Give a Firecracker microVM outbound network access through a host TAP interface with NAT
domain: firecracker-microvm.github.io · 4 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
Create and configure the TAP on the host: sudo ip tuntap add dev tap0 mode tap; sudo ip addr add 172.16.0.1/30 dev tap0; sudo ip link set dev tap0 up
Enable forwarding and NAT for outbound traffic: sudo sh -c 'echo 1 > /proc/sys/net/ipv4/ip_forward'; sudo iptables -t nat -A POSTROUTING -o <host_iface> -j MASQUERADE
Attach the TAP to the VM before start: curl --unix-socket /tmp/firecracker.socket -X PUT http://localhost/network-interfaces/net1 -d '{"iface_id":"net1","guest_mac":"06:00:AC:10:00:02","host_dev_name":"tap0"}'
In the guest set the default route via the host TAP IP and add DNS: ip route add default via 172.16.0.1 dev eth0; echo 'nameserver 8.8.8.8' > /etc/resolv.conf
Known gotchas
The guest IP is typically derived from its MAC; keep the TAP IP and guest MAC consistent with the guest rootfs config
Without ip_forward and the MASQUERADE iptables rule, outbound guest traffic will not route
glibc DNS resolves A/AAAA in parallel and the AAAA query can stall on host NAT, adding seconds per lookup; add 'options single-request-reopen' to resolv.conf
The guest must have eth0 up; on some rootfs images you add the default route explicitly as shown
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