Ubuntu Manpage: tun — tunnel software network interface

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NAME

       tun — tunnel software network interface

SYNOPSIS

       device tuntap

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(4) driver does for terminals.

The tun driver, like the pty(4) driver, provides two interfaces: an interface like the usual facility it
is simulating (a network interface in the case of tun, or a terminal for pty(4)), and a character-special
device “control” interface. A client program transfers IP (by default) packets to or from the tun
“control” interface. The tap(4) interface provides similar functionality at the Ethernet layer: a client
will transfer Ethernet frames to or from a tap(4) “control” interface.

The network interfaces are named “tun0”, “tun1”, etc., one for each control device that has been opened.
These network interfaces persist until the if_tuntap.ko module is unloaded, or until removed with the
ifconfig(8) command.

tun devices are created using interface cloning. This is done using the “ifconfig tunN create” command.
This is the preferred method of creating tun devices. The same method allows removal of interfaces. For
this, use the “ifconfig tunN destroy” command.

If the sysctl(8) variable net.link.tun.devfs_cloning is non-zero, the tun interface permits opens on the
special control device /dev/tun. When this device is opened, tun will return a handle for the lowest
unused tun device (use devname(3) to determine which).

Disabling the legacy devfs cloning functionality may break existing applications which use tun, such as
ppp(8) and ssh(1). It therefore defaults to being enabled until further notice.

Control devices (once successfully opened) persist until if_tuntap.ko is unloaded in the same way that
network interfaces persist (see above).

Each interface supports the usual network-interface ioctl(2)s, such as SIOCAIFADDR 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 as “input”
there); 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 (/dev/tunN) is exclusive-open (it cannot be opened if it is already open). A read(2)
call will return an error (EHOSTDOWN) if the interface is not “ready” (which means that the control
device is open and the interface's address has been set).

Once the interface is ready, read(2) will return a packet if one is available; if not, it will either
block until one is or return EWOULDBLOCK, 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(2), the extra data will be silently
dropped.

If the TUNSLMODE ioctl has been set, packets read from the control device will be prepended with the
destination address as presented to the network interface output routine, tunoutput(). The destination
address is in struct sockaddr format. The actual length of the prepended address is in the member
sa_len. If the TUNSIFHEAD ioctl has been set, packets will be prepended with a four byte address family
in network byte order. TUNSLMODE and TUNSIFHEAD are mutually exclusive. In any case, the packet data
follows immediately.

A write(2) call passes a packet in to be “received” on the pseudo-interface. If the TUNSIFHEAD ioctl has
been set, the address family must be prepended, otherwise the packet is assumed to be of type AF_INET.
Each write(2) call supplies exactly one packet; the packet length is taken from the amount of data
provided to write(2) (minus any supplied address family). 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.

The following ioctl(2) calls are supported (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 anything, 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.

TUNSIFINFO The argument should be a pointer to an struct tuninfo and allows setting the MTU and the
baudrate of the tunnel device. The type must be the same as returned by TUNGIFINFO or set to
IFT_PPP else the ioctl(2) call will fail. The struct tuninfo is declared in <net/if_tun.h>.

The use of this ioctl is restricted to the super-user.

TUNGIFINFO The argument should be a pointer to an struct tuninfo, where the current MTU, type, and
baudrate will be stored.

TUNSIFMODE The argument should be a pointer to an int; its value must be either IFF_POINTOPOINT or
IFF_BROADCAST and should have IFF_MULTICAST OR'd into the value if multicast support is
required. The type of the corresponding “tunN” interface is set to the supplied type. If
the value is outside the above range, an EINVAL error is returned. The interface must be
down at the time; if it is up, an EBUSY error is returned.

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
the network destination address (see above).

TUNSIFPID Will set the pid owning the tunnel device to the current process's pid.

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 family.

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.

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

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 is not 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 asynchronous 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.

AUTHORS

       This manual page was originally obtained from NetBSD.

Debian                                           April 29, 2019                                           TUN(4)

NAME

SYNOPSIS

DESCRIPTION

SEE ALSO

AUTHORS