Re: [Qemu-devel] [PATCH] qemu-bridge-helper: force usage of a very high MAC address for the bridge

[Corey Bryant <coreyb@linux.vnet.ibm.com>]

On 03/22/2013 12:57 PM, Paolo Bonzini wrote:
> Linux uses the lowest enslaved MAC address as the MAC address of
> the bridge.  Set MAC address to a high value so that it does not
> affect the MAC address of the bridge.
>
> Changing the MAC address of the bridge could cause a few seconds
> of network downtime.
>
> Cc: qemu-stable@nongnu.org
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>   qemu-bridge-helper.c | 18 ++++++++++++++++++
>   1 file changed, 18 insertions(+)
>
> diff --git a/qemu-bridge-helper.c b/qemu-bridge-helper.c
> index 287bfd5..6a0974e 100644
> --- a/qemu-bridge-helper.c
> +++ b/qemu-bridge-helper.c
> @@ -367,6 +367,24 @@ int main(int argc, char **argv)
>           goto cleanup;
>       }
>
> +    /* Linux uses the lowest enslaved MAC address as the MAC address of
> +     * the bridge.  Set MAC address to a high value so that it doesn't
> +     * affect the MAC address of the bridge.
> +     */
> +    if (ioctl(ctlfd, SIOCGIFHWADDR, &ifr) < 0) {
> +        fprintf(stderr, "failed to get MAC address of device `%s': %s\n",
> +                iface, strerror(errno));
> +        ret = EXIT_FAILURE;
> +        goto cleanup;
> +    }
> +    ifr.ifr_hwaddr.sa_data[0] = 0xFE;
> +    if (ioctl(ctlfd, SIOCSIFHWADDR, &ifr) < 0) {
> +        fprintf(stderr, "failed to set MAC address of device `%s': %s\n",
> +                iface, strerror(errno));
> +        ret = EXIT_FAILURE;
> +        goto cleanup;
> +    }
> +
>       /* add the interface to the bridge */
>       prep_ifreq(&ifr, bridge);
>       ifindex = if_nametoindex(iface);
>

Is it desirable to change a mac address under the covers?  I know you 
mentioned that libvirt does something like this already.

Also it seems like this might be better if it was optional. 
qemu-bridge-helper can take command line options like this:

-net tap,helper="/usr/local/libexec/qemu-bridge-helper --br=br0"

Perhaps adding a --macaddr option is a better approach?

-- 
Regards,
Corey Bryant