Index: vl.c
===================================================================
RCS file: /cvsroot/qemu/qemu/vl.c,v
retrieving revision 1.152
diff -u -r1.152 vl.c
--- vl.c	5 Dec 2005 20:31:52 -0000	1.152
+++ vl.c	7 Dec 2005 17:46:22 -0000
@@ -2131,8 +2131,14 @@
     int index;
     int packet_len;
     uint8_t buf[4096];
+    struct sockaddr_in dgram_dst; /* contains inet host and port destination iff connectionless (SOCK_DGRAM) */
 } NetSocketState;
 
+static int qemu_socket_is_dgram(const struct NetSocketState *s)
+{
+    return (s->dgram_dst.sin_addr.s_addr!=0);
+}
+
 typedef struct NetSocketListenState {
     VLANState *vlan;
     int fd;
@@ -2145,8 +2151,12 @@
     uint32_t len;
     len = htonl(size);
 
-    unix_write(s->fd, (const uint8_t *)&len, sizeof(len));
-    unix_write(s->fd, buf, size);
+    if (qemu_socket_is_dgram(s)) {
+	sendto(s->fd, buf, size, 0, (struct sockaddr *)&s->dgram_dst, sizeof(s->dgram_dst));
+    } else {
+	unix_write(s->fd, (const uint8_t *)&len, sizeof(len));
+	unix_write(s->fd, buf, size);
+    }
 }
 
 static void net_socket_send(void *opaque)
@@ -2164,6 +2174,11 @@
         qemu_set_fd_handler(s->fd, NULL, NULL, NULL);
         return;
     }
+    if (qemu_socket_is_dgram(s)) {
+	s->index = 0;
+	s->state = 1;
+	s->packet_len = size;
+    }
     buf = buf1;
     while (size > 0) {
         /* reassemble a packet from the network */
@@ -2336,6 +2351,78 @@
     return 0;
 }
 
+static int net_socket_mcast_init(VLANState *vlan, const char *host_str)
+{
+    NetSocketState *s;
+    int fd=-1, ret, val;
+    struct sockaddr_in saddr, bindaddr;
+    struct ip_mreq imr;
+
+    if (parse_host_port(&saddr, host_str) < 0)
+        return -1;
+
+    if (!IN_MULTICAST(ntohl(saddr.sin_addr.s_addr))) {
+	fprintf(stderr, "qemu: error: specified mcastaddr \"%s\" (0x%08x) does not contain a multicast address\n",
+		inet_ntoa(saddr.sin_addr), ntohl(saddr.sin_addr.s_addr));
+	return -1;
+
+    }
+    fd = socket(PF_INET, SOCK_DGRAM, 0);
+    if (fd < 0) {
+        perror("socket(PF_INET, SOCK_DGRAM)");
+        return -1;
+    }
+
+    /* Add host to multicast group */
+    imr.imr_multiaddr = saddr.sin_addr;
+    imr.imr_interface.s_addr = htonl(INADDR_ANY);
+
+    ret = setsockopt(fd, IPPROTO_IP, IP_ADD_MEMBERSHIP, (void *) &imr, sizeof(struct ip_mreq));
+    if (ret < 0) {
+	perror("setsockopt(IP_ADD_MEMBERSHIP)");
+	goto fail;
+    }
+
+    /* Force mcast msgs to loopback (eg. several QEMUs in same host */
+    val = 1;
+    ret=setsockopt(fd, SOL_IP, IP_MULTICAST_LOOP, &val, sizeof(val));
+    if (ret < 0) {
+	perror("setsockopt(SOL_IP, IP_MULTICAST_LOOP)");
+	goto fail;
+    }
+    ret=setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &val, sizeof(val));
+    if (ret < 0) {
+	perror("setsockopt(SOL_SOCKET, SO_REUSEADDR)");
+	goto fail;
+    }
+    
+    bindaddr = saddr;
+    bindaddr.sin_addr.s_addr = htonl(INADDR_ANY);
+
+    ret = bind(fd, (struct sockaddr *)&bindaddr, sizeof(bindaddr));
+    if (ret < 0) {
+        perror("bind");
+        goto fail;
+    }
+    
+    fcntl(fd, F_SETFL, O_NONBLOCK);
+
+    s = net_socket_fd_init(vlan, fd, 1);
+    if (!s)
+        goto fail;
+    
+    /* Connectionless: save mcast address for sendto() */
+    s->dgram_dst = saddr;
+
+    snprintf(s->vc->info_str, sizeof(s->vc->info_str),
+             "socket: mcast at %s:%d", 
+             inet_ntoa(saddr.sin_addr), ntohs(saddr.sin_port));
+    return 0;
+fail:
+    if (fd>=0) close(fd);
+    return -1;
+}
+
 #endif /* !_WIN32 */
 
 static int get_param_value(char *buf, int buf_size,
@@ -2472,6 +2559,8 @@
             ret = net_socket_listen_init(vlan, buf);
         } else if (get_param_value(buf, sizeof(buf), "connect", p) > 0) {
             ret = net_socket_connect_init(vlan, buf);
+        } else if (get_param_value(buf, sizeof(buf), "mcast", p) > 0) {
+            ret = net_socket_mcast_init(vlan, buf);
         } else {
             fprintf(stderr, "Unknown socket options: %s\n", p);
             return -1;
@@ -3810,6 +3899,8 @@
            "                use 'fd=h' to connect to an already opened TAP interface\n"
            "-net socket[,vlan=n][,fd=h][,listen=[host]:port][,connect=host:port]\n"
            "                connect the vlan 'n' to another VLAN using a socket connection\n"
+           "-net socket[,vlan=n][,fd=h][,mcast=maddr:port]\n"
+           "                create a vlan 'n' with another QEMUs on multicast maddr and port\n"
 #endif
            "-net none       use it alone to have zero network devices; if no -net option\n"
            "                is provided, the default is '-net nic -net user'\n"
Index: qemu-doc.texi
===================================================================
RCS file: /cvsroot/qemu/qemu/qemu-doc.texi,v
retrieving revision 1.73
diff -u -r1.73 qemu-doc.texi
--- qemu-doc.texi	19 Nov 2005 17:42:52 -0000	1.73
+++ qemu-doc.texi	7 Dec 2005 17:46:25 -0000
@@ -293,6 +293,22 @@
 qemu linux.img -net nic,macaddr=52:54:00:12:34:57 -net socket,connect=127.0.0.1:1234
 @end example
 
address@hidden -net socket[,vlan=n][,fd=h][,mcast=maddr:port]
+
+Create a VLAN @var{n} shared with another QEMU virtual
+machines using a UDP multicast socket, effectively making a bus for 
+every QEMU with same multicast address @var{maddr} and @var{port}.
+
+Example:
address@hidden
+# launch one QEMU instance
+qemu linux.img -net nic,macaddr=52:54:00:12:34:56 -net socket,mcast=230.0.0.1:1234
+# launch another QEMU instance on same "bus"
+qemu linux.img -net nic,macaddr=52:54:00:12:34:57 -net socket,mcast=230.0.0.1:1234
+# launch yet another QEMU instance on same "bus"
+qemu linux.img -net nic,macaddr=52:54:00:12:34:58 -net socket,mcast=230.0.0.1:1234
address@hidden example
+
 @item -net none
 Indicate that no network devices should be configured. It is used to
 override the default configuration which is activated if no