Updated getting started that enables an interface configured with internet access (works with theirs, and a default `ubuntu` docker image dump).

firecracker.sh

TAP_DEV="tap0"
TAP_IP="172.16.0.1"
MASK_SHORT="/30"

# Setup network interface
sudo ip link del "$TAP_DEV" 2> /dev/null || true
sudo ip tuntap add dev "$TAP_DEV" mode tap
sudo ip addr add "${TAP_IP}${MASK_SHORT}" dev "$TAP_DEV"
sudo ip link set dev "$TAP_DEV" up

# Enable ip forwarding
sudo sh -c "echo 1 > /proc/sys/net/ipv4/ip_forward"

HOST_IFACE="eth0"

# Set up microVM internet access
sudo iptables -t nat -D POSTROUTING -o "$HOST_IFACE" -j MASQUERADE || true
sudo iptables -D FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT \
    || true
sudo iptables -D FORWARD -i tap0 -o "$HOST_IFACE" -j ACCEPT || true
sudo iptables -t nat -A POSTROUTING -o "$HOST_IFACE" -j MASQUERADE
sudo iptables -I FORWARD 1 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
sudo iptables -I FORWARD 1 -i tap0 -o "$HOST_IFACE" -j ACCEPT

API_SOCKET="/tmp/firecracker.socket"
LOGFILE="./firecracker.log"

# Create log file
touch $LOGFILE

# Set log file
sudo curl -X PUT --unix-socket "${API_SOCKET}" \
    --data "{
        \"log_path\": \"${LOGFILE}\",
        \"level\": \"Debug\",
        \"show_level\": true,
        \"show_log_origin\": true
    }" \
    "http://localhost/logger"

KERNEL="./vmlinux-5.10.217" # Dan's modification updated from example
KERNEL_BOOT_ARGS="console=ttyS0 reboot=k panic=1 pci=off"

# Dan's modifications
FC_IP="172.16.0.2"
MASK_LONG="255.255.255.0"
KERNEL_BOOT_ARGS="${KERNEL_BOOT_ARGS} ip=${FC_IP}::${TAP_IP}:${MASK_LONG}::eth0:off"

ARCH=$(uname -m)

if [ ${ARCH} = "aarch64" ]; then
    KERNEL_BOOT_ARGS="keep_bootcon ${KERNEL_BOOT_ARGS}"
fi

# Set boot source
sudo curl -X PUT --unix-socket "${API_SOCKET}" \
    --data "{
        \"kernel_image_path\": \"${KERNEL}\",
        \"boot_args\": \"${KERNEL_BOOT_ARGS}\"
    }" \
    "http://localhost/boot-source"

# Dan's modification
ROOTFS="./rootfs.ext4"

# Set rootfs
sudo curl -X PUT --unix-socket "${API_SOCKET}" \
    --data "{
        \"drive_id\": \"rootfs\",
        \"path_on_host\": \"${ROOTFS}\",
        \"is_root_device\": true,
        \"is_read_only\": false
    }" \
    "http://localhost/drives/rootfs"

# The IP address of a guest is derived from its MAC address with
# `fcnet-setup.sh`, this has been pre-configured in the guest rootfs. It is
# important that `TAP_IP` and `FC_MAC` match this.
FC_MAC="06:00:AC:10:00:02"

# Set network interface
sudo curl -X PUT --unix-socket "${API_SOCKET}" \
    --data "{
        \"iface_id\": \"eth0\",
        \"guest_mac\": \"$FC_MAC\",
        \"host_dev_name\": \"$TAP_DEV\"
    }" \
    "http://localhost/network-interfaces/eth0"

# API requests are handled asynchronously, it is important the configuration is
# set, before `InstanceStart`.
sleep 0.015s

# Start microVM
sudo curl -X PUT --unix-socket "${API_SOCKET}" \
    --data "{
        \"action_type\": \"InstanceStart\"
    }" \
    "http://localhost/actions"

# API requests are handled asynchronously, it is important the microVM has been
# started before we attempt to SSH into it.
sleep 5s

# Setup internet access in the guest
ssh -i ./ubuntu-22.04.id_rsa root@172.16.0.2  "ip route add default via 172.16.0.1 dev eth0"

# Setup DNS resolution in the guest
ssh -i ./ubuntu-22.04.id_rsa root@172.16.0.2  "echo 'nameserver 8.8.8.8' > /etc/resolv.conf"

# SSH into the microVM
ssh -i ./ubuntu-22.04.id_rsa root@172.16.0.2

# Use `root` for both the login and password.
# Run `reboot` to exit.