heyvm-firecracker

name	heyvm-firecracker
description	Build Dockerfiles for Firecracker microVM rootfs images and manage them with `heyvm mvm`. Use when the user wants to create a Firecracker image, write a Dockerfile for a Firecracker VM, or build/list/manage Firecracker rootfs images.
argument-hint	[build|images|fetch-kernel] [args...]
allowed-tools	Bash, Read, Write, Edit, Grep, Glob

firecracker — Build & Manage Firecracker VM Images

See also: load the heyvm-docs skill for an overview of the heyvm platform and an index of the other heyvm-* skills (sandbox, deploy, proxy, api, login, system).

Write Dockerfiles that produce Firecracker-compatible ext4 rootfs images and build them with heyvm mvm.

Commands

Build an image from a Dockerfile

heyvm mvm build --local-only -f <Dockerfile> -n <name>

Flag	Description	Default
-f, --file <FILE>	Path to Dockerfile	Dockerfile
-c, --context <DIR>	Build context directory	Dockerfile's parent dir
-n, --name <NAME>	Image name/tag	derived from Dockerfile
--size-mb <SIZE>	Rootfs size in MB	auto (tar size * 1.2 + 64MB, min 128MB)
--local-only	Skip upload, build locally only	false
--token <TOKEN>	JWT for cloud upload	HEYO_ARCHIVE_TOKEN env var

List local images

heyvm mvm images

Download the Firecracker kernel

heyvm mvm fetch-kernel

The kernel is also auto-downloaded on first sandbox start.

Build pipeline

The build process (implemented in mvm-ctrl/src/image_builder.rs):

1. docker build — builds a Docker image from the Dockerfile
2. docker create + docker export — exports the container filesystem to a tar
3. tar -xf — extracts contents to a staging directory
4. mke2fs -t ext4 -d <contents> — creates an ext4 rootfs image from the extracted tree
5. Cleanup — removes the temporary Docker container and image

Prerequisites: Docker and mke2fs (from e2fsprogs) must be installed.

Images are stored in ~/.heyo/images/firecracker/<name>/rootfs.ext4.

Writing Dockerfiles for Firecracker

Firecracker VMs boot directly into a rootfs with no systemd, no cloud-init, and no container runtime. The Dockerfile must produce a filesystem that can boot standalone with a custom /init.sh entry point.

Required structure

Every Firecracker Dockerfile MUST include:

1. Base image — Use ubuntu:24.04 (or another distro with standard userspace tools).

2. openssh-server — Required for heyvm exec and heyvm sh to work. Pre-generate host keys at build time so sshd doesn't block on entropy at boot.

3. /init.sh boot script — This is the entry point. It must:

   • Mount essential virtual filesystems (proc, sysfs, devtmpfs, devpts)
   • Suppress kernel console messages with dmesg -n 1
   • Configure DNS (/etc/resolv.conf)
   • Set hostname
   • Bring up the network interface (ip link set eth0 up)
   • Start sshd
   • Start your application service(s)
   • Print HEYVM_READY — this marker is required, the host waits for it to confirm the VM is ready
   • End with a bash respawn loop on the serial console (below) — keeps the VM alive and makes heyvm sh land on a usable prompt that survives exit / Ctrl-D without panicking the kernel

4. EXPOSE ports — Always expose 22 (SSH) plus any application ports.

5. CMD ["/init.sh"] — Set the init script as the default command.

Template

Use this as a starting point when writing new Firecracker Dockerfiles:

FROM ubuntu:24.04

# Install openssh-server (required) + your application packages
RUN apt-get update && apt-get install -y --no-install-recommends \
    openssh-server \
    iproute2 \
    # --- add your packages here --- \
    && rm -rf /var/lib/apt/lists/*

# Configure SSH for Firecracker access.
# Pre-generate host keys so sshd doesn't block on entropy at boot.
# Explicitly enable PasswordAuthentication — Ubuntu 24.04 may disable it
# in /etc/ssh/sshd_config.d/ and Firecracker has no cloud-init to fix it.
RUN mkdir -p /run/sshd /etc/ssh/sshd_config.d \
    && echo "PermitRootLogin yes" >> /etc/ssh/sshd_config \
    && echo "PermitEmptyPasswords yes" >> /etc/ssh/sshd_config \
    && echo "PasswordAuthentication yes" > /etc/ssh/sshd_config.d/50-heyo.conf \
    && chmod 644 /etc/ssh/sshd_config.d/50-heyo.conf \
    && passwd -d root \
    && useradd -m -s /bin/bash heyo \
    && echo 'heyo:heyo' | chpasswd \
    && ssh-keygen -A

# Nicer serial-console prompt. `heyvm sh` lands on this PID-1 bash, and the
# default PS1 from Ubuntu's skeleton isn't loaded for a non-interactive parent.
RUN printf '%s\n' \
    'PS1="\[\033[1;32m\]\h\[\033[0m\]:\[\033[1;34m\]\w\[\033[0m\]# "' \
    'export HISTFILE=/root/.bash_history HISTSIZE=2000 HISTCONTROL=ignoredups' \
    > /etc/profile.d/heyvm-prompt.sh

# --- Application setup (config files, build steps, etc.) ---

# Boot script — runs as PID 1 (no systemd).
RUN printf '#!/bin/sh\n\
mount -t proc proc /proc\n\
mount -t sysfs sysfs /sys\n\
mount -t devtmpfs devtmpfs /dev 2>/dev/null\n\
if [ ! -c /dev/null ]; then\n\
    echo "init: devtmpfs unavailable, creating device nodes manually"\n\
    mknod -m 666 /dev/null    c 1 3\n\
    mknod -m 666 /dev/zero    c 1 5\n\
    mknod -m 444 /dev/random  c 1 8\n\
    mknod -m 444 /dev/urandom c 1 9\n\
    mknod -m 666 /dev/tty     c 5 0\n\
    mknod -m 666 /dev/ptmx    c 5 2\n\
    ln -sf /proc/self/fd /dev/fd\n\
fi\n\
mkdir -p /dev/pts && mount -t devpts devpts /dev/pts\n\
dmesg -n 1 2>/dev/null\n\
echo "nameserver 8.8.8.8" > /etc/resolv.conf\n\
hostname firecracker\n\
ip link set eth0 up 2>/dev/null\n\
if ! ip addr show eth0 2>/dev/null | grep -q "inet "; then\n\
    for param in $(cat /proc/cmdline); do\n\
        case "$param" in\n\
            ip=*)\n\
                GUEST_IP="${param#ip=}"; GUEST_IP="${GUEST_IP%%%%::*}"\n\
                TAIL="${param#*::}"; GW="${TAIL%%%%:*}"\n\
                ip addr add "$GUEST_IP/30" dev eth0 2>/dev/null\n\
                [ -n "$GW" ] && ip route add default via "$GW" dev eth0 2>/dev/null\n\
                ;;\n\
        esac\n\
    done\n\
fi\n\
mkdir -p /run/sshd\n\
chown root:root /run/sshd\n\
chmod 755 /run/sshd\n\
chown root:root /etc/ssh/ssh_host_* 2>/dev/null\n\
chmod 600 /etc/ssh/ssh_host_*_key 2>/dev/null\n\
chmod 644 /etc/ssh/ssh_host_*_key.pub 2>/dev/null\n\
/usr/sbin/sshd -D -e 2>/tmp/sshd.log &\n\
# --- start your services here --- \n\
echo "HEYVM_READY"\n\
# Interactive shell on ttyS0 for `heyvm sh`. The loop keeps the VM alive\n\
# even if the user exits the shell (PID 1 exit = kernel panic).\n\
while :; do /bin/bash --login; sleep 0.1; done\n' > /init.sh \
    && chmod +x /init.sh

EXPOSE 22 # add your ports

CMD ["/init.sh"]

Key constraints

• No systemd. Firecracker boots directly to /init.sh. All services must be started manually in the init script.
• No docker-entrypoint. There is no Docker runtime — ENTRYPOINT and CMD are only used by heyvm mvm build to know the default command. The actual boot command is the kernel init= parameter, which points to /init.sh.
• Filesystem mounts are manual. /proc, /sys, /dev, and /dev/pts do not exist at boot. The init script must mount them. Docker-exported rootfs has an empty /dev — you MUST mount devtmpfs or create device nodes manually (/dev/null, /dev/urandom, /dev/ptmx) or sshd will fail to start.
• Network is partially configured. The kernel ip= boot parameter sets an IP, but the interface may need ip link set eth0 up to be fully usable. Include the fallback IP parsing from /proc/cmdline (see template) in case the kernel param didn't fully configure the interface before init runs.
• Create /run/sshd at boot. sshd requires this privilege-separation directory. Even if created in the Dockerfile, it may not survive the Docker export → ext4 conversion. Always mkdir -p /run/sshd before starting sshd.
• Use sshd -D -e 2>/tmp/sshd.log & (foreground mode, errors to log file). Do NOT let sshd write to stderr/serial — stray output corrupts the marker-delimited serial command protocol used by heyvm exec.
• HEYVM_READY is mandatory. The host process watches the serial console for this exact string. Without it, the VM will appear to hang during startup.
• Keep images small. The ext4 image is sized automatically (tar * 1.2 + 64MB, min 128MB). Large images slow down boot. Use --no-install-recommends and clean apt caches.
• Pre-generate SSH host keys. Run ssh-keygen -A at build time. Generating keys at boot time on Firecracker can be very slow due to limited entropy.
• The heyo user (password: heyo) is the standard non-root user. Always create it for compatibility with heyvm exec and heyvm sh.
• Explicitly enable PasswordAuthentication yes. Write /etc/ssh/sshd_config.d/50-heyo.conf at build time. Ubuntu 24.04 may ship with password auth disabled and Firecracker has no cloud-init to fix it at runtime. Without this, deployed sandboxes cannot get a shell.
• Don't end init.sh with exec /bin/sh. That gives you dash with no prompt on the serial console, and exit / Ctrl-D panics the kernel (PID 1 exit). Use the while :; do /bin/bash --login; sleep 0.1; done loop from the template instead.

Reference example

See third-party/Dockerfile.firecracker-nginx for a working example that runs nginx inside a Firecracker VM.

Using built images

After building, create a Firecracker sandbox:

heyvm create --name my-vm --backend-type firecracker --image <name>

Where <name> matches the -n flag used during heyvm mvm build.

Workflow

When the user asks to build a Firecracker image or write a Dockerfile for Firecracker:

1. Understand the service — what application/services should run in the VM.
2. Write the Dockerfile — follow the template above, adding the required packages and service startup to the init script.
3. Save the Dockerfile — place it at third-party/Dockerfile.firecracker-<name> by convention.
4. Build it — run heyvm mvm build --local-only -f third-party/Dockerfile.firecracker-<name> -n <name>.
5. Verify — run heyvm mvm images to confirm the image was created.

When the user provides $ARGUMENTS, interpret them as arguments to heyvm mvm. For example, /firecracker images should run heyvm mvm images, and /firecracker build -f Dockerfile -n myimg --local-only should run heyvm mvm build -f Dockerfile -n myimg --local-only.

If $ARGUMENTS is empty, ask the user what service they want to run in Firecracker and write the Dockerfile for them.