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       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

       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.

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

              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

   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 .


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

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

              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.

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

              Unable to perform operation on an unconnected socket.

              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.

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

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


       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


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


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