Programming interface MAC filter without enabling PROMISC on an interface from user space.

Tom Judge tom at
Mon Jan 14 10:39:31 PST 2008

Bruce M. Simpson wrote:
> Tom Judge wrote:
>> Thanks for the response.  I have a quick grep of the src tree to find 
>> an example of this being used and only found the following from 
>> wpa_supplicant and I have a few questions:
>>     * I am presuming that this will do what I want, am I correct?
> Yes, it will attempt to add the given link layer multicast group to the 
> ifnet's underlying device driver.
>>     * If I was only ever to add the address to an interface an never 
>> delete it would this cause any problems?  I.e. when lldpd ends, or is 
>> restarted and tries to add the address again?
> SIOCADDMULTI is very low level, no resource tracking is performed; I 
> changed its semantics to only allow one userland opener so that 
> in-kernel refcounting would work, as there is no per-process or 
> per-client resource tracking -- so it's a really good idea to clean up 
> after it.
>>     * Alternatively is there a way to query the filter to ask what 
>> addresses it is currently programmed for?
> Nope, there is no userland or kernel API for that unless you hack up the 
> driver.

Ok, so if I can safely assume that the process sending/receiving the 
LLDP frames should always be running would it be safe to use a helper 
program to add the mac on system startup so it is always registered on 
particular interfaces for the uptime of the system rather than having 
the daemon add/remove the address on startup shutdown?  If not what 
problems would this create?

Personally I can't see why this approach would be a problem,  but I am 
not a expert.  The address is defined in IEEE Std 802.1D-2004 as to not 
be forwarded by bridges (which I interpret as it being link local in a 
sense as switches/bridges are not allowed to forward the frame), so I 
can't see it being a problem registered on multiple interfaces.

On a side note does anyone know if if_bridge will respect the standard 
and not forward this frame on to other interfaces?

Thanks again


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