Unikernel: Another paradigm for the cloud

In this cloud era it is hard to imagine a world without access to services in the cloud. From contacting someone through mail, to storing work-related documents on an online drive and accessing it across devices, there are lot of services we use on a daily basis that is in the cloud.

To reduce the cost of compute power, Virtualization has been adapted towards offering more services with less hardware. And then came the concept of containers where you deploy the application in isolated containers with light weight images which has few binaries and libraries to run your application, But still we need the underlying VMs to deploy such solutions. All these VMs comes with a cost. While large data-centers are offering services in the cloud, they are also hungry for electric power, which is becoming a growing concern as our planet is being drained of its resources. So what we need now is less power-hungry solutions.

What if, instead of virtualization of an entire operating system, you were to load an application with only the required components from the operating system? Effectively reducing the size of the virtual machine to its bare minimum resource footprint? This is where unikernels come into play.

Unikernel

Unikernel is a relatively new concept that was first introduced around 2013 by Anil Madhavapeddy in a paper titled “Unikernels: Library Operating Systems for the Cloud” (Madhavapeddy, et al., 2013).

You can find more details on Unilernel by searching the scholarly articles in Google.

Unikernels are defined by the community at Unikernel.org as follows.

“Unikernels are specialized, single-address-space machine images constructed by using library operating systems.”

For more detailed reading about the concepts behind Unikernel, please follow this link,

A Unikernel is an application that has been boiled down to a small, secure, light-weight virtual machine which eliminates general purpose operating systems such as Linux or Windows. Unikernels aims to be a much more secure system than Linux. It does this through several thrusts. Not having the notion of users, running a single process per VM, and limiting the amount of code that is incorporated into each VM. This means that there are no users and no shell to login to and, more importantly, you can’t run more than the one program you want to run inside. Despite their relatively young age, unikernels borrow from age-old concepts rooted in the dawn of the computer era: microkernels and library operating systems. This means that a unikernel holds a single application. Single-address space means that in its core, the unikernel does not have separate user and kernel address space. Library operating systems are the core of unikernel systems. Unikernels are provisioned directly on the hypervisor without a traditional system like Linux. So it can run 1000X more vms/per server.

You can have a look in here for details about Microkernel, Monolithic & Library Operating Systems

Virtual Machines VS Linux Containers VS Unikernel

Virtualization of services can be implemented in various ways. One of the most widespread methods today is through virtual machine, hosted on hypervisors such as VMware’s ESXi or Linux Foundation’s Xen Project.

Hypervisors allow hosting multiple guest operating systems on a single physical machine. These guest operating systems are executed in what is called virtual machines. The widespread use of hypervisors is due to their ability to better distribute and optimize the workload on the physical servers as opposed to legacy infrastructures of one operating system per physical server.

Containers are another method of virtualization, which differentiates from hypervisors by creating virtualized environments and sharing the host’s kernel. This provides a lighter approach to hypervisors which requires each guest to have their copy of the operating system kernel, making a hypervisor-virtualized environment resource heavy in contrast to containers which share parts of the existing operating system.

As aforementioned, unikernels leverage the abstraction of hypervisors in addition to using library operating systems to only include the required kernel routines alongside the application to present the lightest of all three solutions.

The figure above shows the major difference between the three virtualization technologies. Here we can clearly see that virtual machines present a much larger load on the infrastructure as opposed to containers and unikernels.

Additionally, unikernels are in direct “competition” with containers. By providing services in the form of reduced virtual machines, unikernels improve on the container model by its increased security. By sharing the host kernel, containerized applications share the same vulnerabilities as the host operating system. Furthermore, containers do not possess the same level of host/guest isolation as hypervisors/virtual machines, potentially making container breaches more damaging than both virtual machines and unikernels.

TechnologyProsCons
Virtual Machines– Allows deploying different operating systems on a single host
– Complete isolation from host
– Orchestration solutions available
– Requires compute power proportional to number of instances
– Requires large infrastructures
– Each instance loads an entire operating system
Linux Containers– Lightweight virtualization
– Fast boot times
– Ochestration solutions
– Dynamic resource allocation
– Reduced isolation between host and guest due to shared kernel
– Less flexible (i.e.: dependent on host kernel)
– Network is less flexible
Unikernels– Lightweight images
– Specialized application
– Complete isolation from host
– Higher security against absent functionalities (e.g.: remote command execution)
– Not mature enough yet for production
– Requires developing applications from the grounds up
– Limited deployment possibilities
– Lack of complete IDE support
– Static resource allocation
– Lack of orchestration tools
A Comparison of solutions

Docker and containerization technology and the container orchestra-tors like Kubernetes, OpenShift are 2 steps forward for the world of DevOps and that the principles it promotes are forward-thinking and largely on-target for the future of a more secure, performance oriented, and easy-to-manage cloud future. However, an alternative approach leveraging unikernels and immutable servers will result in smaller, easier to manage, more secure containers that will be simpler to adopt by existing enterprises. As DevOps matures, the shortcomings of cloud application deployment and management are becoming clear. Virtual machine image bloat, large attack surfaces, legacy executable, base-OS fragmentation, and unclear division of responsibilities between development and IT for cloud deployments are all causing significant friction (and opportunities for the future).

For Example: It remains virtually impossible to create a Ruby or Python web server virtual machine image that DOESN’T include build tools (gcc), ssh, and multiple latent shell executable. All of these components are detrimental for production systems as they increase image size, increase attack surface, and increase maintenance overhead.

Compared to VMs running Operating systems like Windows and Linux, the unikernel has only a tenth of 1% of the attack surface. So in the case of a unikernel — sysdig, tcpdump, and mysql-client are not installed and you can’t just “apt-get install” them either. You have to bring that with your exploit. To take it further even a simple cat /etc/hosts or grep of /var/log/nginx/access.log simply won’t work — once again they are separate processes.
So unikernels are highly resistant to remote code execution attacks, more specifically shell code exploits.

Immutable Servers & Unikernels

Immutable Servers are a deployment model that mandates that no application updates, security patches, or configuration changes happen on production systems. If any of these layers needs to be modified, a new image is constructed, pushed and cycled into production. Heroku is a great example of immutable servers in action: every change to your application requires a ‘git push’ to overwrite the existing version. The advantages of this approach include higher confidence in the code that is running in production, integration of testing into deployment workflows, easy to verify that systems have not been compromised.

Once you become a believer in the concept of immutable servers, then speed of deployment and minimizing vulnerability surface area become objectives. Containers promote the idea of single-service-per-container (microservices), and unikernels take this idea even further.

Unikernels allow you to compile and link your application code all the way down to and include the operating system. For example, if your application doesn’t require persistent disk access, no device drivers or OS facilities for disk access would even be included in final production images. Since unikernels are designed to run on hypervisors such as Xen, they only need interfaces to standardized resources such as networking and persistence. Device drivers for thousands of displays, disks, network cards are completely unnecessary. Production systems become minimalist — only requiring the application code, the runtime environment, and the OS facilities required by the applications. The net effect is smaller VM images with less surface area that can be deployed faster and maintained more easily.

Traditional Operating Systems (Linux, Windows) will become extinct on servers. They will be replaced with single-user, bare metal hypervisors optimized for the specific hardware, taking decades of multi-user, hardware-agnostic code cruft with them. More mature build-deploy-manage tool set based on these technologies will be truly game changing for hosted and enterprise clouds alike.

UnikernelLanguageTargetsFunctions
ClickOSC++XenNetwork Function Virtualization
HalVMHaskellXen
IncludeOSC++KVM, VirtualBox, ESXi, Google Cloud, OpenStackOrchestration tool available
MirageOSOCamlKVM, Xen, RTOS/MCU
Nanos UnikernelC, C++, Go, Java, Node.js, Python, Rust, Ruby, PHP, etcQEMU/KVMOrchestration tool available
OSvJava, C, C++, Node, RubyVirtualBox, ESXi, KVM, Amazon EC2, Google CloudCloud and IoT (ARM)
RumprunC, C++, Erlan, Go, Java, JavaScript, Node.js, Python, Ruby, RustXen, KVM
UnikGo, Node.js, Java, C, C++, Python, OCamlVirtualBox, ESXi, KVM, XEN, Amazon EC2, Google Cloud, OpenStack, PhotonControllerUnikernel compiler toolbox with orchestration possible through Kubernetes and Cloud Foundry
ToroKernelFreePascalVirtualBox, KVM, XEN, HyperVUnikernel dedicated to run microservices
Comparing few Unikernel solutions from active projects

Out of the various existing projects, some standout due to their wide range of supported languages. Out of the active projects, the above table describes the language they support, the hypervisors they can run on and remarks concerning their functionality.

Currently experimenting with the Unikernel in the AWS and Google Cloud Platform and will update you with another post on that soon.

Source: Medium, github, containerjournal, linuxjournal

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