valid command syntax. The qemu-img command does not have a pool subcommand, and the vms:/vms argument is not a valid image specification5.

touch /vms/.libvirtpool is not a valid command to create a storage pool. The touch command only creates an empty file, and the .libvirtpool file is not recognized by libvirt as a storage pool configuration file6.

References:

1: virsh - difference between pool-define-as and pool-create-as - Stack Overflow

2: dd(1) - Linux manual page - man7.org

3: 12.3.3. Creating a Directory-based Storage Pool with virsh - Red Hat Customer Portal

4: libvirt - Linux Man Pages (3)

5: qemu-img(1) - Linux manual page - man7.org

6: touch(1) - Linux manual page - man7.org

The file procs in a cgroup directory contains the list of processes belonging to this cgroup. Each line in the file shows the PID of a process that is a member of the cgroup. A process can be moved to a cgroup by writing its PID into the cgroup’s procs file. For example, to move the process with PID 24982 to the cgroup cg1, the following command can be used: echo 24982 > /sys/fs/cgroup/cg1/procs1. The file procs is different from the file tasks, which lists the threads belonging to the cgroup. The file procs can be used to move all threads in a thread group at once, while the file tasks can be used to move individual threads2. References:

Creating and organizing cgroups · cgroup2 - GitHub Pages

Control Groups — The Linux Kernel documentation

 The command vagrant init is used to initialize the current directory to be a Vagrant environment by creating an initial Vagrantfile if one does not already exist1. The Vagrantfile contains the configuration settings for the Vagrant box, such as the box name, box URL, network settings, synced folders, provisioners, etc. The command vagrant init does not execute any provisioning tool, start any box, install Vagrant on a Linux host, or download any box. Those actions are performed by other Vagrant commands, such as vagrant provision, vagrant up, vagrant install, and vagrant box add, respectively. References:

1: vagrant init - Command-Line Interface | Vagrant | HashiCorp Developer