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vsock notes

vsock notes

about vsocks

Vsocks are a means of providing socket communication (either stream or datagram) directly between VMs and their host operating system. The host and each VM have a 32 bit CID (Context IDentifier) and may connect or bind to a 32 bit port number. Ports < 1024 are privileged ports.

special values

name value description
VMADDR_PORT_ANY -1 Bind to a random available port.
VMADDR_CID_ANY -1 Bind to any CID. This seems to work inside VMs only.
VMADDR_CID_HYPERVISOR 0 The hypervisor's CID.
VMADDR_CID_RESERVED 1 Reserved; this must not be used.
VMADDR_CID_HOST 2 The host's CID.

how to launch a vm with a vsock device

This creates a VM with a vsock device with CID 123.

qemu-system-x86_64 -device vhost-vsock-pci,guest-cid=123

vsock server on host

c

#include <sys/socket.h>
#include <linux/vm_sockets.h>
#include <string.h>
#include <stdio.h>

int main()
{
	int s = socket(AF_VSOCK, SOCK_STREAM, 0);

	struct sockaddr_vm addr;
	memset(&addr, 0, sizeof(struct sockaddr_vm));
	addr.svm_family = AF_VSOCK;
	addr.svm_port = 9999;
	addr.svm_cid = VMADDR_CID_HOST;

	bind(s, &addr, sizeof(struct sockaddr_vm));

	listen(s, 0);

	struct sockaddr_vm peer_addr;
	socklen_t peer_addr_size = sizeof(struct sockaddr_vm);
	int peer_fd = accept(s, &peer_addr, &peer_addr_size);

	char buf[64];
	size_t msg_len;
	while ((msg_len = recv(peer_fd, &buf, 64, 0)) > 0) {
		printf("Received %lu bytes: %.*s\n", msg_len, msg_len, buf);
	}

	return 0;
}

python

#!/usr/bin/env python3

import socket

CID = socket.VMADDR_CID_HOST
PORT = 9999

s = socket.socket(socket.AF_VSOCK, socket.SOCK_STREAM)
s.bind((CID, PORT))
s.listen()
(conn, (remote_cid, remote_port)) = s.accept()

print(f"Connection opened by cid={remote_cid} port={remote_port}")

while True:
    buf = conn.recv(64)
    if not buf:
        break

    print(f"Received bytes: {buf}")

vsock client in vm

c

#include <sys/socket.h>
#include <linux/vm_sockets.h>
#include <string.h>

int main()
{
	int s = socket(AF_VSOCK, SOCK_STREAM, 0);

	struct sockaddr_vm addr;
	memset(&addr, 0, sizeof(struct sockaddr_vm));
	addr.svm_family = AF_VSOCK;
	addr.svm_port = 9999;
	addr.svm_cid = VMADDR_CID_HOST;

	connect(s, &addr, sizeof(struct sockaddr_vm));

	send(s, "Hello, world!", 13, 0);

	close(s);

	return 0;
}

python

#!/usr/bin/env python3

import socket

CID = socket.VMADDR_CID_HOST
PORT = 9999

s = socket.socket(socket.AF_VSOCK, socket.SOCK_STREAM)
s.connect((CID, PORT))
s.sendall(b"Hello, world!")
s.close()

wireshark

ip link add type vsockmon
ip link set vsockmon0 up
wireshark -k -i vsockmon0
ip link set vsockmon0 down
ip link del vsockmon0

see also

@tristone13th
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@stefano-garzarella Get it, many thanks!

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