QEMU Sockets Networking Backend Multicast Networking Fix

[Faisal Al-Humaimidi]

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

qemu-4.2.0-fix_socket_mcast.patch
Description: Binary data