Problem receiving UDP broadcast packets.

Grant Edwards invalid at invalid.invalid
Wed Apr 20 17:50:17 CEST 2011

On 2011-04-20, Grant Edwards <invalid at invalid.invalid> wrote:
> On 2011-04-20, Heiko Wundram <modelnine at> wrote:
>> Am 20.04.2011 01:54, schrieb Grant Edwards:
>>> I guess the problem is that I expected to receive a packet on an
>>> interface anytime a packet was received with a destination IP address
>>> that matched that of the the interface.  Apprently there's some
>>> filtering in the network stack based on the _source_ address as well
>>> (that seems very counter-intuitive to me).
>> Just to pitch in here (because nobody's mentioned it yet AFAICT): yes,
>> there's a filtering done (at least under Linux, and I'd guess something
>> similar on xBSD too) to packets based on the source address coming in on
>> an interface, and it's called the reverse path filter and is on by
>> default (the tunable on Linux is /proc/sys/net/ipv4/conf/*/rp_filter).
> Brilliant!  While I had determined that such filtering took place, I'd
> been unable to figure out if it was configurable.


Turning off reverse-path filtering solved my original problem (not
receiving broadcasts sent to because the source IP
wasn't reachable via the receiving interface).

It doesn't help with the "new" requirement of receiving subnet
broadcasts that don't match the interface on which they're received,
since it's destination IP filtering that's dropping those.

Apparently that requirement is due to the fact that on some OSes (some
flavors of BSD and Windows) applications can't broadcast to, they can only do subnet broadcasts.

Grant Edwards               grant.b.edwards        Yow! I wish I was a
                                  at               sex-starved manicurist
                                found dead in the Bronx!!

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