vsock(7)

NAME#

vsock - Linux VSOCK address family

SYNOPSIS#

#include <sys/socket.h>\n
\n#include <linux/vm_sockets.h>

stream_socket\n = socket(AF_VSOCK, SOCK_STREAM, 0);\n
\ndatagram_socket\n = socket(AF_VSOCK, SOCK_DGRAM, 0);

DESCRIPTION#

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

bash

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.

ERRORS#

EACCES

Unable to bind to a privileged port without the CAP_NET_BIND_SERVICE capability.

EADDRINUSE

Unable to bind to a port that is already in use.

EADDRNOTAVAIL

Unable to find a free port for binding or unable to bind to a nonlocal CID.

EINVAL

Invalid parameters. This includes: attempting to bind a socket that is already bound, providing an invalid struct sockaddr_vm, and other input validation errors.

ENOPROTOOPT

Invalid socket option in setsockopt(2) or getsockopt(2).

ENOTCONN

Unable to perform operation on an unconnected socket.

EOPNOTSUPP

Operation not supported. This includes: the MSG_OOB flag that is not implemented for the send(2) family of syscalls and MSG_PEEK for the recv(2) family of syscalls.

EPROTONOSUPPORT

Invalid socket protocol number. The protocol should always be 0.

ESOCKTNOSUPPORT

Unsupported socket type in socket(2). Only SOCK_STREAM and SOCK_DGRAM are valid.

VERSIONS#

Support for VMware (VMCI) has been available since Linux 3.9. KVM (virtio) is supported since Linux 4.8. Hyper-V is supported since Linux 4.14.

VMADDR_CID_LOCAL is supported since Linux 5.6. Local communication in the guest and on the host is available since Linux 5.6. Previous versions supported only local communication within a guest (not on the host), and with only some transports (VMCI and virtio).

SEE ALSO#

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