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Re: Problem receiving UDP broadcast packets.

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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).

The idea behind the reverse path filter is that your machine won't
accept packets coming in over an interface when a return packet (i.e.,
the presumed response) won't be routed over the same interface, and from
what I gather, this is what makes the TCP/IP stack drop the packets
because your machine will not route packets to 192.168.x.x over the same
interface it sees the packet coming in. This is a _security_ feature,
because it makes address spoofing harder.

If you need to see the packets regardless, either use a promiscuous mode
sniffer (i.e., tcpdump, but that's relatively easy to mirror in Python
using SOCK_RAW, capturing packets at the ethernet level), or add a route
on your system for the 192.168.x.x network on the same interface.

HTH!

-- 
--- Heiko.

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Thread

Problem receiving UDP broadcast packets. Grant Edwards <invalid@invalid.invalid> - 2011-04-19 22:21 +0000
  Re: Problem receiving UDP broadcast packets. Irmen de Jong <irmen.NOSPAM@xs4all.nl> - 2011-04-20 01:13 +0200
    Re: Problem receiving UDP broadcast packets. Grant Edwards <invalid@invalid.invalid> - 2011-04-19 23:21 +0000
      Re: Problem receiving UDP broadcast packets. Irmen de Jong <irmen.NOSPAM@xs4all.nl> - 2011-04-20 01:40 +0200
        Re: Problem receiving UDP broadcast packets. Dan Stromberg <drsalists@gmail.com> - 2011-04-19 16:52 -0700
          Re: Problem receiving UDP broadcast packets. Grant Edwards <invalid@invalid.invalid> - 2011-04-20 00:09 +0000
            Re: Problem receiving UDP broadcast packets. Chris Angelico <rosuav@gmail.com> - 2011-04-20 10:53 +1000
              Re: Problem receiving UDP broadcast packets. Grant Edwards <invalid@invalid.invalid> - 2011-04-20 01:15 +0000
                Re: Problem receiving UDP broadcast packets. Chris Angelico <rosuav@gmail.com> - 2011-04-20 11:24 +1000
                Re: Problem receiving UDP broadcast packets. Grant Edwards <invalid@invalid.invalid> - 2011-04-20 03:05 +0000
                Re: Problem receiving UDP broadcast packets. Dan Stromberg <drsalists@gmail.com> - 2011-04-19 20:12 -0700
                Re: Problem receiving UDP broadcast packets. Grant Edwards <invalid@invalid.invalid> - 2011-04-20 14:31 +0000
                Re: Problem receiving UDP broadcast packets. Grant Edwards <invalid@invalid.invalid> - 2011-04-20 15:42 +0000
                Re: Problem receiving UDP broadcast packets. Dan Stromberg <drsalists@gmail.com> - 2011-04-19 20:16 -0700
                Re: Problem receiving UDP broadcast packets. Grant Edwards <invalid@invalid.invalid> - 2011-04-20 14:21 +0000
                Re: Problem receiving UDP broadcast packets. Dan Stromberg <drsalists@gmail.com> - 2011-04-20 18:35 -0700
                Re: Problem receiving UDP broadcast packets. Grant Edwards <invalid@invalid.invalid> - 2011-04-21 14:22 +0000
                Re: Problem receiving UDP broadcast packets. Heiko Wundram <modelnine@modelnine.org> - 2011-04-21 08:22 +0200
            Re: Problem receiving UDP broadcast packets. Roy Smith <roy@panix.com> - 2011-04-19 23:35 -0400
              Re: Problem receiving UDP broadcast packets. Grant Edwards <invalid@invalid.invalid> - 2011-04-20 14:23 +0000
            Re: Problem receiving UDP broadcast packets. Sherm Pendley <sherm.pendley@gmail.com> - 2011-04-20 06:07 -0400
              Re: Problem receiving UDP broadcast packets. Adam Tauno Williams <awilliam@whitemice.org> - 2011-04-20 06:20 -0400
                Re: Problem receiving UDP broadcast packets. Grant Edwards <invalid@invalid.invalid> - 2011-04-20 15:37 +0000
        Re: Problem receiving UDP broadcast packets. Grant Edwards <invalid@invalid.invalid> - 2011-04-19 23:54 +0000
          Re: Problem receiving UDP broadcast packets. Heiko Wundram <modelnine@modelnine.org> - 2011-04-20 08:45 +0200
            Re: Problem receiving UDP broadcast packets. Grant Edwards <invalid@invalid.invalid> - 2011-04-20 14:30 +0000
              Re: Problem receiving UDP broadcast packets. Heiko Wundram <modelnine@modelnine.org> - 2011-04-20 16:53 +0200
                Re: Problem receiving UDP broadcast packets. Grant Edwards <invalid@invalid.invalid> - 2011-04-20 15:24 +0000
                Re: Problem receiving UDP broadcast packets. Grant Edwards <invalid@invalid.invalid> - 2011-04-20 15:32 +0000
              Re: Problem receiving UDP broadcast packets. Grant Edwards <invalid@invalid.invalid> - 2011-04-20 15:50 +0000
  Re: Problem receiving UDP broadcast packets. Dan Stromberg <drsalists@gmail.com> - 2011-04-19 16:40 -0700
    Re: Problem receiving UDP broadcast packets. Grant Edwards <invalid@invalid.invalid> - 2011-04-20 00:00 +0000
      Re: Problem receiving UDP broadcast packets. Chris Angelico <rosuav@gmail.com> - 2011-04-20 10:32 +1000
  Re: Problem receiving UDP broadcast packets. Thomas Heller <theller@ctypes.org> - 2011-04-20 12:18 +0200
    Re: Problem receiving UDP broadcast packets. Grant Edwards <invalid@invalid.invalid> - 2011-04-20 14:35 +0000
  Re: Problem receiving UDP broadcast packets. Grant Edwards <invalid@invalid.invalid> - 2011-04-20 22:25 +0000

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