Set up host-to-guest and guest-to-host communication in a Firecracker microVM over virtio-vsock
domain: firecracker-microvm.github.io · 5 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
Add a vsock device at VM config time via the API: PUT /vsock with {"guest_cid":3, "uds_path":"./v.sock"}
Ensure kernels have vsock support (host CONFIG_VHOST_VSOCK=m, guest CONFIG_VIRTIO_VSOCKETS=y; verify inside guest with ls /dev/vsock)
Host->guest: guest listens on an AF_VSOCK port; host connects to the AF_UNIX uds_path and sends 'CONNECT <port>\n'; Firecracker replies 'OK <host_port>\n'
Guest->host: host listens on AF_UNIX at <uds_path>_<port>; guest connects to HOST_CID (always 2) and the port
Verify with socat: host 'socat - UNIX-CONNECT:./v.sock' then 'CONNECT 52', guest 'socat VSOCK-LISTEN:52,fork -'
Known gotchas
HOST_CID is always 2 for guest-initiated connections
Host-initiated uses one UDS with a text CONNECT command; guest-initiated uses per-port UDS paths (<uds_path>_<port>)
If no guest is listening, Firecracker terminates the host connection; if no host socket exists for a guest connect, an RST is sent back to the guest
On snapshot restore use the vsock_override parameter to remap the UDS path and avoid path collisions
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