Re: Problem receiving UDP broadcast packets.

From	Grant Edwards <invalid@invalid.invalid>
Newsgroups	comp.lang.python
Subject	Re: Problem receiving UDP broadcast packets.
Date	2011-04-20 00:09 +0000
Organization	PANIX Public Access Internet and UNIX, NYC
Message-ID	<iol875$ah2$3@reader1.panix.com> (permalink)
References	<iol1tv$8dn$1@reader1.panix.com> <4dae172e$0$65870$e4fe514c@news.xs4all.nl> <iol5dm$rdb$2@reader1.panix.com> <4dae1d82$0$81483$e4fe514c@news.xs4all.nl> <mailman.592.1303257124.9059.python-list@python.org>

Show all headers | View raw

On 2011-04-19, Dan Stromberg <drsalists@gmail.com> wrote:
> On Tue, Apr 19, 2011 at 4:40 PM, Irmen de Jong <irmen.NOSPAM@xs4all.nl> wrote:
>> On 20-4-2011 1:21, Grant Edwards wrote:
>>>
>>> If I don't call bind(), then the broadcast packets go out the wrong
>>> interface on the sending machine.
>>>
>>
>> Fair enough.
>>
>> Next issue then: as far as I know, broadcast packets are by default not routed across
>> subnets by gateways. Which is a good thing.
>>
>> That would explain why your receiver doesn't see the packets unless its interface IP
>> address is in the same subnet as the sender's.
>>
>> However it doesn't explain (for me) why the tcpdump program running on that same
>> receiver machine still happily spits out received packets. Unless the routing between
>> the subnets is somehow done on the receiving machine itself? My knowledge of networks
>> and TCP/IP ends here I'm afraid.
>
> I'm guessing there are two different subnets on the same physical
> cable

Yes -- though technically they're on the same Ethernet segment rather
than physical cable, since there's an intervening Ethernet switch.

> - which is a little unusual, but not impossible.

OK, here's some background...

I'm trying to implement a device discovery/configuration protocol that
uses UDP broadcast packets to discover specific types of devices on
the local Ethernet segment.  The management program broadcasts a
discovery command to a particular UDP port.  All devices who get that
packet are expected to answer regardless of thier current IP address.

The management program can then send another broadcast packet to
configure the IP address of a device. After that, the management
program switches over to normal unicast TCP and UDP protocols (HTTP,
TFTP, etc.) to set up the device.

I had ignorantly assumed that an UDP broadcast sent to IP address
255.255.255.255 would be received by everybody who could hear it.

Apparently I'm going to have to use RAW packets and implement UDP
myself. :/

-- 
Grant Edwards               grant.b.edwards        Yow! BELA LUGOSI is my
                                  at               co-pilot ...
                              gmail.com

Back to comp.lang.python | Previous | Next — Previous in thread | Next in thread | Find similar

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

csiph-web