Issue: How to Establish Bidirectional Communication Between Host and Guest Using virtio-vsock

[sntaoo]

Issue: How to Establish Bidirectional Communication Between Host and Guest Using virtio-vsock

Background

I'm trying to set up bidirectional communication between host and guest using vsock. According to the documentation at https://github.com/crc-org/vfkit/blob/main/doc/usage.md#virtio-vsock-communication, I understand that:

This allows virtio-vsock communication from the host to the guest over vsock port 6:

--device virtio-vsock,port=6,socketURL=/Users/virtuser/vfkit-6.sock,connect

The socket can be created on the guest with nc --vsock --listen 3 6, and the host can connect to it with nc -U /Users/virtuser/vfkit-6.sock

My Goal

I want to achieve the following:

1. Allow the guest VM to connect to Service A on the host via VSOCK port 1023
2. Allow the host to connect to Service B in the guest VM via a Unix domain socket

My Configuration

Here is my vfkit start command:

vfkit \
  -c 4 \
  -m 4096 \
  -b linux,kernel=vmlinux,initrd=initrd,cmdline="console=hvc0 root=/dev/vda" \
  -d virtio-blk,path=disk.img \
  -d virtio-serial,stdio \
  -d virtio-net,nat \
  -d virtio-fs,sharedDir=/,mountTag=host \
  -d virtio-vsock,port=1023,socketURL=/tmp/sock-a.sock \
  -d virtio-vsock,port=3300,socketURL=/tmp/vfkit-3300.sock,connect \
  -d virtio-fs,sharedDir="$SHARED_DIR",mountTag=mytag

Observed Behavior

Observation 1: Socket Creation

• /tmp/sock-a.sock is successfully created automatically
• /tmp/vfkit-3300.sock is not created automatically. I had to manually create this socket file using nc

Observation 2: Guest to Host Communication (Working ✅)

From within the guest VM, I can successfully connect to the host service on VSOCK port 1023:

# Command in guest VM
echo "Hello From Guest" | socat - VSOCK-CONNECT:2:1023

# Output
Hello From Host

This works as expected.

Observation 3: Host to Guest Communication (Not Working ❌)

I started a vsock listener in the guest VM:

ncat --vsock --listen 3 3300 -c 'cat'

However, when I try to connect from the host, it fails:

# Command on host
echo "Hello From Host" | socat - UNIX-CONNECT:/tmp/vfkit-3300.sock

# Error output
2026/03/03 22:19:13 socat[52745] E connect(, LEN=31 AF=1 "/tmp/vfkit-3300.sock", 22): Connection refused

I also tried using nc -U as shown in the documentation, but there is no output at all. Additionally, the listener in the guest VM does not receive any data.

Question

Could you please advise on the correct configuration to achieve my goal? Specifically:

1. Why is /tmp/vfkit-3300.sock not being created automatically when using the connect flag?
2. What is the proper way to establish a connection from the host to a vsock listener in the guest?
3. Is there a specific order or additional configuration required for bidirectional vsock communication?

Thank you for your assistance!

[bittermandel]

I managed to FINALLY get guest-to-host working. Must've taken me all day.

host→guest vsock under vfkit required two fixes: use connect for the vfkit vsock bridge, and attach a log-backed virtio-serial device so the guest-side becomes stable enough to accept connections.

[cfergeau]

-d virtio-serial,stdio can unforunately cause issues if this is what you mean by " attach a log-backed virtio-serial device so the guest-side becomes stable enough to accept connections". Another option is not to add the serial device at all.

host->guest vsock communication indeed needs to use connect as documented here https://github.com/crc-org/vfkit/blob/main/doc/usage.md#virtio-vsock-communication.

[cfergeau]

• Why is /tmp/vfkit-3300.sock not being created automatically when using the connect flag?

This is unexpected, when connect is used, I expect vfkit to create the socket. When listen (the default) is used, the user needs to create the socket. This is consistent with the documentation https://github.com/crc-org/vfkit/blob/main/doc/usage.md#virtio-vsock-communication

• What is the proper way to establish a connection from the host to a vsock listener in the guest?

• Is there a specific order or additional configuration required for bidirectional vsock communication?

You need 2 different vsock ports if you need to connect in both directions, but this is what you did. Apart from that, this should work as in your description. I’ll test it when I have access to a mac.