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

[John Weekes <lists.xen@nuclearfallout.net>]

Ian,

This appears to have the side effect on dom0 of also causing both eth0 
and peth0 to have MAC fe:ff:ff:ff:ff:ff. I had to revert it to make 
networking work properly again for me.

-John

> # HG changeset patch
> # User Ian Campbell<ian.campbell@citrix.com>
> # 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@citrix.com>
>
> 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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