Re: [Bridge] Hardware requirements for bridging wired+wireless together

["John W. Linville" <linville@tuxdriver.com>]

On Wed, May 19, 2010 at 10:15:35AM -0700, Umar Qureshey wrote:

> What about bridging in Ad-Hoc mode?  Would that technically work?

No.
 
> I guess what I am trying to figure out is why bridging would work in WDS mode?  What is it about that mode that allows bridging to work?

It has to do with the MAC-layer addressing on wireless LANs.  Wireless
frames can use 2, 3, or 4 MAC addresses to identify the transmitter,
receiver, sender, and destination.  For most frames and most modes,
3 MAC addresses are used.  The FromDS and ToDS bits in the header
are used to allow one of the MAC address fields to specify either
the transmitter and sender or the destination and receiver.  This is
sufficient for non-bridged cases since the wireless station is either
an endpoint of the communication or possibly a router (and therefore
a Layer-2 endpoint).

WDS (or 4 address) mode removes this limitation by using 4 MAC
addresses to identify all 4 roles independently.  So, the wireless
station is able to forward frames received off the air to the
appropriate destination with the correct sender information intact.

mac80211-based devices can have interfaces created with support for
4 address mode using the iw command.  For this to work, your AP has
to be willing to accept and forward those frames appropriately --
some do, others don't.  This is only supported for "managed" mode
interfaces AFAIK.

> If one were to try to modify the kernel code to allow MAC-level NAT, which area of the kernel code would one look at?

netfilter -- I thought there was already some ebtables code to
do this...?

John
-- 
John W. Linville		Someday the world will need a hero, and you
linville@tuxdriver.com			might be all we have.  Be ready.