vsock - Linux VSOCK address family

#include <sys/socket.h>
#include <linux/vm_sockets.h>
stream_socket = socket(AF_VSOCK, SOCK_STREAM, 0);
datagram_socket = socket(AF_VSOCK, SOCK_DGRAM, 0);

The VSOCK address family facilitates communication between virtual machines and the host they are running on. This address family is used by guest agents and hypervisor services that need a communications channel that is independent of virtual machine network configuration.

Valid socket types are SOCK_STREAM and SOCK_DGRAM. SOCK_STREAM provides connection-oriented byte streams with guaranteed, in-order delivery. SOCK_DGRAM provides a connectionless datagram packet service with best-effort delivery and best-effort ordering. Availability of these socket types is dependent on the underlying hypervisor.

A new socket is created with

socket(AF_VSOCK, socket_type, 0);

When a process wants to establish a connection, it calls connect(2) with a given destination socket address. The socket is automatically bound to a free port if unbound.

A process can listen for incoming connections by first binding to a socket address using bind(2) and then calling listen(2).

Data is transmitted using the send(2) or write(2) families of system calls and data is received using the recv(2) or read(2) families of system calls.

Address format

A socket address is defined as a combination of a 32-bit Context Identifier (CID) and a 32-bit port number. The CID identifies the source or destination, which is either a virtual machine or the host. The port number differentiates between multiple services running on a single machine.

struct sockaddr_vm {
    sa_family_t    svm_family;    /* Address family: AF_VSOCK */
    unsigned short svm_reserved1;
    unsigned int   svm_port;      /* Port # in host byte order */
    unsigned int   svm_cid;       /* Address in host byte order */
    unsigned char  svm_zero[sizeof(struct sockaddr) -
                            sizeof(sa_family_t) -
                            sizeof(unsigned short) -
                            sizeof(unsigned int) -
                            sizeof(unsigned int)];
};

svm_family is always set to AF_VSOCK. svm_reserved1 is always set to 0. svm_port contains the port number in host byte order. The port numbers below 1024 are called privileged ports. Only a process with the CAP_NET_BIND_SERVICE capability may bind(2) to these port numbers. svm_zero must be zero-filled.

There are several special addresses: VMADDR_CID_ANY (-1U) means any address for binding; VMADDR_CID_HYPERVISOR (0) is reserved for services built into the hypervisor; VMADDR_CID_LOCAL (1) is the well-known address for local communication (loopback); VMADDR_CID_HOST (2) is the well-known address of the host.

The special constant VMADDR_PORT_ANY (-1U) means any port number for binding.

Live migration

Sockets are affected by live migration of virtual machines. Connected SOCK_STREAM sockets become disconnected when the virtual machine migrates to a new host. Applications must reconnect when this happens.

The local CID may change across live migration if the old CID is not available on the new host. Bound sockets are automatically updated to the new CID.

Ioctls

The following ioctls are available on the /dev/vsock device.

IOCTL_VM_SOCKETS_GET_LOCAL_CID

Get the CID of the local machine. The argument is a pointer to an unsigned int.

ioctl(fd, IOCTL_VM_SOCKETS_GET_LOCAL_CID, &cid);

Consider using VMADDR_CID_ANY when binding instead of getting the local CID with IOCTL_VM_SOCKETS_GET_LOCAL_CID.

Local communication

VMADDR_CID_LOCAL (1) directs packets to the same host that generated them. This is useful for testing applications on a single host and for debugging.

The local CID obtained with IOCTL_VM_SOCKETS_GET_LOCAL_CID can be used for the same purpose, but it is preferable to use VMADDR_CID_LOCAL.

bind(2), connect(2), listen(2), recv(2), send(2), socket(2), capabilities(7)