Re: [Discuss-gnuradio] Compiler preprocessor defines

[Greg Troxel]

Eric Blossom <address@hidden> writes:

> On Mon, Nov 03, 2008 at 03:46:56PM -0500, Ed Criscuolo wrote:
>> The tun/tap pseudo device is implemented very differently
>> on OSX vs LINUX, UNIX, et al.
>
> OK.
>
> Take a look at how we handle the "Fast USB" technique selection.
> See config/usrp_fusb_tech.m4.
>
> It would probably be a good idea to abstract the whole "open the
> tap/tun device" operation into a separate function, then swig it up so
> that it's accessible from python.

Ed: while you are working on this, it would be good to plan for N
different implementations of user-space tunneling network interfaces
rather than 2.  I suspect that most of *BSD is similar, and I've
enclosed the NetBSD man pages.  Darwin may or may not be like *BSD; they
seem to do things in the Apple way sometimes, but from your description
it seems similar.

I would have code that handles each major API, and then select which api
with autoconf tests, looking for api-specific ioctls being defined in
headers, and relying on OS names as a last resort.  I have enclosed
net/if_tap.h from NetBSD which defines the TAPGIFNAME ioctl.


TAP(4)                  NetBSD Kernel Interfaces Manual                 TAP(4)

NAME
     tap -- virtual Ethernet device

SYNOPSIS
     pseudo-device tap

DESCRIPTION
     The tap driver allows the creation and use of virtual Ethernet devices.
     Those interfaces appear just as any real Ethernet NIC to the kernel, but
     can also be accessed by userland through a character device node in order
     to read frames being sent by the system or to inject frames.

     In that respect it is very similar to what tun(4) provides, but the added
     Ethernet layer allows easy integration with machine emulators or virtual
     Ethernet networks through the use of bridge(4) with tunneling.

   INTERFACE CREATION
     Interfaces may be created in two different ways: using the ifconfig(8)
     create command with a specified device number, or its ioctl(2) equiva-
     lent, SIOCIFCREATE, or using the special cloning device /dev/tap.

     The former works the same as any other cloning network interface: the
     administrator can create and destroy interfaces at any time, notably at
     boot time.  This is the easiest way of combining tap and bridge(4).
     Later, userland will actually access the interfaces through the specific
     device nodes /dev/tapN.

     The latter is aimed at applications that need a virtual Ethernet device
     for the duration of their execution.  A new interface is created at the
     opening of /dev/tap, and is later destroyed when the last process using
     the file descriptor closes it.

   CHARACTER DEVICES
     Whether the tap devices are accessed through the special cloning device
     /dev/tap or through the specific devices /dev/tapN, the possible actions
     to control the matching interface are the same.

     When using /dev/tap though, as the interface is created on-the-fly, its
     name is not known immediately by the application.  Therefore the
     TAPGIFNAME ioctl is provided.  It should be the first action an applica-
     tion using the special cloning device will do.  It takes a pointer to a
     struct ifreq as an argument.

     Ethernet frames sent out by the kernel on a tap interface can be obtained
     by the controlling application with read(2).  It can also inject frames
     in the kernel with write(2).  There is absolutely no validation of the
     content of the injected frame, it can be any data, of any length.

     One call of write(2) will inject a single frame in the kernel, as one
     call of read(2) will retrieve a single frame from the queue, to the
     extent of the provided buffer.  If the buffer is not large enough, the
     frame will be truncated.

     tap character devices support the FIONREAD ioctl which returns the size
     of the next available frame, or 0 if there is no available frame in the
     queue.

     They also support non-blocking I/O through the FIONBIO ioctl.  In that
     mode, EWOULDBLOCK is returned by read(2) when no data is available.

     Asynchronous I/O is supported through the FIOASYNC, FIOSETOWN, and
     FIOGETOWN ioctls.  The first will enable SIGIO generation, while the two
     other configure the process group that will receive the signal when data
     is ready.

     Synchronisation may also be achieved through the use of select(2),
     poll(2), or kevent(2).

   ETHERNET ADDRESS
     When a tap device is created, it is assigned an Ethernet address of the
     form f2:0b:a4:xx:xx:xx.  This address can later be changed in two ways:
     through a sysctl node, or an ioctl call.

     The sysctl node is net.link.tap.<iface>.  Any string of six colon-sepa-
     rated hexadecimal numbers will be accepted.  Reading that node will pro-
     vide a string representation of the current Ethernet address.

     The address can also be changed with the SIOCSIFPHYADDR ioctl, which is
     used the same way as with gif(4).  The difference is in the family of the
     address which is passed inside the struct ifreqalias argument, which
     should be set to AF_LINK.  This ioctl call should be made on a socket, as
     it is not available on the ioctl handler of the character device inter-
     face.

FILES
     /dev/tap        cloning device
     /dev/tap[0-9]*  individual character device nodes

SEE ALSO
     bridge(4), gif(4), tun(4), ifconfig(8)

HISTORY
     The tap driver first appeared in NetBSD 3.0.

NetBSD 4.0_RC1                 January 10, 2005                 NetBSD 4.0_RC1

TUN(4)                  NetBSD Kernel Interfaces Manual                 TUN(4)

NAME
     tun -- tunnel software network interface

SYNOPSIS
     pseudo-device tun

DESCRIPTION
     The tun interface is a software loopback mechanism that can be loosely
     described as the network interface analog of the pty(4), that is, tun
     does for network interfaces what the pty driver does for terminals.

     The tun driver, like the pty driver, provides two interfaces: an inter-
     face like the usual facility it is simulating (a network interface in the
     case of tun, or a terminal for pty), and a character-special device
     ``control'' interface.

     To use a tun device, the administrator must first create the interface.
     This can be done by using the ifconfig(8) create command, or via the
     SIOCIFCREATE ioctl.  An open() call on /dev/tunN, will also create a net-
     work interface with the same unit number of that device if it doesn't
     exists yet.

     The network interfaces should be named tun0, tun1, etc.  Each interface
     supports the usual network-interface ioctl(2)s, such as SIOCSIFADDR and
     SIOCSIFNETMASK, and thus can be used with ifconfig(8) like any other
     interface.  At boot time, they are POINTOPOINT interfaces, but this can
     be changed; see the description of the control device, below.  When the
     system chooses to transmit a packet on the network interface, the packet
     can be read from the control device (it appears there as ``output'');
     writing a packet to the control device generates an input packet on the
     network interface, as if the (non-existent) hardware had just received
     it.

     The tunnel device, normally /dev/tunN, is exclusive-open (it cannot be
     opened if it is already open) and is restricted to the super-user
     (regardless of file system permissions).  A read() call will return an
     error (EHOSTDOWN) if the interface is not ``ready'' (which means that the
     interface address has not been set).  Once the interface is ready, read()
     will return a packet if one is available; if not, it will either block
     until one is or return EAGAIN, depending on whether non-blocking I/O has
     been enabled.  If the packet is longer than is allowed for in the buffer
     passed to read(), the extra data will be silently dropped.

     Packets can be optionally prepended with the destination address as pre-
     sented to the network interface output routine (`tunoutput').  The desti-
     nation address is in `struct sockaddr' format.  The actual length of the
     prepended address is in the member `sa_len'.  The packet data follows
     immediately.  A write(2) call passes a packet in to be ``received'' on
     the pseudo-interface.  Each write() call supplies exactly one packet; the
     packet length is taken from the amount of data provided to write().
     Writes will not block; if the packet cannot be accepted for a transient
     reason (e.g., no buffer space available), it is silently dropped; if the
     reason is not transient (e.g., packet too large), an error is returned.
     If ``link-layer mode'' is on (see TUNSLMODE below), the actual packet
     data must be preceded by a `struct sockaddr'.  The driver currently only
     inspects the `sa_family' field.  The following ioctl(2) calls are sup-
     ported (defined in <net/if_tun.h>):

     TUNSDEBUG   The argument should be a pointer to an int; this sets the
                 internal debugging variable to that value.  What, if any-
                 thing, this variable controls is not documented here; see the
                 source code.

     TUNGDEBUG   The argument should be a pointer to an int; this stores the
                 internal debugging variable's value into it.

     TUNSIFMODE  The argument should be a pointer to an int; its value must be
                 either IFF_POINTOPOINT or IFF_BROADCAST (optionally
                 IFF_MULTICAST may be or'ed into the value).  The type of the
                 corresponding tunn interface is set to the supplied type.  If
                 the value is anything else, an EINVAL error occurs.  The
                 interface must be down at the time; if it is up, an EBUSY
                 error occurs.

     TUNSLMODE   The argument should be a pointer to an int; a non-zero value
                 turns off ``multi-af'' mode and turns on ``link-layer'' mode,
                 causing packets read from the tunnel device to be prepended
                 with network destination address.

     TUNGIFHEAD  The argument should be a pointer to an int; the ioctl sets
                 the value to one if the device is in ``multi-af'' mode, and
                 zero otherwise.

     TUNSIFHEAD  The argument should be a pointer to an int; a non-zero value
                 turns off ``link-layer'' mode, and enables ``multi-af'' mode,
                 where every packet is preceded with a four byte address fam-
                 ily.

     FIONBIO     Turn non-blocking I/O for reads off or on, according as the
                 argument int's value is or isn't zero (Writes are always
                 nonblocking).

     FIOASYNC    Turn asynchronous I/O for reads (i.e., generation of SIGIO
                 when data is available to be read) off or on, according as
                 the argument int's value is or isn't zero.

     FIONREAD    If any packets are queued to be read, store the size of the
                 first one into the argument int; otherwise, store zero.

     TIOCSPGRP   Set the process group to receive SIGIO signals, when asyn-
                 chronous I/O is enabled, to the argument int value.

     TIOCGPGRP   Retrieve the process group value for SIGIO signals into the
                 argument int value.

     The control device also supports select(2) for read; selecting for write
     is pointless, and always succeeds, since writes are always non-blocking.

     On the last close of the data device, by default, the interface is
     brought down (as if with ``ifconfig tunn down'').  All queued packets are
     thrown away.  If the interface is up when the data device is not open
     output packets are always thrown away rather than letting them pile up.

SEE ALSO
     inet(4), intro(4)

HISTORY
     IPv6 support comes mostly from FreeBSD and was added in NetBSD 4.0 by
     Rui Paulo <address@hidden>.

NetBSD 4.0_RC1                   April 8, 2006                  NetBSD 4.0_RC1


/*      $NetBSD: if_tap.h,v 1.3 2005/12/10 23:21:38 elad Exp $  */

/*
 *  Copyright (c) 2004 The NetBSD Foundation.
 *  All rights reserved.
 *
 *  This code is derived from software contributed to the NetBSD Foundation
 *   by Quentin Garnier.
 *
 *  Redistribution and use in source and binary forms, with or without
 *  modification, are permitted provided that the following conditions
 *  are met:
 *  1. Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *  2. Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *  3. All advertising materials mentioning features or use of this software
 *     must display the following acknowledgement:
 *         This product includes software developed by the NetBSD
 *         Foundation, Inc. and its contributors.
 *  4. Neither the name of The NetBSD Foundation nor the names of its
 *     contributors may be used to endorse or promote products derived
 *     from this software without specific prior written permission.
 *
 *  THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
 *  ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 *  TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 *  PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
 *  BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 *  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 *  SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 *  INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 *  CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 *  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 *  POSSIBILITY OF SUCH DAMAGE.
 */

#ifndef _NET_IF_TAP_H_
#define _NET_IF_TAP_H_

/* 'e' comes from former name 'ethfoo' */
#define TAPGIFNAME      _IOR('e', 0, struct ifreq)

#endif /* !_NET_IF_TAP_H_ */

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