Re: QEMU Sockets Networking Backend Multicast Networking Fix

[Markus Armbruster]

Jason, please have a look.

Faisal Al-Humaimidi <address@hidden> 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