how to set multicast MAC ligitemately?

how to set multicast MAC ligitemately?

[Alexey V. Yurchenko]

Hi, everybody

In view of include/linux/etherdevice.h:

...
/**
 * is_valid_ether_addr - Determine if the given Ethernet address is valid
 * @addr: Pointer to a six-byte array containing the Ethernet address
 *
 * Check that the Ethernet address (MAC) is not 00:00:00:00:00:00, is not
 * a multicast address, and is not FF:FF:FF:FF:FF:FF.  The multicast
 * and FF:FF:... tests are combined into the single test "!(addr[0]&1)".
 *
 * Return true if the address is valid.
 */
static inline int is_valid_ether_addr( u8 *addr )
{
        const char zaddr[6] = {0,};
                                                                          
        return !(addr[0]&1) && memcmp( addr, zaddr, 6);
}
...

how it is possible to set up multicast MAC address legitemately if NIC driver resorts to use of this function. Why are multicast addresses forbidden and who is the right person to ask?

(I really do need to set a multicast MAC address, and if multicast MAC addresses exist, why are they forbidden?)

Regards,
Alex

Re: how to set multicast MAC ligitemately?

[Stephen Hemminger]

On Fri, 26 Sep 2003 21:53:26 +0300
"Alexey V. Yurchenko" <ayurchen@path.emicnetworks.com> wrote:

> 
> Hi, everybody
> 
> In view of include/linux/etherdevice.h:
> 
> ...
> /**
>  * is_valid_ether_addr - Determine if the given Ethernet address is valid
>  * @addr: Pointer to a six-byte array containing the Ethernet address
>  *
>  * Check that the Ethernet address (MAC) is not 00:00:00:00:00:00, is not
>  * a multicast address, and is not FF:FF:FF:FF:FF:FF.  The multicast
>  * and FF:FF:... tests are combined into the single test "!(addr[0]&1)".
>  *
>  * Return true if the address is valid.
>  */
> static inline int is_valid_ether_addr( u8 *addr )
> {
>         const char zaddr[6] = {0,};
>                                                                           
>         return !(addr[0]&1) && memcmp( addr, zaddr, 6);
> }
> ...
> 
> how it is possible to set up multicast MAC address legitemately if NIC driver resorts to use of this function. Why are multicast addresses forbidden and who is the right person to ask?
> 
> (I really do need to set a multicast MAC address, and if multicast MAC addresses exist, why are they forbidden?)
> 
> Regards,
> Alex

Not interface should have a multicast MAC address. A multicast address
should only exist as a destination address, never a source.

Re: how to set multicast MAC ligitemately?

[Alexey V. Yurchenko]

On Fri, 26 Sep 2003 12:03:45 -0700
Stephen Hemminger <shemminger@osdl.org> wrote:

> 
> Not interface should have a multicast MAC address. A multicast address
> should only exist as a destination address, never a source.

Well, that's in theory. In practice I need several computers connected to a switch to share a single interface and look to the rest of LAN as a single node. All those computers must receive all packets desitned to that interface. Using non-multicast MAC confuses many switches.

Any suggestions? (Except not using a switch ;))

Regards,
Alex

PS isn't this approach (forbidding certain addresses) a tad Microsoftish? Like saving users from themselves? If

RE: how to set multicast MAC ligitemately?

[Jeroen Massar]

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Alexey V. Yurchenko wrote:

> On Fri, 26 Sep 2003 12:03:45 -0700
> Stephen Hemminger <shemminger@osdl.org> wrote:
> 
> > 
> > Not interface should have a multicast MAC address. A 
> multicast address
> > should only exist as a destination address, never a source.
> 
> Well, that's in theory. In practice I need several computers 
> connected to a switch to share a single interface and look to 
> the rest of LAN as a single node. All those computers must 
> receive all packets desitned to that interface. Using 
> non-multicast MAC confuses many switches.
> 
> Any suggestions? (Except not using a switch ;))

Enable the ports to be mirror ports and then they will get
all the traffic targetted at the switch.
But you do have to have a real switch to support that feature ;)

Greets,
 Jeroen

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Re: how to set multicast MAC ligitemately?

[Pekka Savola]

On Sat, 27 Sep 2003, Alexey V. Yurchenko wrote:
> On Fri, 26 Sep 2003 12:03:45 -0700
> Stephen Hemminger <shemminger@osdl.org> wrote:
> 
> > 
> > Not interface should have a multicast MAC address. A multicast address
> > should only exist as a destination address, never a source.
> 
> Well, that's in theory. In practice I need several computers connected
> to a switch to share a single interface and look to the rest of LAN as a
> single node. All those computers must receive all packets desitned to
> that interface. Using non-multicast MAC confuses many switches.

This likely breaks with IGMP snooping switches, btw.

> Any suggestions? (Except not using a switch ;))
> 
> PS isn't this approach (forbidding certain addresses) a tad
> Microsoftish? Like saving users from themselves?

The rules are there to prevent users from shooting themselves in the foot, 
or the users thinking using multicast MAC address as source is a right way 
to solve a problem.

But Linux is Free Software and Open Source.  You can change these things 
if you really want.

-- 
Pekka Savola                 "You each name yourselves king, yet the
Netcore Oy                    kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings