routing broadcast mac ethernet frames

routing broadcast mac ethernet frames

[Joris]

Dear netfilter team,

I'd like to make a linux setup (both 2.4 and 2.6) accept and route
packets on a lan with a broadcast mac.

Maybe it's a setting somewhere we haven't discovered yet, perhaps it
can be done with an iptables rule, or perhaps someone can whip up a
kernel patch?


This is why: certain embedded devices, such as some wifi accesspoints
and some DSL modems, seem to have a common bug/feature.
They keep an arp table, and any traffic from any host not in that
table is re-sent with a broadcast mac address (ff:ff:ff:ff:ff:ff).

As far as I know, linux will happily accept packets like that when
they're adressed to itself, but will not route them. This makes
communication impossible in certain setups with linux as a router.
A lot of people, especially wireless communities, are (knowingly or
more likely unknowningly) suffering from this problem.

Of course, those devices are buggy and should use ARP, but making
linux 'compatible' is a lot easier than getting multiple hardware
manufacturers to rewrite their firmware.




Many thanks in advance,
Joris & zombie


A highly similar problem description I found on the linux-net list:
http://marc.theaimsgroup.com/?l=linux-net&m=108263333208537&w=2
Read this too: http://marc.theaimsgroup.com/?l=linux-net&m=108264197315176&w=2

Re: routing broadcast mac ethernet frames

[Mike Ireton]

Joris -

Very astute of you. Yes, you're right on the money and is a recurring 
'compatibillity' issue between wireless and wired networks.

You can try using the ebtables kernel patch, which will allow you to do 
address translation on the ethernet destination address, so if you see a 
broadcast frame, you can transparently re-write it with your nic's 
ethernet address, thus your ip stack will receive and process the frame. 
There may be some undesired side effects of doing this, depending on 
your specfic configuration and goals, but since you got this far I'm 
sure you can probbly use tcpdump effectively and hash those out.


Mike


>This is why: certain embedded devices, such as some wifi accesspoints
>and some DSL modems, seem to have a common bug/feature.
>They keep an arp table, and any traffic from any host not in that
>table is re-sent with a broadcast mac address (ff:ff:ff:ff:ff:ff).
>
>As far as I know, linux will happily accept packets like that when
>they're adressed to itself, but will not route them. This makes
>communication impossible in certain setups with linux as a router.
>A lot of people, especially wireless communities, are (knowingly or
>more likely unknowningly) suffering from this problem.
>
>Of course, those devices are buggy and should use ARP, but making
>linux 'compatible' is a lot easier than getting multiple hardware
>manufacturers to rewrite their firmware.
>  
>