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What is Firecracker? Firecracker is an open-source virtual machine monitor (VMM) that uses Linux KVM to create lightweight virtual machines called microVMs. Its deliberately small device model is intended to retain a virtual-machine kernel boundary while reducing the startup and resource costs associated with a conventional, feature-rich VM. AWS developed Firecracker for services including Lambda and Fargate.
In practical terms, Linux runs on the host, KVM supplies hardware-assisted virtualization, Firecracker configures and launches the microVM, and a guest Linux kernel plus root filesystem run inside it. The result is not a container: the workload has its own guest kernel, although the host and its configuration remain part of the security model.
Firecracker in one sentence
Firecracker is the user-space VMM; a Firecracker microVM is the virtual machine that VMM creates. The project is designed for dense, frequently created workloads such as serverless functions, where a provider needs stronger isolation than a process or container alone but less emulated hardware than a traditional VM.
The canonical project overview and source code are maintained in the Firecracker repository. Firecracker is software you can build and operate yourself; it is not the same thing as the managed AWS Lambda MicroVM offering.
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How the Firecracker stack works
1. Linux is the host
A Linux operating system owns the physical machine, manages drivers and networking, and exposes KVM through /dev/kvm. Host kernel configuration, permissions, updates and resource limits therefore affect the security and reliability of every microVM.
2. KVM creates the virtualization boundary
Kernel-based Virtual Machine (KVM) uses the processor’s virtualization extensions to execute guest code in a separate virtual-machine context. KVM is the low-level mechanism; it does not by itself provide Firecracker’s narrow device model, API or sandbox policy.
3. Firecracker is the VMM process
Firecracker’s API defines the guest’s virtual CPUs and memory, kernel and boot arguments, root and data drives, networking, logging and metrics. The design intentionally omits many devices found in general-purpose VMMs. Fewer emulated interfaces mean a smaller attack surface and less work during creation, but also fewer assumptions about what guest operating systems can do. See the project’s design document for the architecture and supported controls.
4. The guest runs inside the microVM
A guest kernel boots from a host-provided kernel image and mounts a root filesystem. Applications see a virtual machine environment, not the host’s kernel. Operators can add virtual block devices and a network interface, but should not expect the broad hardware compatibility of a desktop or enterprise hypervisor.
Is a microVM a container?
No. Containers package processes and share the host kernel; a microVM boots a separate guest kernel behind KVM. Containers can be extremely efficient, but a kernel vulnerability or misconfiguration can have different consequences when many tenants share one kernel. MicroVMs add a VM boundary while keeping the device model and operating profile intentionally small.
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That distinction does not make Firecracker automatically safe or “unhackable.” Isolation is layered, and a weak host configuration, excessive privileges, unsafe networking or an unpatched kernel can undermine a deployment.
| Characteristic | Container | Firecracker microVM | Traditional VM |
|---|---|---|---|
| Kernel | Shares the host kernel | Boots a guest kernel | Boots a guest kernel |
| Device model | Usually host namespaces and virtual devices | Deliberately minimal | Broad, general-purpose hardware model |
| Isolation boundary | Namespaces, cgroups and kernel controls | KVM plus Firecracker sandboxing and host controls | Hypervisor boundary with more emulated functionality |
| Operational burden | Typically lower | Requires kernels, filesystems, networking and host hardening | Often higher, but with broader guest compatibility |
Why AWS Lambda uses Firecracker
AWS’s 2018 launch announcement said, “AWS Lambda uses Firecracker as the foundation for provisioning and running sandboxes upon which we execute customer code.” That is the launch-era statement about Lambda’s architecture, not a promise that every current internal detail is unchanged. AWS also says Firecracker virtualization powers more than 15 trillion Lambda invocations per month; the cited documentation does not attach a year to that figure. Read the AWS announcement alongside the current project documentation.
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What AWS Lambda MicroVMs do
AWS also documents a named, managed Lambda MicroVMs capability. In that service, you upload a zip containing a Dockerfile and application artifacts. Lambda builds the environment and captures a Firecracker snapshot; run-microvm restores that snapshot. AWS describes dedicated HTTPS endpoints and suspend/resume behavior that preserves memory and disk state. The managed workflow should not be confused with downloading the open-source VMM and assembling an equivalent service yourself. AWS explains the lifecycle in its core concepts documentation.
Performance: what the published number actually means
The Firecracker design document specifies a scenario—not a universal latency guarantee—in which a minimal Linux kernel, one guest CPU and 128 MiB of RAM support a steady mutation rate of five microVMs per host core per second. Its example is 180 microVMs per second on a 36-physical-core host. The result depends on the stated kernel, memory size, host hardware and workload; it should not be read as a Lambda cold-start figure or as a ranking against other hypervisors.
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An AWS 2018 announcement reported memory overhead below 5 MiB. That is a historical, launch-era figure. Treat current overhead as version- and configuration-dependent rather than carrying the old number into a present-day capacity plan.
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Security is a layered responsibility
KVM and the VMM boundary
KVM separates guest execution from host execution, while Firecracker restricts the virtual hardware exposed to the guest. Together they form the first layer, not the whole security story.
Process and resource controls
The project documents per-thread seccomp filters, cgroups and Linux namespaces. These controls limit system calls, CPU and memory consumption, and process visibility. Firecracker also provides the jailer, which can drop privileges and arrange a restricted runtime environment; the design documentation recommends starting production workloads through it.
Host configuration still matters
The project repository states: “The overall security of Firecracker microVMs, including the ability to meet the criteria for safe multi-tenant computing, depends on a well configured Linux host operating system.” Keep the host patched, minimize privileges, isolate management paths, apply resource limits, and design networking as if the host were part of the trusted-computing base. Firecracker alone does not make arbitrary untrusted code safe.
What you need to run Firecracker yourself
The official getting-started guide requires a Linux host with KVM enabled and read/write access to /dev/kvm. It describes x86_64 and aarch64 Linux support. Before a useful deployment, prepare:
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- A tested host kernel, CPU virtualization support and permission to access
/dev/kvm. - A compatible guest kernel image and root filesystem.
- Host networking, commonly a TAP interface connected to the required bridge or routing setup.
- Storage, logging and metrics paths with appropriate ownership and quotas.
- A production isolation design using the jailer, seccomp, cgroups, namespaces and least-privilege service accounts.
Firecracker’s tested-platform table changes as kernels and hardware evolve. Check the current repository table before selecting an instance type or kernel. Do not treat the i3.metal example in the 2018 blog as a current prescription.
A safe conceptual launch sequence
- Verify hardware virtualization and confirm that the service account can open
/dev/kvm. - Build or obtain a guest kernel and root filesystem that match the workload.
- Start Firecracker under the jailer with a dedicated chroot, dropped privileges and the project’s recommended seccomp policy.
- Use the Firecracker API to configure boot source, drives, vCPUs, memory, network interfaces and logging before starting the instance.
- Connect networking through a controlled TAP/bridge arrangement and apply cgroup quotas.
- Exercise shutdown, restart, log collection and cleanup paths before accepting untrusted tenants.
A demo that boots a guest is not a production multi-tenant platform. Follow the project’s production host guidance and validate the complete host, kernel and network configuration.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Common failure modes and fixes
/dev/kvm is missing or inaccessible
Virtualization may be disabled in firmware, the host may not expose KVM, or the service account may lack read/write permission. Enable virtualization where appropriate, load the host KVM module, and grant only the required device access.
The guest does not boot
Check that the kernel image, boot arguments and root filesystem are compatible. A conventional distribution image may expect devices Firecracker does not emulate; use a guest configuration designed for its minimal model.
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The guest boots but has no network
Validate the TAP interface, bridge or routing rules, guest interface name, addresses and firewall policy. Confirm that the host-side interface is attached before starting workloads.
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A workload consumes too many resources
Apply cgroup CPU and memory limits, impose quotas on drives and processes, and monitor Firecracker metrics. Do not infer capacity from the five-per-core design scenario unless your kernel, memory size and host are comparable.
A “secure” setup still exposes risk
Review jailer privileges, seccomp filters, namespaces, host patching, management sockets and network reachability. Recheck the host whenever the kernel, Firecracker release or deployment topology changes.
When to choose Firecracker
- Choose it when you need many short-lived, isolated Linux environments and can operate the host, guest images and networking.
- Prefer containers when sharing a host kernel is acceptable and the lower operational burden matters more than a guest-kernel boundary.
- Prefer a general-purpose VM when you need broad device support, conventional operating-system installers or mature desktop/server hardware compatibility.
- Use managed Lambda MicroVMs when AWS’s build, snapshot, HTTPS endpoint and suspend/resume workflow fits your application and you do not want to run the VMM fleet.
Optional tooling for documenting a deployment
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FAQ
Does Firecracker replace KVM?
No. KVM supplies the kernel virtualization mechanism; Firecracker is the user-space VMM that configures and manages the microVM.
Can any Linux distribution run unchanged?
Not necessarily. The guest must work with Firecracker’s limited device model and the kernel, boot arguments and root filesystem must be compatible.
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No. You can build and operate Firecracker yourself. AWS Lambda MicroVMs are a separate managed offering documented by AWS.
The Bottom Line
Firecracker is a deliberately small VMM that combines KVM’s guest-kernel boundary with a serverless-oriented device model. It is a strong building block for dense, short-lived isolation, but production safety and reliability depend on the Linux host, jailer, resource controls, networking and operational discipline around it.
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