Some help with broadcasts through a Linux system

Nicolas Williams Nicolas.Williams at
Wed Feb 3 16:13:35 GMT 1999

But (assuming there's a 16.153.112/24 network) is a
*directed* broadcast.

Commercial routers usually make directed broadcast forwarding an

The forwarding of UDP broadcasts to the all-1s address (
is almost never desirable, except for some specific UDP services with
well-known ports allocated to them. To enable the forwarding of all-1s
broadcasts to such services it is usually best to use a UDP helper; UDP
helpers can be implemented in userland and, in fact, are best left to
user-level processes.

The forwarding of directed broadcasts, when turned on for ALL services
(possibly including ICMP echo requests and the like) then the
implementation of it is best left to the kernel; otherwise it's best
left to a user-land UDP helper.

Having said all of that, it is rarely desirable for a multi-homed host
to do any IP forwarding whatsoever, except when such a host is meant to
be used as a router.

My 2 cents.


On Wed, Feb 03, 1999 at 11:48:54PM +1100, Alan Cox wrote:
> > However, when I ping from, responds,
> > and I see nothing on the subnet.
> Correct
> > It seems that Linux feels that it should not have to forward the broadcast
> > datagrams.
> Correct. You don't forward broadcasts. Imagine what ping would
> do if you did
> Alan

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