netdev - Re: [Xen-devel] [RFC v2 3/4] xen-netback: use a random MAC address

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Message-ID: <5301E496.40802@citrix.com>
Date:	Mon, 17 Feb 2014 10:29:42 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	"Luis R. Rodriguez" <mcgrof@...not-panic.com>
CC:	<netdev@...r.kernel.org>, Wei Liu <wei.liu2@...rix.com>,
	Ian Campbell <ian.campbell@...rix.com>, <kvm@...r.kernel.org>,
	"Luis R. Rodriguez" <mcgrof@...e.com>,
	<linux-kernel@...r.kernel.org>,
	Paul Durrant <Paul.Durrant@...rix.com>,
	<xen-devel@...ts.xenproject.org>
Subject: Re: [Xen-devel] [RFC v2 3/4] xen-netback: use a random MAC address

On 15/02/14 02:59, Luis R. Rodriguez wrote:
> From: "Luis R. Rodriguez" <mcgrof@...e.com>
> 
> The purpose of using a static MAC address of FE:FF:FF:FF:FF:FF
> was to prevent our backend interfaces from being used by the
> bridge and nominating our interface as a root bridge. This was
> possible given that the bridge code will use the lowest MAC
> address for a port once a new interface gets added to the bridge.
> The bridge code has a generic feature now to allow interfaces
> to opt out from root bridge nominations, use that instead.
[...]
> --- a/drivers/net/xen-netback/interface.c
> +++ b/drivers/net/xen-netback/interface.c
> @@ -42,6 +42,8 @@
>  #define XENVIF_QUEUE_LENGTH 32
>  #define XENVIF_NAPI_WEIGHT  64
>  
> +static const u8 xen_oui[3] = { 0x00, 0x16, 0x3e };

You shouldn't use a vendor prefix with a random MAC address.  You should
set the locally administered bit and clear the multicast/unicast bit and
randomize the remaining 46 bits.

(If existing VIF scripts are doing something similar, they also need to
be fixed.)

David
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