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