diff options
Diffstat (limited to 'Documentation/networking/tuntap.rst')
| -rw-r--r-- | Documentation/networking/tuntap.rst | 259 |
1 files changed, 259 insertions, 0 deletions
diff --git a/Documentation/networking/tuntap.rst b/Documentation/networking/tuntap.rst new file mode 100644 index 000000000..4d7087f72 --- /dev/null +++ b/Documentation/networking/tuntap.rst @@ -0,0 +1,259 @@ +.. 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 ) + strscpy_pad(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. |