BSSID and MAC address

Tobias Heer heer at cs.rwth-aachen.de
Wed Sep 30 17:47:40 EDT 2009


Hi.

I hope this is the right place to ask this question. I tried to  
structure my question a bit:

My goal:
I am trying to set up a wireless "network" consisting of a number of  
access points+routers (one device) with the same ssid at the same  
frequency without encryption. A client should be able to roam between  
these two networks/APs. The two networks are not connected on layer 2.  
There should be a default service running at each access point  
(reachable via the APs/routers IP addresses in both networks e.g.  
192.168.1.1 - wlan0). When roaming between the networks the host  
should be able to constantly reach that service (e.g., via some other  
stateless protocol - let's assume it is ping from the client to  
192.168.1.1).

My problem:

a) If set the same bssid (same mac for both routers on wlan0) for both  
APs, both will receive the ping messages and answer accordingly. This  
is bad since I want the client to explicitly roam between the APs.

b) If I use different bssids for the wlan0 interfaces of both routers,  
the roaming "works" (i.e., I can see the ping messages via tcpdump on  
either the first or the second AP and the client switches the AP if  
the signal gets bad) BUT the second router's interface wlan0 has the  
wrong mac address and drops the packet (packet was addressed to the  
mac address of the first AP not the second). An arp lookup fixes the  
situation eventually but that takes too much time.

My question:
Is there a way to set the bssid independently of the mac address of  
the router's wlan0 interface? Is there some other way to decouple the  
destination mac address of the ping packets from the bssid of the AP?

My hardware:
Two linux routers (ubuntu server) with atheros (ath5k) wifi cards and  
hostapd 0.6.9-3.


I have been searching in the archives of the last 2 years but did not  
find anything that solved my problem, forgive me if I missed  
something. I am rather new to the topic - I hope I am not asking for  
something obvious or something impossible.

Thanks in advance,

Tobias




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