Re: [Qemu-devel] [PATCH v2] net/net: Change the default mac address of nic

[Stefan Weil <sw@weilnetz.de>]

Am 15.10.2013 07:57, schrieb mike:
> On 10/15/2013 01:07 PM, Stefan Weil wrote:
>> Am 15.10.2013 06:17, schrieb Mike Qiu:
>>> Changelog to v1:
>>>     Find remainder of macaddr->a[5] by modulo 256,
>>>     otherwise it may be overflow by add index++.
>>>
>>> The default mac address is 52:54:00:12:34:56 + index, this will
>>> cause problem that when we boot up more than one guest with all
>>> mac addresses unset by default, assume that each guest has one
>>> nic. In this situation, all the guest's nic has the same mac address.
>>>
>>> This patch is to solve this bug.
>>>
>>> Signed-off-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>
>>> ---
>>>   net/net.c | 9 ++++++---
>>>   1 file changed, 6 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/net/net.c b/net/net.c
>>> index c330c9a..9e72764 100644
>>> --- a/net/net.c
>>> +++ b/net/net.c
>>> @@ -21,6 +21,8 @@
>>>    * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
>>> DEALINGS IN
>>>    * THE SOFTWARE.
>>>    */
>>> +#include <time.h>
>>> +
>>>   #include "config-host.h"
>>>     #include "net/net.h"
>>> @@ -147,12 +149,13 @@ void qemu_macaddr_default_if_unset(MACAddr
>>> *macaddr)
>>>         if (memcmp(macaddr, &zero, sizeof(zero)) != 0)
>>>           return;
>>> +    srand((unsigned)time(NULL));
>>>       macaddr->a[0] = 0x52;
>>>       macaddr->a[1] = 0x54;
>>>       macaddr->a[2] = 0x00;
>>> -    macaddr->a[3] = 0x12;
>>> -    macaddr->a[4] = 0x34;
>>> -    macaddr->a[5] = 0x56 + index++;
>>> +    macaddr->a[3] = rand() % 256;
>>> +    macaddr->a[4] = rand() % 256;
>>> +    macaddr->a[5] = (rand() % 256 + index++) % 256;
>>>   }
>>>     /**
>> There is no overflow which must be handled because a[5] is an uint8_t
>> value, so the assignment automatically limits the range to 0...255.
> OK, you are right, but I think we'd better to ensure this,
> even though a[5] is an uint8_t.
>> Is it reasonable to get a random mac address in your guest? I don't
>> think so. It would no longer be possible to connect to a guest using
>> ssh, restart that guest and connect again with ssh.
> Why not? I have do the experiment, after reboot, the mac is not changed.
> and the ip address always the same.
>
> And can be login to the guest after reboot.

"restart" means terminate QEMU and start it again.