docs: networking: convert tuntap.txt to ReST

- add SPDX header;
- use copyright symbol;
- adjust titles and chapters, adding proper markups;
- mark code blocks and literals as such;
- adjust identation, whitespaces and blank lines where needed;
- add to networking/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 259 insertions, 0 deletions

diff --git a/Documentation/networking/tuntap.rst b/Documentation/networking/tuntap.rst
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+.. SPDX-License-Identifier: GPL-2.0
+.. include:: <isonum.txt>
+
+===============================
+Universal TUN/TAP device driver
+===============================
+
+Copyright |copy| 1999-2000 Maxim Krasnyansky <max_mk@yahoo.com>
+
+  Linux, Solaris drivers
+  Copyright |copy| 1999-2000 Maxim Krasnyansky <max_mk@yahoo.com>
+
+  FreeBSD TAP driver
+  Copyright |copy| 1999-2000 Maksim Yevmenkin <m_evmenkin@yahoo.com>
+
+  Revision of this document 2002 by Florian Thiel <florian.thiel@gmx.net>
+
+1. Description
+==============
+
+  TUN/TAP provides packet reception and transmission for user space programs.
+  It can be seen as a simple Point-to-Point or Ethernet device, which,
+  instead of receiving packets from physical media, receives them from
+  user space program and instead of sending packets via physical media
+  writes them to the user space program.
+
+  In order to use the driver a program has to open /dev/net/tun and issue a
+  corresponding ioctl() to register a network device with the kernel. A network
+  device will appear as tunXX or tapXX, depending on the options chosen. When
+  the program closes the file descriptor, the network device and all
+  corresponding routes will disappear.
+
+  Depending on the type of device chosen the userspace program has to read/write
+  IP packets (with tun) or ethernet frames (with tap). Which one is being used
+  depends on the flags given with the ioctl().
+
+  The package from http://vtun.sourceforge.net/tun contains two simple examples
+  for how to use tun and tap devices. Both programs work like a bridge between
+  two network interfaces.
+  br_select.c - bridge based on select system call.
+  br_sigio.c  - bridge based on async io and SIGIO signal.
+  However, the best example is VTun http://vtun.sourceforge.net :))
+
+2. Configuration
+================
+
+  Create device node::
+
+     mkdir /dev/net (if it doesn't exist already)
+     mknod /dev/net/tun c 10 200
+
+  Set permissions::
+
+     e.g. chmod 0666 /dev/net/tun
+
+  There's no harm in allowing the device to be accessible by non-root users,
+  since CAP_NET_ADMIN is required for creating network devices or for
+  connecting to network devices which aren't owned by the user in question.
+  If you want to create persistent devices and give ownership of them to
+  unprivileged users, then you need the /dev/net/tun device to be usable by
+  those users.
+
+  Driver module autoloading
+
+     Make sure that "Kernel module loader" - module auto-loading
+     support is enabled in your kernel.  The kernel should load it on
+     first access.
+
+  Manual loading
+
+     insert the module by hand::
+
+	modprobe tun
+
+  If you do it the latter way, you have to load the module every time you
+  need it, if you do it the other way it will be automatically loaded when
+  /dev/net/tun is being opened.
+
+3. Program interface
+====================
+
+3.1 Network device allocation
+-----------------------------
+
+``char *dev`` should be the name of the device with a format string (e.g.
+"tun%d"), but (as far as I can see) this can be any valid network device name.
+Note that the character pointer becomes overwritten with the real device name
+(e.g. "tun0")::
+
+  #include <linux/if.h>
+  #include <linux/if_tun.h>
+
+  int tun_alloc(char *dev)
+  {
+      struct ifreq ifr;
+      int fd, err;
+
+      if( (fd = open("/dev/net/tun", O_RDWR)) < 0 )
+	 return tun_alloc_old(dev);
+
+      memset(&ifr, 0, sizeof(ifr));
+
+      /* Flags: IFF_TUN   - TUN device (no Ethernet headers)
+       *        IFF_TAP   - TAP device
+       *
+       *        IFF_NO_PI - Do not provide packet information
+       */
+      ifr.ifr_flags = IFF_TUN;
+      if( *dev )
+	 strncpy(ifr.ifr_name, dev, IFNAMSIZ);
+
+      if( (err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0 ){
+	 close(fd);
+	 return err;
+      }
+      strcpy(dev, ifr.ifr_name);
+      return fd;
+  }
+
+3.2 Frame format
+----------------
+
+If flag IFF_NO_PI is not set each frame format is::
+
+     Flags [2 bytes]
+     Proto [2 bytes]
+     Raw protocol(IP, IPv6, etc) frame.
+
+3.3 Multiqueue tuntap interface
+-------------------------------
+
+From version 3.8, Linux supports multiqueue tuntap which can uses multiple
+file descriptors (queues) to parallelize packets sending or receiving. The
+device allocation is the same as before, and if user wants to create multiple
+queues, TUNSETIFF with the same device name must be called many times with
+IFF_MULTI_QUEUE flag.
+
+``char *dev`` should be the name of the device, queues is the number of queues
+to be created, fds is used to store and return the file descriptors (queues)
+created to the caller. Each file descriptor were served as the interface of a
+queue which could be accessed by userspace.
+
+::
+
+  #include <linux/if.h>
+  #include <linux/if_tun.h>
+
+  int tun_alloc_mq(char *dev, int queues, int *fds)
+  {
+      struct ifreq ifr;
+      int fd, err, i;
+
+      if (!dev)
+	  return -1;
+
+      memset(&ifr, 0, sizeof(ifr));
+      /* Flags: IFF_TUN   - TUN device (no Ethernet headers)
+       *        IFF_TAP   - TAP device
+       *
+       *        IFF_NO_PI - Do not provide packet information
+       *        IFF_MULTI_QUEUE - Create a queue of multiqueue device
+       */
+      ifr.ifr_flags = IFF_TAP | IFF_NO_PI | IFF_MULTI_QUEUE;
+      strcpy(ifr.ifr_name, dev);
+
+      for (i = 0; i < queues; i++) {
+	  if ((fd = open("/dev/net/tun", O_RDWR)) < 0)
+	     goto err;
+	  err = ioctl(fd, TUNSETIFF, (void *)&ifr);
+	  if (err) {
+	     close(fd);
+	     goto err;
+	  }
+	  fds[i] = fd;
+      }
+
+      return 0;
+  err:
+      for (--i; i >= 0; i--)
+	  close(fds[i]);
+      return err;
+  }
+
+A new ioctl(TUNSETQUEUE) were introduced to enable or disable a queue. When
+calling it with IFF_DETACH_QUEUE flag, the queue were disabled. And when
+calling it with IFF_ATTACH_QUEUE flag, the queue were enabled. The queue were
+enabled by default after it was created through TUNSETIFF.
+
+fd is the file descriptor (queue) that we want to enable or disable, when
+enable is true we enable it, otherwise we disable it::
+
+  #include <linux/if.h>
+  #include <linux/if_tun.h>
+
+  int tun_set_queue(int fd, int enable)
+  {
+      struct ifreq ifr;
+
+      memset(&ifr, 0, sizeof(ifr));
+
+      if (enable)
+	 ifr.ifr_flags = IFF_ATTACH_QUEUE;
+      else
+	 ifr.ifr_flags = IFF_DETACH_QUEUE;
+
+      return ioctl(fd, TUNSETQUEUE, (void *)&ifr);
+  }
+
+Universal TUN/TAP device driver Frequently Asked Question
+=========================================================
+
+1. What platforms are supported by TUN/TAP driver ?
+
+Currently driver has been written for 3 Unices:
+
+  - Linux kernels 2.2.x, 2.4.x
+  - FreeBSD 3.x, 4.x, 5.x
+  - Solaris 2.6, 7.0, 8.0
+
+2. What is TUN/TAP driver used for?
+
+As mentioned above, main purpose of TUN/TAP driver is tunneling.
+It is used by VTun (http://vtun.sourceforge.net).
+
+Another interesting application using TUN/TAP is pipsecd
+(http://perso.enst.fr/~beyssac/pipsec/), a userspace IPSec
+implementation that can use complete kernel routing (unlike FreeS/WAN).
+
+3. How does Virtual network device actually work ?
+
+Virtual network device can be viewed as a simple Point-to-Point or
+Ethernet device, which instead of receiving packets from a physical
+media, receives them from user space program and instead of sending
+packets via physical media sends them to the user space program.
+
+Let's say that you configured IPv6 on the tap0, then whenever
+the kernel sends an IPv6 packet to tap0, it is passed to the application
+(VTun for example). The application encrypts, compresses and sends it to
+the other side over TCP or UDP. The application on the other side decompresses
+and decrypts the data received and writes the packet to the TAP device,
+the kernel handles the packet like it came from real physical device.
+
+4. What is the difference between TUN driver and TAP driver?
+
+TUN works with IP frames. TAP works with Ethernet frames.
+
+This means that you have to read/write IP packets when you are using tun and
+ethernet frames when using tap.
+
+5. What is the difference between BPF and TUN/TAP driver?
+
+BPF is an advanced packet filter. It can be attached to existing
+network interface. It does not provide a virtual network interface.
+A TUN/TAP driver does provide a virtual network interface and it is possible
+to attach BPF to this interface.
+
+6. Does TAP driver support kernel Ethernet bridging?
+
+Yes. Linux and FreeBSD drivers support Ethernet bridging.