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SubjectRe: Where do packets sent to 255.255.255.255 go?
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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>

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