TUN(4) OpenBSD Programmer's Manual TUN(4)NAMEtun - network tunnel pseudo-device
SYNOPSIS
pseudo-device tun
#include <sys/types.h>
#include <net/if_tun.h>
DESCRIPTION
The tun driver provides a network interface pseudo-device. Packets sent
to this interface can be read by a userland process and processed as
desired. Packets written by the userland process are injected back into
the kernel networking subsystem.
A tun interface can be created at runtime using the ifconfig tunN create
command or by opening the character special device /dev/tunN.
Both layer 3 and layer 2 tunneling is supported; layer 3 tunneling is the
default mode. To enable layer 2 tunneling mode, where the tun interface
simulates an Ethernet network interface, the link0 flag needs to be set
with ifconfig(8) or by setting up a hostname.if(5) configuration file for
netstart(8). Note that setting or unsetting the link0 flag causes tun to
lose any configuration settings, and that it is not advisable to use the
flag with any other parameters.
Each device has the exclusive open property; it cannot be opened if it is
already open and in use by another process. Each read returns at most
one packet; if insufficient buffer space is provided, the packet is
truncated. Each write supplies exactly one packet. Each packet read or
written is prefixed with a tunnel header consisting of a 4-byte network
byte order integer containing the address family in the case of layer 3
tunneling. In layer 2 mode the 4-byte tunnel header is replaced with an
Ethernet header. On the last close of the device, all queued packets are
discarded. If the device was created by opening /dev/tunN, it will be
automatically destroyed. Devices created via ifconfig(8) are only marked
as not running and traffic will be dropped returning EHOSTDOWN.
Writes never block. If the protocol queue is full, the packet is
dropped, a ``collision'' is counted, and ENOBUFS is returned.
In addition to the usual network interface ioctl commands described in
netintro(4), the following special commands defined in <net/if_tun.h> are
supported:
TUNGIFINFO struct tuninfo *
TUNSIFINFO struct tuninfo *
Get or set the interface characteristics.
/* iface info */
struct tuninfo {
u_int mtu;
u_short type;
u_short flags;
u_int baudrate;
};
flags sets the interface flags, and can include one or more of
IFF_UP, IFF_POINTOPOINT, IFF_MULTICAST, IFF_BROADCAST. Flags
given will be set; flags omitted will be cleared; flags not in
this list will not be changed even when given. Flags default to
IFF_POINTOPOINT for layer 3 and to IFF_BROADCAST | IFF_MULTICAST
for layer 2 mode. It is an error to set both IFF_POINTOPOINT and
IFF_BROADCAST. type defaults to IFT_TUNNEL for layer 3 and
IFT_ETHER for layer 2 tunneling mode. This sets the interface
media address header type.
TUNSIFMODE int *flags
Set just the interface flags. The same restrictions as for
TUNSIFINFO apply.
FIONBIO int *flag
Set non-blocking I/O.
FIOASYNC int *flag
Cause signal SIGIO to be sent when a packet can be read.
TIOCSPGRP int *pgrp
TIOCGPGRP int *pgrp
Get or set the process group to which signals might be sent via
FIOASYNC.
FIONREAD int *count
Get the byte count of the next packet available to be read.
SIOCGIFADDR struct ether_addr *addr
SIOCSIFADDR struct ether_addr *addr
Get or set the Ethernet address of the device in layer 2 mode.
FILES
/dev/tun*
ERRORS
If open fails, errno(2) may be set to one of:
[ENXIO] Not that many devices configured.
[EBUSY] Device was already open.
If a write(2) call fails, errno(2) is set to one of:
[EMSGSIZE] The packet supplied was too small or too large. The
maximum sized packet allowed is currently 16384 bytes.
[ENOBUFS] There were no mbufs, or the queue for the outgoing protocol
is full.
[EAFNOSUPPORT]
The address family specified in the tunnel header was not
recognized.
Ioctl commands may fail with:
[EINVAL] Attempt to set both IFF_POINTOPOINT and IFF_BROADCAST with
TUNSIFMODE or using SIOCGIFADDR or SIOCSIFADDR in layer 3
mode.
[ENOTTY] Unrecognized ioctl command.
A read(2) call may fail because of:
[EHOSTDOWN] The device is not ready. The device must have an inet(4)
interface address assigned to it, such as via SIOCSIFADDR.
[EWOULDBLOCK]
Non-blocking I/O was selected and no packets were
available.
An attempt to send a packet out via the interface may fail with:
[EHOSTDOWN] The device is not ready. The device must have an inet(4)
interface address assigned to it, such as via SIOCSIFADDR.
SEE ALSOinet(4), intro(4), netintro(4), hostname.if(5), ifconfig(8), netstart(8)OpenBSD 4.9 September 23, 2010 OpenBSD 4.9