package hvsock

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Bindings for Hyper-V AF_VSOCK


Dune Dependency






Build Status (Linux) Build status (Windows)

These bindings allow Host <-> VM communication on Hyper-V systems on both Linux and Windows.

Warning: the AF_HYPERV patches for Linux are not yet merged and hence the definition of AF_HYPERV is not yet stable. If other address families are merged before this one then the value of AF_HYPERV will change!

Please read the API documentation.

Published: 20 Jun 2022


ocaml-hvsock -- bindings for Hypervisor sockets

These bindings allow Host <-> VM communication

  • on Windows with Hyper-V sockets

  • on Linux with AF_VSOCK

  • on macOS with VMs running with hyperkit.

Please read the API documentation and see the example src/


The addressing scheme used is hypervisor-dependent. Windows identifies a VM (partition) with a GUID and a service with a GUID. Linux identifies a VM with a 32-bit int and a port as a 32-bit int. When running on a Windows host, the Linux kernel maps a 32-bit port number onto a special range of Hyper-V socket GUIDs using the format string "%08lx-FACB-11E6-BD58-64006A7986D3".

This library exposes the underlying addressing schemes in the modules

  • Hvsock.Af_hyperv: corresponding to the AF_HYPERV address family on Windows

  • Hvsock.Af_vsock: corresponding to the AF_VSOCK address family on Linux

  • Hvsock.Hyperkit: corresponding to the macOS backend implementation of the AF_VSOCK address family on Linux.

For utilities that wish to be cross-platform, there is a generic interface in Hvsock.Socket which understands addresses encoded as URIs, suitable for command-line interfaces (and compatible with those used by linuxkit/virtsock). The URIs look like:

  • vsock://:80: listen on AF_VSOCK port 80. If run in a Linux VM on a Windows host then the Windows service GUID will be 00000050-FACB-11E6-BD58-64006A7986D3.

  • hvsock://<VM GUID>/3049197C-9A4E-4FBF-9367-97F792F16994: connect to service 3049.. on a Windows VM <VM GUID> as displayed by the powershell (Get-VM Name).Id. This requires the service 3049... to be pre-registered in the registry but the connection itself can be run as a non-Administrator user.

  • hvsock://<VM name>/3049197C-9A4E-4FBF-9367-97F792F16994: connect to a service 3049.. in the Windows VM <VM name>. Note the library will shell out to powershell to resolve the name to a GUID, which unfortunately requires Administrator.

  • vsock://2:80/: connect to a Linux VM with id 2 on port 80 via AF_VSOCK

  • hyperkit://:80/Users/foo/Library/Containers/com.docker.docker/Data/vms/0: connect to AF_VSOCK port 80 on the Hyperkit VM running at the given filesystem path.


Consider the Linux VM running on a Windows host started by Docker Desktop. The VM name in Hyper V manager is MobyLinuxVM.

First build and run this code in a privileged Linux container in the VM:

docker build -t hvsock .
docker run -it -v <path>:/src --privileged hvsock sh
./_build/default/src/hvcat.exe -l vsock://:255

Next build and run the code on the host:

./_build/default/src/hvcat hvsock://MobyLinuxVM/000000ff-FACB-11E6-BD58-64006A7986D3 --register

The --register flag will run a powershell fragment to register the serviceid in the registry. This requires Administrator but only needs to be done once.


Although the connections use the regular socket APIs, current Windows kernels don't support calls like select so we must always use blocking I/O from background threads, rather than regular asynchronous I/O. The Hvsock_lwt.Socket module can create a Lwt-compatible I/O layer on top of regular threads.

The client connect call seems to block forever if the server calls listen after the client calls connect. The Af_hyperv.connect works around this with a self-imposed 1s timeout after which time it will raise ECONNREFUSED.

Depending on the Windows and Linux kernel versions, in-flight data can be lost after a shutdown_write or close on a Windows host.


For background, see the MSDN article on making an integration service

Dev Dependencies (1)

  1. alcotest with-test & >= "0.4.0"

Used by (3)

  1. datakit >= "0.10.0"
  2. datakit-bridge-github >= "0.10.0"
  3. vpnkit >= "0.1.1"




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