Set a stable & high MAC addr for guest TAP devices on host

A Linux software bridge will assume the MAC address of the enslaved
interface with the numerically lowest MAC addr. When the bridge
changes MAC address there is a period of network blackout, so a
change should be avoided. The kernel gives TAP devices a completely
random MAC address. Occassionally the random TAP device MAC is lower
than that of the physical interface (eth0, eth1etc) that is enslaved,
causing the bridge to change its MAC.

This change sets an explicit MAC address for all TAP devices created
using the configured MAC from the XML, but with the high byte set
to 0xFE. This should ensure TAP device MACs are higher than any
physical interface MAC.

* src/qemu/qemu_conf.c, src/uml/uml_conf.c: Pass in a MAC addr
  for the TAP device with high byte set to 0xFE
* src/util/bridge.c, src/util/bridge.h: Set a MAC when creating
  the TAP device to override random MAC

src/qemu/qemu_conf.c

@@ -1608,6 +1608,7 @@ qemudNetworkIfaceConnect(virConnectPtr conn,
     int tapfd = -1;
     int vnet_hdr = 0;
     int template_ifname = 0;
+    unsigned char tapmac[VIR_MAC_BUFLEN];
 
     if (net->type == VIR_DOMAIN_NET_TYPE_NETWORK) {
         int active, fail = 0;
@@ -1675,8 +1676,14 @@ qemudNetworkIfaceConnect(virConnectPtr conn,
         net->model && STREQ(net->model, "virtio"))
         vnet_hdr = 1;
 
-    if ((err = brAddTap(driver->brctl, brname,
-                        &net->ifname, vnet_hdr, &tapfd))) {
+    memcpy(tapmac, net->mac, VIR_MAC_BUFLEN);
+    tapmac[0] = 0xFE; /* Discourage bridge from using TAP dev MAC */
+    if ((err = brAddTap(driver->brctl,
+                        brname,
+                        &net->ifname,
+                        tapmac,
+                        vnet_hdr,
+                        &tapfd))) {
         if (errno == ENOTSUP) {
             /* In this particular case, give a better diagnostic. */
             qemuReportError(VIR_ERR_INTERNAL_ERROR,

src/uml/uml_conf.c

@@ -113,6 +113,7 @@ umlConnectTapDevice(virDomainNetDefPtr net,
     int tapfd = -1;
     int template_ifname = 0;
     int err;
+    unsigned char tapmac[VIR_MAC_BUFLEN];
 
     if ((err = brInit(&brctl))) {
         virReportSystemError(err, "%s",
@@ -130,8 +131,14 @@ umlConnectTapDevice(virDomainNetDefPtr net,
         template_ifname = 1;
     }
 
-    if ((err = brAddTap(brctl, bridge,
-                        &net->ifname, BR_TAP_PERSIST, &tapfd))) {
+    memcpy(tapmac, net->mac, VIR_MAC_BUFLEN);
+    tapmac[0] = 0xFE; /* Discourage bridge from using TAP dev MAC */
+    if ((err = brAddTap(brctl,
+                        bridge,
+                        &net->ifname,
+                        tapmac,
+                        0,
+                        &tapfd))) {
         if (errno == ENOTSUP) {
             /* In this particular case, give a better diagnostic. */
             umlReportError(VIR_ERR_INTERNAL_ERROR,

src/util/bridge.c

@@ -284,6 +284,38 @@ brDeleteInterface(brControl *ctl ATTRIBUTE_UNUSED,
 }
 # endif
 
+/**
+ * ifSetInterfaceMac:
+ * @ctl: bridge control pointer
+ * @ifname: interface name to set MTU for
+ * @macaddr: MAC address (VIR_MAC_BUFLEN in size)
+ *
+ * This function sets the @macaddr for a given interface @ifname. This
+ * gets rid of the kernel's automatically assigned random MAC.
+ *
+ * Returns 0 in case of success or an errno code in case of failure.
+ */
+static int ifSetInterfaceMac(brControl *ctl, const char *ifname,
+                             const unsigned char *macaddr)
+{
+    struct ifreq ifr;
+
+    if (!ctl || !ifname)
+        return EINVAL;
+
+    memset(&ifr, 0, sizeof(struct ifreq));
+    if (virStrcpyStatic(ifr.ifr_name, ifname) == NULL)
+        return EINVAL;
+
+    /* To fill ifr.ifr_hdaddr.sa_family field */
+    if (ioctl(ctl->fd, SIOCGIFHWADDR, &ifr) != 0)
+        return errno;
+
+    memcpy(ifr.ifr_hwaddr.sa_data, macaddr, VIR_MAC_BUFLEN);
+
+    return ioctl(ctl->fd, SIOCSIFHWADDR, &ifr) == 0 ? 0 : errno;
+}
+
 /**
  * ifGetMtu
  * @ctl: bridge control pointer
@@ -430,6 +462,7 @@ brProbeVnetHdr(int tapfd)
  * @ctl: bridge control pointer
  * @bridge: the bridge name
  * @ifname: the interface name (or name template)
+ * @macaddr: desired MAC address (VIR_MAC_BUFLEN long)
  * @vnet_hdr: whether to try enabling IFF_VNET_HDR
  * @tapfd: file descriptor return value for the new tap device
  *
@@ -447,6 +480,7 @@ int
 brAddTap(brControl *ctl,
          const char *bridge,
          char **ifname,
+         const unsigned char *macaddr,
          int vnet_hdr,
          int *tapfd)
 {
@@ -478,6 +512,14 @@ brAddTap(brControl *ctl,
     if (ioctl(fd, TUNSETIFF, &ifr) < 0)
         goto error;
 
+    /* We need to set the interface MAC before adding it
+     * to the bridge, because the bridge assumes the lowest
+     * MAC of all enslaved interfaces & we don't want it
+     * seeing the kernel allocate random MAC for the TAP
+     * device before we set our static MAC.
+     */
+    if ((errno = ifSetInterfaceMac(ctl, ifr.ifr_name, macaddr)))
+        goto error;
     /* We need to set the interface MTU before adding it
      * to the bridge, because the bridge will have its
      * MTU adjusted automatically when we add the new interface.

src/util/bridge.h

@@ -68,7 +68,8 @@ enum {
 int     brAddTap                (brControl *ctl,
                                  const char *bridge,
                                  char **ifname,
-                                 int features,
+                                 const unsigned char *macaddr,
+                                 int vnet_hdr,
                                  int *tapfd);
 
 int     brDeleteTap             (brControl *ctl,