Date: Thu, 15 May 2008 14:49:26 -0400 From: "Bob McConnell" <rvm@CBORD.com> To: <freebsd-questions@freebsd.org> Subject: RE: Unable to talk to tap(4) Message-ID: <FF8482A96323694490C194BABEAC24A002B841D1@Email.cbord.com> References: <FF8482A96323694490C194BABEAC24A002B83B78@Email.cbord.com> <20080514234101.P2316@wojtek.tensor.gdynia.pl>
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From: Bob McConnell=20
>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 =3D 19 (ENODEV - Operation =
not
>>> supported?).
>>>
>>> /dev/tap0 returns errno =3D 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 =3D 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 =3D (unsigned char*)malloc(1514);
>>> if (buffer =3D NULL) {
>>> fprintf (stderr, "No memory available.\n");
>>> close (tapFD);
>>> exit(3);
>>> }
When I replace the malloc with an automatic array, the
error goes away and I get the data I am looking for. i.e.:
unsigned char buffer[1514];
So why can't I use malloc to create that buffer?
>>> int lenth =3D 0;
>>>
>>> again:
>>> lenth =3D read(tapFD, buffer, 1514);
>>> if (lenth < 0) {
>>> int error =3D errno;
>>> if (error =3D=3D EINTR)
>>> goto again;
>>> fprintf (stderr, "tap read error: %d\n", error);
>>> }
>>> else {
>>> int index;
>>>
>>> fprintf (stdout, "%d bytes received.\n", lenth);
>>> for (index =3D 0; index < lenth; ++index) {
>>> fprintf (stdout, " %02x", buffer[index]);
>>> if (index % 16 =3D=3D 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
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