Network programming question

[Andrew Falanga]

On Thu, Mar 13, 2008 at 11:57 PM, Patrick Mahan <mahan at mahan.org> wrote:
>
>
>
>  inet_pton() clobbered the fields you pointed out.  In fact the sin_family
>  field was being set to 0x01 which caused your initial EADDRNOTSUPPORT error
>  you were seeing.  You quick change fixed that problem.  However, (depending
>  on how sockaddr_in structure is actually allocated) the sin_addr field was
>  0.0.0.0.  This is actually an accepted form of the broadcast address for UDP
>  packets.  I forget exactly who the culprit was (Sun comes to mind) but there
>  was a need to allow broadcasts to 0.0.0.0 (which is also know as INADDR_ANY).
>  So, therefore, sendto() succeeded, just not in the way you expected.  Looking
>  at in_pcbconnect_setup() in the kernel shows that actually the packet is sent
>  to the local primary interface address.
>
>  Let's look at what really happen to that packet -
>
>    "192.168.0.1" after being mangled by inet_pton() gives
>    the field sin_addr.s_addr of 0x0100A8C0.  This should make
>    your sockaddr_in structure look like -
>
>      sa.sin_len = 0x01
>      sa.sin_family = 0x00
>      sa.sin_port = 0xA8C0 (which is port 49320)
>      sa.sin_addr.s_addr = 0x00000000
>
>  So the sendto() call was sending a packet to your local interface for port 49320.
>  And since UDP is a connectionless protocol, you don't have a way (unless it is
>  builtin to your application protocol) to determine an error.  For example, TFTP
>  sends back notification for every dgram received.
>
>  I hope this helps with your understanding.  I highly recommend if you are going
>  to do more network programming that you obtain at least some books on the subject.
>
>
>  Patrick
>

Thanks much for this explanation.  Books would be good, yes.  I guys
got to learn somehow.  Thanks for taking the time to explain it.
That's interesting that a broadcast may be sent to 0.0.0.0.  I knew
that 0.0.0.0 is equal to INADDR_ANY.  However, I thought it wasn't
possible to send to that address, only to bind to it locally for a
server application.

Andy

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