problem receiving udp broadcasts

2 messages, 1 author, 2011-07-13 · open the first message on its own page

problem receiving udp broadcasts

From: Pieter <hidden>
Date: 2011-07-13 08:26:35 

Hi All

Im running Linux 2.6.32.14 on an embedded  MPC4858.
One of the Application (CLI) must reciece a UDP broadcast packet.

The problem is that I only receive packets broadcast from within the=20
same subnet  (192.168.0.x)  even though the broadcast app sends the=20
broadcast to  255.255.255.255.

when i run tcpdump on the embedded machine it shows both broadcasts from =

the same subnet well as different subsets: as shown below:

     SDH-0A000221> /tmp/tcpdump port 23 or port 33334
     tcpdump: verbose output suppressed, use -v or -vv for full protocol =

decode
     listening on eth0, link-type EN10MB (Ethernet), capture size 65535=20
bytes
(1)    07:48:35.787826 IP 172.16.63.184.cli > 255.255.255.255.cli: UDP,=20
length 58
(2)   07:48:43.832539 IP 192.168.0.1.cli > 255.255.255.255.cli: UDP,=20
length 60
(3)    07:48:43.840627 IP 192.168.0.254.cli > 192.168.0.1.cli: UDP,=20
length 112

The embedded machine is configured with:
ip 192.168.0.254
mask 255.255.0.0

(1) is a broadcast from a different subnet
(2) is broadcast from within same subnet
(3) the imbeded applications reply to the broadcast from same subnet


The service has been delared in  "etc/xinetd.d/cli" as follows:
service cli
{
     socket_type        =3D dgram
     protocol        =3D udp
     port            =3D 23
     wait            =3D yes
     user            =3D root
     server            =3D /usr/vastech/in.cli
     disable            =3D no
     cps                =3D 10 5
     flags            =3D IPv4
}

and the port assigned in "/etc/xinetd.d/services" as follows (port 23=20
for back comparability with older products)

     #telnet        23/udp
     cli                23/udp
     cli                33334/udp

Is there something basic that I have overlooked ? should the interface=20
be set and left in
promiscuous mode to do what I require?

thanks pieter

--=20
VASTech PTY (LTD)
021 880 9807

Re: Soved problem receiving udp broadcasts

From: Pieter <hidden>
Date: 2011-07-13 10:09:39 

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    <font face="Arial"></font>Hi<br>
    <br>
    The problem was that "reverse-path filtering" was enabled. after
    disabling reverse-path filtering I could receive broadcasts as
    expected.<br>
    <br>
    As a side note - interface does not need to be in promiscuous mode<br>
    <br>
    cheers pieter<br>
    <br>
    On 13/07/2011 10:03, Pieter wrote:
    <blockquote cite="mid:4E1D513F.9030405@vastech.co.za" type="cite">Hi
      All
      <br>
      <br>
      Im running Linux 2.6.32.14 on an embedded&nbsp; MPC4858.
      <br>
      One of the Application (CLI) must reciece a UDP broadcast packet.
      <br>
      <br>
      The problem is that I only receive packets broadcast from within
      the same subnet&nbsp; (192.168.0.x)&nbsp; even though the broadcast app
      sends the broadcast to&nbsp; 255.255.255.255.
      <br>
      <br>
      when i run tcpdump on the embedded machine it shows both
      broadcasts from the same subnet well as different subsets: as
      shown below:
      <br>
      <br>
      &nbsp;&nbsp;&nbsp; SDH-0A000221&gt; /tmp/tcpdump port 23 or port 33334
      <br>
      &nbsp;&nbsp;&nbsp; tcpdump: verbose output suppressed, use -v or -vv for full
      protocol decode
      <br>
      &nbsp;&nbsp;&nbsp; listening on eth0, link-type EN10MB (Ethernet), capture size
      65535 bytes
      <br>
      (1)&nbsp;&nbsp;&nbsp; 07:48:35.787826 IP 172.16.63.184.cli &gt;
      255.255.255.255.cli: UDP, length 58
      <br>
      (2)&nbsp;&nbsp; 07:48:43.832539 IP 192.168.0.1.cli &gt; 255.255.255.255.cli:
      UDP, length 60
      <br>
      (3)&nbsp;&nbsp;&nbsp; 07:48:43.840627 IP 192.168.0.254.cli &gt; 192.168.0.1.cli:
      UDP, length 112
      <br>
      <br>
      The embedded machine is configured with:
      <br>
      ip 192.168.0.254
      <br>
      mask 255.255.0.0
      <br>
      <br>
      (1) is a broadcast from a different subnet
      <br>
      (2) is broadcast from within same subnet
      <br>
      (3) the imbeded applications reply to the broadcast from same
      subnet
      <br>
      <br>
      <br>
      The service has been delared in&nbsp; "etc/xinetd.d/cli" as follows:
      <br>
      service cli
      <br>
      {
      <br>
      &nbsp;&nbsp;&nbsp; socket_type&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; = dgram
      <br>
      &nbsp;&nbsp;&nbsp; protocol&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; = udp
      <br>
      &nbsp;&nbsp;&nbsp; port&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; = 23
      <br>
      &nbsp;&nbsp;&nbsp; wait&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; = yes
      <br>
      &nbsp;&nbsp;&nbsp; user&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; = root
      <br>
      &nbsp;&nbsp;&nbsp; server&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; = /usr/vastech/in.cli
      <br>
      &nbsp;&nbsp;&nbsp; disable&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; = no
      <br>
      &nbsp;&nbsp;&nbsp; cps&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; = 10 5
      <br>
      &nbsp;&nbsp;&nbsp; flags&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; = IPv4
      <br>
      }
      <br>
      <br>
      and the port assigned in "/etc/xinetd.d/services" as follows (port
      23 for back comparability with older products)
      <br>
      <br>
      &nbsp;&nbsp;&nbsp; #telnet&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 23/udp
      <br>
      &nbsp;&nbsp;&nbsp; cli&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 23/udp
      <br>
      &nbsp;&nbsp;&nbsp; cli&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 33334/udp
      <br>
      <br>
      Is there something basic that I have overlooked ? should the
      interface be set and left in
      <br>
      promiscuous mode to do what I require?
      <br>
      <br>
      thanks pieter
      <br>
      <br>
    </blockquote>
    <br>
    <pre class="moz-signature" cols="72">-- 
VASTech PTY (LTD)
021 880 9807</pre>
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