Re: Linux Broadcast packet routing

On 23.2.10 5:24 , Arimus wrote:
Hi,

Currently encountering a problem with handling broadcast (subnet
broadcast rather than global broadcast):

Setup:
3 terminals each connected to a linux based router. Each linux based
router is connected to a radio net.

The radio net exists on its own subnet address (172.17.32.0/24), and
the terminals are on their own subnets (e.g 10.16.1.1 for the first
router's terminal interface, 10.16.1.2 for the terminal, x.x.2.1 for
the second router and so on).

We've got some messaging software which can work out that, for
instance, when sending from Terminal 1 to both terminals 2 and 3
they're on a common radio subnet and so can send to 172.17.32.255
instead of having to send two individual messages.

The translation from 172.17.32.255 to terminal addresses on the
receiving side is fine - however traffic from terminal 1 is not being
routed out when sent as broadcast from router 1.

Terminal 1> eth 10.16.x.x/16> router> slip 172.16.32.x/24> radio

Terminal 2> eth 10.16.y.y/16> router> slip 172.16.32.x/24> radio

Terminal 3> eth 10.16.y.y/16> router> slip 172.16.32.x/24> radio

Its possible to broadcast ping 172.16.32.255 from any router to get to
the other routers (or with the nat rule in place the terminals). The
problem occurs when Terminal 1 tries sending to 172.16.32.255.

On router 1 we see the packet come in on eth0 and then gets to the
INPUT queue rather than being forwarded out to sl0 (172.16.32.1/24
which has bcast address of 172.16.32.255).

Reading various articles this in a way is expected behaviour as it
appears linux does not route broadcast traffic between different
subnets. Unfortunately for our purposes we need this ability.


Your problem is that the routers are not allowed to forward/NAT
the broadcasts of the radio subnet (172.16.32.255).

You need to set the routers bridging instead of routing so that
the final peer hosts are in the intended subnet 172.16.32.0/24.
Of course, you have to change the addresses of the peer hosts
to match.

--

Tauno Voipio
tauno voipio (at) iki fi


.

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