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LinuxKit with HyperKit (macOS)

We recommend using LinuxKit in conjunction with Docker for Mac. For the time being it's best to be on the latest edge release. linuxkit run uses HyperKit and VPNKit and the edge release ships with updated versions of both.

Alternatively, you can install HyperKit and VPNKit standalone and use it without Docker for Mac.

Boot

The HyperKit backend currently supports booting:

  • kernel+initrd output from linuxkit build.
  • kernel+squashfs output from linuxkit build.
  • EFI ISOs using the EFI firmware.

You need to select the boot method manually using the command line options. The default is kernel+initrd. kernel+squashfs can be selected using -squashfs and to boot a ISO with EFI you have to specify -iso -uefi.

The kernel+initrd uses a RAM disk for the root filesystem. If you have RAM constraints or large images we recommend using either the kernel+squashfs or the EFI ISO boot.

Console

With linuxkit run on HyperKit the serial console is redirected to stdio, providing interactive access to the VM. The output of the VM can be re-directed to a file or pipe, but then stdin is not available. HyperKit does not provide a console device.

Disks

The HyperKit backend support configuring a persistent disk using the standard linuxkit -disk syntax. Multiple disks are supported and the disks are in raw format.

Power management

HyperKit sends an ACPI power event when it receives SIGTERM to allow the VM to shut down properly. The VM has to be able to receive ACPI events to initiate the shutdown. This is provided by the acpid package. An example is available in the Docker for Mac example.

Networking

By default, linuxkit run creates a VM with a single network interface which, logically, is attached to a L2 bridge. The bridge also has the VM used by Docker for Mac attached to it. This means that the LinuxKit VMs, created with linuxkit run, can be accessed from containers running on Docker for Mac.

The LinuxKit VMs have IP addresses on the 192.168.65.0/24 subnet assigned by a DHCP server part of VPNKit. 192.168.65.1 is reserved for VPNKit as the default gateway and 192.168.65.2 is used by the Docker for Mac VM.

By default, LinuxKit VMs get incrementally increasing IP addresses, but you can assign a fixed IP address with linuxkit run -ip. It's best to choose an IP address from the DHCP address range above, but care must be taken to avoid clashes of IP address.

NOTE: The LinuxKit VMs can not be directly accessed by IP address from the host. Enabling this would require use of the macOS vmnet framework, which requires the VMs to run as root. We don't consider this option palatable, and provide alternative options to access the VMs over the network below.

Accessing network services

hyperkit offers a number of ways for accessing network services running inside the LinuxKit VM from the host. These depend on the networking mode selected via -networking. The default mode is docker-for-mac where the same VPNkit instance is shared between LinuxKit VMs and the VM running as part of Docker for Mac.

Access from the Docker for Mac VM (-networking docker-for-mac)

The simplest way to access networking services exposed by a LinuxKit VM is to use a Docker for Mac container. For example, to access an ssh server in a LinuxKit VM, create a ssh client container from:

FROM alpine:edge
RUN apk add --no-cache openssh-client

and then run

docker build -t ssh .
docker run --rm -ti -v ~/.ssh:/root/.ssh  ssh ssh <IP address of VM>

Forwarding ports with socat (-networking docker-for-mac)

A socat container on Docker for Mac can be used to proxy between the LinuxKit VM's ports and localhost. For example, to expose the redis port from the RedisOS example, use this Dockerfile:

FROM alpine:edge
RUN apk add --no-cache socat
ENTRYPOINT [ "/usr/bin/socat" ]

and then:

docker build -t socat .
docker run --rm -t -d -p 6379:6379 socat tcp-listen:6379,reuseaddr,fork tcp:<IP address of VM>:6379

Port forwarding with VPNKit (-networking docker-for-mac)

There is experimental support for exposing selected ports of the guest on localhost using the -publish command line option. For example, using -publish 2222:22/tcp exposes the guest TCP port 22 on localhost on port 2222. Multiple -publish options can be specified. For example, the image build from the sshd example can be started with:

linuxkit run -publish 2222:22/tcp sshd

and then you can log into the LinuxKit VM with ssh -p 2222 root@localhost.

Note, this mode is experimental and may cause the VPNKit instance shared with Docker for Mac being confused about which ports are currently in use, in particular if the LinuxKit VM does not exit gracefully. This can typically be fixed by restarting Docker for Mac.

Port forwarding with VPNKit (-networking vpnkit)

An alternative to the previous method is to start your own copy of vpnkit (or connect to an already running instance). This can be done using the -networking vpnkit command line option.

VPNKit uses a 9P mount in /port for coordination between components. The first VM on a VPNKit instance currently needs mount the 9P filesystem and also needs to run the vpnkit-forwarder service to enable port forwarding to localhost. A full example with vpnkit forwarding of sshd is available in examples/vpnkit-forwarder.yml.

To run this example with its own instance of VPNKit, use:

linuxkit run -networking vpnkit -publish 2222:22/tcp vpnkit-forwarder

You can then access it via:

ssh -p 2222 root@localhost

More details about the VPNKit forwarding mechanism is available in the VPNKit documentation.

Integration services and Metadata

There are no special integration services available for HyperKit, but there are a number of packages, such as vsudd, which enable tighter integration of the VM with the host (see below).

The HyperKit backend also allows passing custom userdata into the metadata package using either the -data or -data-file command-line option. This attaches a CD device with the data on.

vsudd unix domain socket forwarding

The vsudd package provides a daemon that exposes unix domain socket inside the VM to the host via virtio or Hyper-V sockets. With HyperKit, the virtio sockets can be exposed as unix domain sockets on the host, enabling access to other daemons, like containerd and dockerd, from the host. An example configuration file is available in examples/vsudd-containerd.yml.

After building the example, run it with linuxkit run hyperkit -vsock-ports 2374 vsudd. This will create a unix domain socket in the state directory that maps to the containerd control socket. The socket is called guest.00000946.

If you install the ctr tool on the host you should be able to access the containerd running in the VM:

$ go get -u -ldflags -s github.com/containerd/containerd/cmd/ctr
...
$ ctr -a vsudd-state/guest.00000946 list
ID        IMAGE     PID       STATUS
vsudd               466       RUNNING
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