Unable to talk to tap(4)

[Bob McConnell]

From: Wojciech Puchar
>>
>> The basic setup sequence is:
>>
>>  ifconfig tap0 create
>>  ifconfig tap0 inet 10.3.4.254/24
>>  route -v add 10.3.4.0/24 10.3.4.254
>
> ifconfig tap0 up
>
> ?
>

'ifconfig' already showed the interface flag UP. Adding this command
to the sequence has no effect on it. I also tried 'ifconfig tap0
promisc'.

Is EFAULT really a memory access exception?

>>
>> At this point, I can ping that address and my application can open
>> either /dev/net/tap0 or /dev/tap0. But when I try to read() from
those
>> devices, I have problems.
>>
>> /dev/net/tap0 always returns with errno = 19 (ENODEV - Operation not
>> supported?).
>>
>> /dev/tap0 returns errno = 14 (EFAULT - bad address). At this point,
>> 'ifconfig' shows that the inet address is no longer attached and
>> 'netstat -rn' shows the route I added above has been dropped.
>>
>> I have been searching for several days to find more information about
>> this device, but have not found anything specific to FreeBSD. All of
the
>> examples and instructions are for Linux or tun(4), both of which are
>> significantly different devices.
>>
>> My code so far:
>>
>> ----------------- tear along dotted line -----------------
>>  tapFD = open ("/dev/tap0", O_RDWR);
>>  if (tapFD < 0) {
>>    fprintf (stderr, "Failed to open /dev/tap0: %d.\n", tapFD);
>>    exit (2);
>>  }
>>
>>  fprintf (stderr, "Successfully opened /dev/tap0.\n");
>>
>>  unsigned char * buffer = (unsigned char*)malloc(1514);
>>  if (buffer = NULL) {
>>    fprintf (stderr, "No memory available.\n");
>>    close (tapFD);
>>    exit(3);
>>  }
>>  int lenth = 0;
>>
>> again:
>>  lenth = read(tapFD, buffer, 1514);
>>  if (lenth < 0) {
>>    int error = errno;
>>    if (error == EINTR)
>>      goto again;
>>    fprintf (stderr, "tap read error: %d\n", error);
>>  }
>>  else {
>>    int index;
>>
>>    fprintf (stdout, "%d bytes received.\n", lenth);
>>    for (index = 0; index < lenth; ++index) {
>>      fprintf (stdout, " %02x", buffer[index]);
>>      if (index % 16 == 15)
>>        fprintf (stdout, "\n");
>>    }
>>    fprintf (stdout, "\n");
>>  }
>>
>>  close (tapFD);
>> ----------------- tear along dotted line -----------------
>>
>> Just in the interest of full disclosure, I am running a stock
>> installation of FreeBSD 7.0 in a VMWare 5.5.4 session on WinXP. There
>> are also two virtual Ethernet cards, one connected to a host only
>> subnet, the other bridged onto a real Ethernet segment. I am using
IPFW
>> with DummyNet to inject some measure of reality into this system.
>>
>> This is the beginnings of a test bench for several commercial
>> applications. My goal, once I get this device working, is to write an
>> application for tap(4) that will emulate a few hundred embedded
devices,
>> each opening a socket directly to a server, which currently resides
in
>> another VM session on the host only network. This setup, coupled with
>> real devices on the external network should give us a much more
>> realistic environment for stress testing our systems.
>>
>> Thank you,
>>
>> Bob McConnell