TUN(4)

NAME
tun - 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 ALSO
inet(4), intro(4), netintro(4), hostname.if(5), ifconfig(8), netstart(8)