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TUN(4) DragonFly Kernel Interfaces Manual TUN(4)
tun - tunnel software network interface
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_tun.ko module is unloaded, or until removed with the
ifconfig(8) command (see below).
The 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 by
using the "ifconfig tunN destroy" command.
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).
Control devices (once successfully opened) persist until the if_tun.ko
module is unloaded or the interface is destroyed.
Each interface supports the usual network-interface ioctl(2)s 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 (i.e., "link-layer" mode), packets
read from the control device will be prepended with the destination
address as presented to the network interface output routine. 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 (i.e., "multi-af" mode), packets will be prepended with a
4-byte address family in network byte order. TUNSLMODE and TUNSIFHEAD
are mutually exclusive. In any case, the packet data follows
A write(2) call passes a packet in to be "received" on the pseudo-
interface. 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.
If the TUNSLMODE ioctl has been set (i.e., "link-layer" mode), the actual
packet data must be preceded by a struct sockaddr. The driver currently
only inspects the sa_family field. If the TUNSIFHEAD ioctl has been set
(i.e., "multi-af" mode), the address family must be prepended, otherwise
the packet is assumed to be of type AF_INET.
The following ioctl(2) calls are supported (defined in
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, otherwise the ioctl(2) call
will fail. struct tuninfo is declared in
TUNGIFINFO The argument should be a pointer to an struct tuninfo,
where the current MTU, type, and baudrate will be stored.
TUNGIFNAME Retrieve the name of the network interface that is
associated with the control device. The argument should
be a pointer to a struct ifreq. The interface name will
be returned in the ifr_name field.
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 anything else, 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
TUNSIFPID Will set the PID owning the tunnel device to the current
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 4-byte
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.
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
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.
ioctl(2), devname(3), inet(4), tap(4), ifconfig(8)
DragonFly 6.3-DEVELOPMENT August 3, 2018 DragonFly 6.3-DEVELOPMENT