grain
Install

guides

Overlay network between VMs

Share an L2 segment across grain VMs with network overlay mode.

By default each grain VM uses SLIRP (user networking): isolated from other VMs, with host→guest port forwards.

For multi-VM labs that need guest-to-guest connectivity, set network: overlay. grain attaches a second NIC using QEMU’s multicast socket backend so VMs on the same host join a shared L2 segment (230.0.0.1:4242) while keeping SLIRP for SSH and published ports.

Enable

Per VM

bash 
grain new -n a --network overlay --wait agent
grain new -n b --network overlay --wait agent

Config default (~/.grain/config.yaml)

yaml 
network: overlay   # or slirp

What you get

InterfaceRole
First NIC (SLIRP)Host access, hostfwd, proxy via 10.0.2.2
Second NIC (overlay)Guest↔guest on the shared multicast LAN

Inside the guest, configure addresses on the second interface yourself (static IP, mDNS, etc.). grain does not run DHCP on the overlay.

Limits

  • Same host only (multicast socket, not a routable multi-host fabric)
  • Firewall/OS multicast restrictions can block the overlay
  • Firecracker backend does not use this path

See also