Jason, please have a look.
Faisal Al-Humaimidi <falhuma...@gmail.com> writes:
> Hello QEMU developers,
>
> I have noticed a bug in the `mcast` option of the `socket` networking
> backend, where I simply cannot join a multicast group (tested in Windows 10
> with QEMU 4.2.0 release). I have found a fix to the problem. The problem
> was mainly due to the fact that QEMU was binding to the multicast address,
> and not the local address or the default INADDR_ANY (0.0.0.0) if no local
> address is used.
>
> Here's the patch text (as well as attached with this email), that outlines
> my fix:
>
> ```
> diff -uarN qemu-4.2.0.original/net/socket.c qemu-4.2.0.modified/net/socket.c
> --- qemu-4.2.0.original/net/socket.c 2019-12-12 10:20:48.000000000 -0800
> +++ qemu-4.2.0.modified/net/socket.c 2020-02-14 10:30:16.395973453 -0800
> @@ -253,6 +253,15 @@
> goto fail;
> }
>
> + /* Preserve the multicast address, and bind to a non-multicast group
> (e.g. a
> + * local address).
> + */
> + struct in_addr group_addr = mcastaddr->sin_addr;
> + if (localaddr) {
> + mcastaddr->sin_addr = *localaddr;
> + } else {
> + mcastaddr->sin_addr.s_addr = htonl(INADDR_ANY);
> + }
> ret = bind(fd, (struct sockaddr *)mcastaddr, sizeof(*mcastaddr));
> if (ret < 0) {
> error_setg_errno(errp, errno, "can't bind ip=%s to socket",
> @@ -260,7 +269,10 @@
> goto fail;
> }
>
> - /* Add host to multicast group */
> + /* Restore the multicast address. */
> + mcastaddr->sin_addr = group_addr;
> +
> + /* Add host to multicast group. */
> imr.imr_multiaddr = mcastaddr->sin_addr;
> if (localaddr) {
> imr.imr_interface = *localaddr;
> @@ -277,7 +289,7 @@
> goto fail;
> }
>
> - /* Force mcast msgs to loopback (eg. several QEMUs in same host */
> + /* Force mcast msgs to loopback (eg. several QEMUs in same host). */
> loop = 1;
> ret = qemu_setsockopt(fd, IPPROTO_IP, IP_MULTICAST_LOOP,
> &loop, sizeof(loop));
> @@ -287,7 +299,7 @@
> goto fail;
> }
>
> - /* If a bind address is given, only send packets from that address */
> + /* If a bind address is given, only send packets from that address. */
> if (localaddr != NULL) {
> ret = qemu_setsockopt(fd, IPPROTO_IP, IP_MULTICAST_IF,
> localaddr, sizeof(*localaddr));
> ```
>
> Regards,
> Faisal Al-Humaimidi
