This is a very simple quick-start guide to getting a Firecracker guest connected to the network. If you're using Firecracker in production, or even want to run multiple guests, you'll need to adapt this setup.

Note Currently firecracker supports only TUN/TAP network backend with no multi queue support.

The simple steps in this guide assume that your internet-facing interface is eth0, you have nothing else using tap0 and no other iptables rules. Check out the Advanced: sections if that doesn't work for you.

The first step on the host is to create a tap device:

sudo ip tuntap add tap0 mode tap

Then you have a few options for routing traffic out of the tap device, through your host's network interface. One option is NAT, set up like this:

sudo ip addr add 172.16.0.1/24 dev tap0
sudo ip link set tap0 up
sudo sh -c "echo 1 > /proc/sys/net/ipv4/ip_forward"
sudo iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
sudo iptables -A FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
sudo iptables -A FORWARD -i tap0 -o eth0 -j ACCEPT

Note: The IP of the TAP device should be chosen such that it's not in the same subnet as the IP address of the host.

Advanced: If you are running multiple Firecracker MicroVMs in parallel, or have something else on your system using tap0 then you need to create a tap for each one, with a unique name.

Advanced: You also need to do the iptables set up for each new tap. If you have iptables rules you care about on your host, you may want to save those rules before starting.

sudo iptables-save > iptables.rules.old

Before starting the guest, configure the network interface using Firecracker's API:

curl --unix-socket /tmp/firecracker.socket -i \
  -X PUT 'http://localhost/network-interfaces/eth0' \
  -H 'Accept: application/json' \
  -H 'Content-Type: application/json' \
  -d '{
      "iface_id": "eth0",
      "guest_mac": "AA:FC:00:00:00:01",
      "host_dev_name": "tap0"
    }'

If you are using a configuration file instead of the API, add a section to your configuration file like this:

"network-interfaces": [
  {
    "iface_id": "eth0",
    "guest_mac": "AA:FC:00:00:00:01",
    "host_dev_name": "tap0"
  }
],

Alternatively, if you are using firectl, add --tap-device=tap0/AA:FC:00:00:00:01` to your command line.

Once you have booted the guest, bring up networking within the guest:

ip addr add 172.16.0.2/24 dev eth0
ip link set eth0 up
ip route add default via 172.16.0.1 dev eth0

Now your guest should be able to route traffic to the internet (assuming that your host can get to the internet). To do anything useful, you probably want to resolve DNS names. In production, you'd want to use the right DNS server for your environment. For testing, you can add a public DNS server to /etc/resolv.conf by adding a line like this:

nameserver 8.8.8.8

1. Create a bridge interface

   sudo ip link add name br0 type bridge

2. Add tap interface created above to the bridge

   sudo ip link set dev tap0 master br0

3. Define an IP address in your network for the bridge.

   For example, if your gateway were on 192.168.1.1 and you wanted to use this for getting dynamic IPs, you would want to give the bridge an unused IP address in the 192.168.1.0/24 subnet.

   sudo ip address add 192.168.1.7/24 dev br0

4. Add firewall rules to allow traffic to be routed to the guest

   sudo iptables -t nat -A POSTROUTING -o br0 -j MASQUERADE

1. Define an unused IP address in the bridge's subnet e.g., 192.168.1.169/24.

   Note: Alternatively, you could rely on DHCP for getting a dynamic IP address from your gateway.

   ip addr add 192.168.1.169/24 dev eth0

2. Set the interface up.

   ip link set eth0 up

3. Create a route to the bridge device

   ip r add 192.168.1.1 via 192.168.1.7 dev eth0

4. Create a route to the internet via the bridge

   ip r add default via 192.168.1.7 dev eth0

   When done, your route table should look similar to the following:

   ip r
   default via 192.168.1.7 dev eth0
   192.168.1.0/24 dev eth0 scope link
   192.168.1.1 via 192.168.1.7 dev eth0

5. Add your nameserver to resolve.conf

   # cat /etc/resolv.conf
   nameserver 192.168.1.1

The first step to cleaning up is deleting the tap device:

sudo ip link del tap0

If you don't have anything else using iptables on your machine, clean up those rules:

sudo iptables -F
sudo sh -c "echo 0 > /proc/sys/net/ipv4/ip_forward" # usually the default

If you have an existing iptables setup, you'll want to be more careful about cleaning up.

Advanced: If you saved your iptables rules in the first step, then you can restore them like this:

if [ -f iptables.rules.old ]; then
    sudo iptables-restore < iptables.rules.old
fi

Advanced: If you created a bridge interface, delete it using the following:

sudo ip link del br0