husker 0.4.4

microVM manager built on Firecracker (Linux) and Apple Virtualization.framework (macOS)

Husker

An open-source microVM manager built on Firecracker (Linux) and Apple Virtualization.framework (macOS).

• Boot lightweight VMs in milliseconds
• Execute commands, transfer files, open interactive shells
• Stream serial console logs
• Port forwarding with nftables NAT (Linux)
• REST API + CLI
• Cloud-init style userdata scripts

Status

Pre-1.0. The core feature set works and has test coverage, but:

• The HTTP API, CLI flags, config schema, and on-disk state layout may change without a deprecation period.
• The Linux/Firecracker backend is more mature than the macOS/Apple VZ backend.
• It has not been run at scale or under production workloads.
• Security features (token auth, rate limiting, encrypted secrets) exist but have had limited review. Don't expose the daemon to an untrusted network.

Useful for experimentation, local development, and CI. Not recommended for production.

Quick Start

Install

macOS (Homebrew):

brew install rvben/tap/husker

Linux & macOS (installer script):

curl -sSfL https://raw.githubusercontent.com/rvben/husker/main/install.sh | sh

Cross-platform (PyPI):

pip install husker

Pinning a version: curl -sSfL https://raw.githubusercontent.com/rvben/husker/main/install.sh | HUSKER_VERSION=v0.1.2 sh or pip install husker==0.1.2.

First boot

# Fetch the default kernel + rootfs for this host
husker images pull

# Start the daemon
husker daemon &

# Boot a VM
husker run --name hello --cpus 2 --memory 512

# Interact
husker exec hello -- uname -a
husker shell hello
husker cp local.txt hello:/tmp/local.txt
husker logs hello -f

# Clean up
husker destroy hello

On Linux, husker run needs firecracker on PATH. If it's missing, husker prompts to download a pinned release into the data directory. For non-interactive use (CI, scripts), set HUSKER_AUTO_INSTALL_FIRECRACKER=1 to skip the prompt.

husker images pull fetches the latest signed image set from the images-* GitHub Releases. If no image release is published yet for this arch, the command will fail — use the BYO path below in the meantime.

BYO kernel / rootfs

If you want to use your own images, pass --kernel and the rootfs path:

husker run /path/to/rootfs.ext4 --kernel /path/to/vmlinux

Configuration

Copy config.example.toml to one of the discovery paths:

1. ~/.config/husker/config.toml (user)
2. /etc/husker/config.toml (system)

Or pass --config /path/to/config.toml explicitly. See config.example.toml for all available fields.

Platform Support

Platform	Backend	Networking	Status
Linux x86_64	Firecracker	TAP + nftables NAT, port forwarding	Usable
macOS ARM64	Apple Virtualization.framework	Shared NAT (VZ-managed)	Experimental
Linux x86_64	QEMU/KVM	TAP + nftables NAT, port forwarding	Experimental

QEMU/KVM backend (Linux)

Besides Firecracker microVMs, husker can run full QEMU/KVM VMs on Linux. Select it with vmm = "qemu" in the config file or HUSKER_VMM=qemu. Requires /dev/kvm, /dev/vhost-vsock (load the vhost_vsock kernel module), and qemu-system-x86_64 on PATH (override with HUSKER_QEMU_BIN).

Current scope is direct-kernel boot (same kernel + rootfs model as the Firecracker backend), with the guest agent reachable over vsock. Booting stock cloud images via UEFI/OVMF is not yet supported.

Guest kernel requirement: the QEMU backend uses the q35 machine, which puts the root disk, NIC, and vsock device on the PCI bus, so the guest kernel must have CONFIG_VIRTIO_PCI (and an initramfs if virtio_blk is a module). The default husker images satisfy this. Firecracker's own kernels are built for virtio-MMIO only and will panic under QEMU with Cannot open root device "vda". The husker default images and most distro kernels work as-is.

Alternatives

husker is one of several ways to run microVMs. Rough positioning:

Tool	Backend	Focus	Notes
husker	Firecracker (Linux) + Apple VZ (macOS)	Single-host VM manager with a REST API and CLI	SQLite-backed state, port forwarding, guest agent over vsock.
Ignite	Firecracker	Docker-image-style workflow on Firecracker	Archived by Weaveworks; Linux only.
firecracker-containerd	Firecracker	containerd runtime backed by microVMs	Kubernetes-friendly; Linux only.
krunvm	libkrun	OCI-image microVMs on macOS	No daemon; ephemeral per-command VMs.
Lima	QEMU (+ VZ on macOS)	Full Linux VMs as a dev environment	Heavier than Firecracker; broader guest support.

Pick husker if you want Firecracker on Linux with a matching Apple VZ path on macOS, driven by a single CLI + daemon.

Architecture

CLI (husker) ──> REST API (husker-api) ──> Core (husker-core)
                                           ├── VMM (husker-vmm)      Firecracker / Apple VZ
                                           ├── State (husker-state)  SQLite persistence
                                           ├── Net (husker-net)      TAP devices, IP allocation
                                           └── Storage (husker-storage) Rootfs cloning
                                       Guest Agent (husker-agent) ←── Proto (husker-agent-proto)

Host-guest communication uses vsock (port 52). Messages are length-prefixed JSON with base64-encoded binary payloads.

Security

• Threat model — STRIDE analysis and residual risks.
• Hardening guide — deployment posture and config recommendations.
• Report vulnerabilities privately via GitHub Security Advisories. See SECURITY.md.

The daemon defaults to loopback-only. Don't bind it on a public interface without a bearer token and a terminating reverse proxy.

Troubleshooting

husker run can't find firecracker (Linux) On a TTY, husker prompts to download a pinned release. For scripted/CI use, set HUSKER_AUTO_INSTALL_FIRECRACKER=1 to skip the prompt, or install Firecracker yourself from the releases page.

husker run reports kvm_init: permission denied (Linux) Add your user to the kvm group: sudo usermod -aG kvm $USER and re-login.

husker run fails on macOS with a virtualization entitlement error The binary needs the com.apple.security.virtualization entitlement. pip, brew, and the installer script all ship a codesigned binary. If you built from source, run make install — it ad-hoc signs via husker.entitlements.

macOS: VM boots but exits immediately Apple VZ needs an initramfs to mount the rootfs. If you're using custom images, make sure --initrd points at a matching initramfs (see guest/build-initramfs.sh).

husker daemon binds but the CLI can't connect The CLI defaults to http://127.0.0.1:7777. If the daemon listens elsewhere, pass --api-url http://host:port (and --api-token if auth is enabled).

Rootfs edits don't take effect on macOS Use make update-rootfs to inject changes via debugfs (works in LXC and macOS). Loop-mounting doesn't work on macOS.

Development

Requires Rust 1.90+ and cargo-nextest.

make build          # Debug build
make build-release  # Release build (LTO, stripped)
make build-agent    # Static musl agent for guest VMs
make test           # Full test suite
make test-unit      # Unit tests only
make lint           # fmt-check + clippy
make check          # Type check
make install        # Install (auto-detects macOS, signs binary)

Running a Development VM

make run-daemon                     # Start daemon
make build-agent-aarch64            # Build ARM64 agent (macOS guests)
make update-rootfs                  # Inject agent into rootfs image

Systemd

A systemd unit file is provided at contrib/husker.service.

Contributing

Issues and pull requests welcome. See CONTRIBUTING.md for the dev workflow and commit conventions.

License

See LICENSE.