[Xen-devel] [PATCH] tools/hotplug/Linux: Ensure tap devices receive a dummy MAC address

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

# HG changeset patch
# User Ian Campbell <ian.campbell@xxxxxxxxxx>
# Date 1291741813 0
# Node ID ca87e5178d11a91092a5c0f8e192e6dcc41d9d45
# Parent  5e4a8db26d5addf1cb73d6d2310101e67d96c7ba
tools/hotplug/Linux: Ensure tap devices receive a dummy MAC address.

If a tap device is not given an explicit MAC address it will generate
one randomly.

The behaviour of the Linux bridge is to pickup the lowest MAC address
of any port for use in for ARP, STP etc. If the tap device's randomly
generated MAC address happens to be the lowest then this can cause all
manner of strange networking glitches in both domain 0 and guests when
the bridge suddenly takes over from the previously used MAC address.

We choose FE:FF:FF:FF:FF:FF as it the numerically largest
non-broadcast address. This ensures that the physical NIC device's
port will have the lowest MAC address and therefore be the one picked
up by the bridge.

vif devices already have a MAC address of FE:FF:FF:FF:FF:FF set by
netback already but there is no harm in forcing it a second time in
the hotplug script.

tap devices are added by the "add" event and therefore we should call
setup_bridge_port then as well as for "online" which is caused by vif
devices.

Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>

diff -r 5e4a8db26d5a -r ca87e5178d11 tools/hotplug/Linux/vif-bridge
--- a/tools/hotplug/Linux/vif-bridge    Mon Dec 06 15:23:49 2010 +0000
+++ b/tools/hotplug/Linux/vif-bridge    Tue Dec 07 17:10:13 2010 +0000
@@ -91,6 +91,7 @@ case "$command" in
         ;;
 
     add)
+        setup_bridge_port "$dev"
         add_to_bridge "$bridge" "$dev"
         ;;
 esac
diff -r 5e4a8db26d5a -r ca87e5178d11 tools/hotplug/Linux/xen-network-common.sh
--- a/tools/hotplug/Linux/xen-network-common.sh Mon Dec 06 15:23:49 2010 +0000
+++ b/tools/hotplug/Linux/xen-network-common.sh Tue Dec 07 17:10:13 2010 +0000
@@ -86,6 +86,12 @@ setup_bridge_port() {
     # take interface down ...
     ip link set ${dev} down
 
+    # Initialise a dummy MAC address. We choose the numerically
+    # largest non-broadcast address to prevent the address getting
+    # stolen by an Ethernet bridge for STP purposes.
+    # (FE:FF:FF:FF:FF:FF)
+    ip link set ${dev} address fe:ff:ff:ff:ff:ff
+
     # ... and configure it
     ip addr flush ${dev}
 }

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