Re: UDP packets not delivered from USB modem

FROM : Jamie Curmi
DATE : Fri Apr 29 22:25:25 2005

On 28/04/2005, at 6:18 PM, Daniel Eggert wrote:

> Hi Jamie,
>
> On 11 Apr 2005, at 01:39, Jamie Curmi wrote:
>

>> I have a USB Modem (actually a wireless broadband CDMA modem).  I'm
>> on a Mac running OS X 10.3.8.  The modem has windows software, but
>> nothing for the Mac, so I'm attempting to develop stuff to work with
>> it on the Mac.

>
> I'm not altogether sure, what your setup is and how your modem
> connects to you mac. Maybe you could clarify which part is wireless
> and wether the modem is on your side or the other side of the wireless
> connection.
>

>> I can start a PPP connection with the modem, and use the internet
>> fine.  The problem I have is communicating with the modem while in a
>> PPP session.
>>
>> The manufacturer sent me details on how to do this.  Basically, you
>> can send commands to the modem via UDP packets sent to 1.1.1.1:33333.
>>  And the modem responds sending UDP packets to 127.0.0.1:33333.
>>
>> Sending to the modem works fine.  Using tcpdump, I can actually see
>> the packets delivered, and view the return packets with -w (they are
>> definitely responses, and the modem can be controlled).  tcpdump
>> gives me lines such as:
>>
>> 09:52:32.705221 IP 1.1.1.1.65530 > 127.0.0.1.33333: UDP, length: 13

>
> If you see this, you're definitely receiving the UDP packets.
>
> Could you run 'ifconfig -a'?
>
> Maybe ipfw is blocking the packets at some point...? Try to check the
> output of 'ipfw list'.


Thanks for the reply Daniel (and many others).

A few people have suggested that the lower levels of the networking
stack reject all incoming packets addressed to 127/8 as being bogus. 
This seems to be the case, which at least explains why I can't see the
packets.  Unfortunately the manufacturer provides no way to change the
receive address (next revision apparently), so the only solution I can
think of now is to use pcap (I've asked if they have a USB
specification of the interface, but that doesn't look promising -
Windows apparently doesn't drop the packets).

The only problem with using the pcap library seems to be that you need
to have the executable run as root to use it.  Unless there is
something I'm missing (anyone used pcap before and could offer any
suggestions)?

Thanks once again.

Jamie