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In this commit we deprecate the `vsock_id` field in PUTs on /vsock by
making it optional.

Signed-off-by: George Pisaltu <>
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Using the Firecracker Virtio-vsock Device

Table of Contents


This document assumes the reader is familiar with running Firecracker and issuing API commands over its API socket. For a more details on how to run Firecracker, check out the getting started guide.

Familiarity with socket programming, in particular Unix sockets, is also assumed.

Firecracker Virtio-vsock Design

The Firecracker vsock device aims to provide full virtio-vsock support to software running inside the guest VM, while bypassing vhost kernel code on the host. To that end, Firecracker implements the virtio-vsock device model, and mediates communication between AF_UNIX sockets (on the host end) and AF_VSOCK sockets (on the guest end).

In order to provide channel multiplexing the guest AF_VSOCK ports are mapped 1:1 to AF_UNIX sockets on the host. The virtio-vsock device must be configured with a path to an AF_UNIX socket on the host (e.g. /path/to/v.sock). There are two scenarios to be considered, depending on where the connection is initiated.

Host-Initiated Connections

When a microvm having a vsock device attached is started, Firecracker will begin listening on an AF_UNIX socket (e.g. /path/to/v.sock). When the host needs to initiate a connection, it should connect to that Unix socket, then send a connect command, in text form, specifying the destination AF_VSOCK port: "CONNECT PORT\n". Where PORT is the decimal port number, and "\n" is EOL (ASCII 0x0A). Following that, the same connection will be forwarded by Firecracker to the guest software listening on that port, thus establishing the requested channel. If the connection has been established, Firecracker will send an acknowledgement message to the connecting end (host-side), in the form "OK PORT\n", where PORT is the vsock port number assigned to the host end. If no one is listening, Firecracker will terminate the host connection.

Client A initiates connection to Server A in figure below:

  1. Host: At VM configuration time, add a virtio-vsock device, with some path specified in uds_path;
  2. Guest: create an AF_VSOCK socket and listen() on <port_num>;
  3. Host: connect() to AF_UNIX at uds_path.
  4. Host: send() "CONNECT <port_num>\n".
  5. Guest: accept() the new connection.
  6. Host: read() "OK <assigned_hostside_port>\n".

The channel is established between the sockets obtained at steps 3 (host) and 5 (guest).

Guest-Initiated Connections

When the virtio-vsock device model in Firecracker detects a connection request coming from the guest (a VIRTIO_VSOCK_OP_REQUEST packet), it tries to forward the connection to an AF_UNIX socket listening on the host, at /path/to/v.sock_PORT (or whatever path was configured via the uds_path property of the vsock device), where PORT is the destination port (in decimal), as specified in the connection request packet. If no such socket exists, or no one is listening on it, a connection cannot be established, and a VIRTIO_VSOCK_OP_RST packet will be sent back to the guest.

Client B initiates connection to Server B in figure below:

  1. Host: At VM configuration time, add a virtio-vsock device, with some uds_path (e.g. /path/to/v.sock).
  2. Host: create and listen on an AF_UNIX socket at /path/to/v.sock_PORT.
  3. Guest: create an AF_VSOCK socket and connect to HOST_CID (i.e. integer value 2) and PORT;
  4. Host: accept() the new connection.

The channel is established between the sockets obtained at steps 4 (host) and 3 (guest).

Vsock Connections

Setting up the virtio-vsock device

The virtio-vsock device will require a CID, and the path to a backing AF_UNIX socket:

curl --unix-socket /tmp/firecracker.socket -i \
  -X PUT 'http://localhost/vsock' \
  -H 'Accept: application/json' \
  -H 'Content-Type: application/json' \
  -d '{
      "guest_cid": 3,
      "uds_path": "./v.sock"

Once the microvm is started, Firecracker will create and start listening on the AF_UNIX socket at uds_path. Incoming connections will get forwarded to the guest microvm, and translated to AF_VSOCK. The destination port is expected to be specified by sending the text command "CONNECT <port_num>\n", immediately after the AF_UNIX connection is established. Connections initiated from within the guest will be forwarded to AF_UNIX sockets expected to be listening at ./v.sock_<port_num>. I.e. a guest connection to port 52 will get forwarded to ./v.sock_52.


The examples below assume a running microvm, with a vsock device configured as shown above.

Using External Socket Tools (nc-vsock and socat)

Connecting From Host to Guest

First, make sure the vsock port is bound and listened to on the guest side. Say, port 52:

nc-vsock -l 52

On the host side, connect to ./v.sock and issue a connection request to that port:

$ socat - UNIX-CONNECT:./v.sock

socat will display the connection acknowledgement message:

OK 1073741824

The connection should now be established (in the above example, between nc-vsock on the guest side, and socat on the host side).

Connecting From Guest To Host

First make sure the AF_UNIX corresponding to your desired port is listened to on the host side:

socat - UNIX-LISTEN:./v.sock_52

On the guest side, create an AF_VSOCK socket and connect it to the previously chosen port on the host (CID=2):

nc-vsock 2 52

Known issues

Vsock snapshot support is currently limited. Please see Snapshotting vsock limitations.