banger/README.md

banger

Minimal Firecracker launcher.

Requirements

• Linux host with KVM (/dev/kvm access)
• sudo, ip, curl, ssh
• dmsetup, losetup, blockdev (device-mapper snapshot for rootfs)
• e2cp, e2rm (writes hostname and resolv.conf into rootfs snapshot)

Files

• firecracker: Firecracker binary
• vmlinux: guest kernel
• rootfs.ext4: guest root filesystem
• id_ed25519: SSH key for root

Run

./run.sh

Run Options

./run.sh --name calm_otter --vcpu 4 --ram 2048 --home-size 6G

• --name: must be unique and match [a-z0-9][a-z0-9-]{0,63}.
• --vcpu: defaults to 2, max 16.
• --ram: MiB, defaults to 1024, max 32768.
• --rootfs: path to the base rootfs image (default: ./rootfs.ext4).
• --kernel: path to the kernel image (default: ./vmlinux).
• --home-size: M/G suffixes supported (default: 2G).
• --var-size: M/G suffixes supported (default: 2G).

Storage Layout

• rootfs.ext4 is used as the read-only origin for a per-VM device-mapper snapshot mounted as /.
• Each VM gets writable ext4 disks mounted at /home and /var.
• The base image must include /etc/fstab entries for /dev/vdb → /home and /dev/vdc → /var.
• /run and /tmp should be tmpfs via /etc/fstab.

SSH

ssh -i "./id_ed25519" root@<guest_ip>

Internet Access

VMs do not get internet access by default. You must enable forwarding and NAT:

./nat.sh up <id-or-name-prefix>

This enables net.ipv4.ip_forward=1 and installs per-VM NAT rules for the VM's guest IP and TAP device. To remove rules:

./nat.sh down <id-or-name-prefix>

Check status with:

./nat.sh status <id-or-name-prefix>

Shutdown

reboot

VM Info File

Each VM writes a metadata file at state/vms/<id>/info with the following fields:

• id: unique identifier for the VM instance.
• name: VM name.
• pid: Firecracker process ID.
• created_at: timestamp when the VM was launched.
• rootfs: root filesystem image path used to launch the VM.
• kernel: kernel image path used to launch the VM.
• guest_ip: IP address assigned to the guest.
• tap: host TAP interface name attached to the bridge.
• api_sock: path to the Firecracker API socket (stored under $XDG_RUNTIME_DIR/banger/ when available).
• log: path to the Firecracker log file.
• base_loop: loop device backing the base rootfs (if present).
• cow_file: copy-on-write image file (if present).
• cow_loop: loop device for the COW image (if present).
• dm_name: device-mapper name for the merged rootfs (if present).
• dm_dev: device-mapper device path for the merged rootfs (if present).

Log Notes

• PCI: Fatal: No config space access function found and MissingAddressRange lines are expected with pci=off in run.sh.
• SELinux: Could not open policy file ... is expected in the minimal rootfs.