diff --git a/net.c b/net.c
index 3fdf1e6..c6eb93c 100644
--- a/net.c
+++ b/net.c
@@ -280,6 +280,20 @@ void qemu_format_nic_info_str(VLANClientState *vc, uint8_t macaddr[6])
              macaddr[3], macaddr[4], macaddr[5]);
 }
 
+void qemu_macaddr_default_if_unset(macaddr_t macaddr)
+{
+    macaddr_t zero = { 0,0,0,0,0,0 };
+
+    if (memcmp(macaddr, zero, sizeof(zero)) != 0)
+        return;
+    macaddr[0] = 0x52;
+    macaddr[1] = 0x54;
+    macaddr[2] = 0x00;
+    macaddr[3] = 0x12;
+    macaddr[4] = 0x34;
+    macaddr[5] = 0x56;
+}
+
 static char *assign_name(VLANClientState *vc1, const char *model)
 {
     VLANState *vlan;
diff --git a/net.h b/net.h
index 1479826..50630a1 100644
--- a/net.h
+++ b/net.h
@@ -7,6 +7,7 @@
 
 /* VLANs support */
 
+typedef uint8_t macaddr_t[6];
 typedef struct VLANClientState VLANClientState;
 
 typedef int (NetCanReceive)(VLANClientState *);
@@ -75,6 +76,7 @@ ssize_t qemu_send_packet_async(VLANClientState *vc, const uint8_t *buf,
 void qemu_purge_queued_packets(VLANClientState *vc);
 void qemu_flush_queued_packets(VLANClientState *vc);
 void qemu_format_nic_info_str(VLANClientState *vc, uint8_t macaddr[6]);
+void qemu_macaddr_default_if_unset(macaddr_t macaddr);
 void qemu_check_nic_model(NICInfo *nd, const char *model);
 void qemu_check_nic_model_list(NICInfo *nd, const char * const *models,
                                const char *default_model);