Skip to main content

Flintlockd

Error: misleading network_interfaces error when creating a MicroVM​

Example error:

ERRO[0003] failed to reconcile microvm ns1/mvm0:
  creating microvm:
    creating firecracker config:
      updating firecracker network-interfaces config:
        firecracker api rejected config: network_interfaces: invalid or missing

The exact wording depends on the Firecracker version, but the error points at network interfaces in a way that looks like a flintlock configuration mistake with the network-interfaces/network_interfaces field names. It isn't: those names are correct and unrelated to the real problem.

The actual cause is that one of the MicroVM's NetworkInterface entries has its device_id set to eth0. Firecracker reserves eth0 for its own use, so a guest interface named eth0 clashes with it and the create fails with this misleading error.

This only happens when guest_mac is not set on that interface: without a MAC address to match on, flintlock falls back to matching the guest interface by name, using device_id as that name. If guest_mac is set, device_id is just an opaque identifier and this problem does not occur.

Fix​

Either:

  • Use a different device_id for the interface, e.g. eth1, or
  • Set guest_mac on the interface so it is matched by MAC address instead of by name.