Re: Where do packets sent to 255.255.255.255 go?

[Daniel Brockman <daniel@brockman.se>]

Hi Eric,

>> If I understand correctly, packets sent to the all-ones
>> broadcast address only go out through a single interface.
>
> I have some blur memories about this kind of issue, so my
> answer my be wrong on some points...

It turns out you are exactly right on all points. :-)

>> My question is threefold:
>>
>>  1. Why doesn't Linux send 255.255.255.255 packages
>>     through all network interfaces?  (I realize that
>>     this is probably not a Linux-specific question.)
>
> IIRC, Linux treats 255.255.255.255 as a normal IP address.
> Therefore it will follow the route for such an address and
> select the interface it is associated (probably eth0 if
> you are on a LAN).

That makes a lot of sense, and it appears to be the case.

>>  2. How does it choose which interface to send through?
>>     My first guess was that it just took the first
>>     Ethernet interface and used that for broadcasting.
>>     But playing around with nameif, this seems not to
>>     be the case.
>
> cf 1
>
>>  3. Can I set the default broadcast interface explicitly?
>>     For example, say I wanted broadcasts to go out over
>>     eth1 by default, instead of over eth0.  What if I
>>     wanted them to get sent through tap0?
>
> Again, I'm not sure, but I think that you can force the
> interface by adding a special route for IP 255.255.255.255
> and with mask 255.255.255.255 to the interface you want.

Yes, this works!  It's so simple --- I can't believe I
didn't try it before.  I did mess around with iptables,
trying to add some weird PREROUTEing DNAT that would
redirect the packets, but I didn't know what I was doing.

> Hope this help, even if my memory is a bit confused,

Yes, it did help.  Thanks a bunch, Eric!  Your memory seems
to be in great shape. :-)


Regards,

-- 
Daniel Brockman <daniel@brockman.se>