|TUN(4)||Device Drivers Manual||TUN(4)|
tun — tunnel
software network interface
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
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
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
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
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
thus can be used with ifconfig(8) like any other
interface. At boot time, they are
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
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.
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,
The destination address is in struct sockaddr format.
The actual length of the prepended address is in the member
sa_len. If the
ioctl has been set, packets will be prepended with a four byte address
family in network byte order.
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
- 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.
- The argument should be a pointer to an int; this stores the internal debugging variable's value into it.
- 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
TUNGIFINFOor set to
IFT_PPPelse the ioctl(2) call will fail. The struct tuninfo is declared in
The use of this ioctl is restricted to the super-user.
- The argument should be a pointer to an struct tuninfo, where the current MTU, type, and baudrate will be stored.
- The argument should be a pointer to an int; its
value must be either
IFF_BROADCASTand should have
IFF_MULTICASTOR'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
EINVALerror is returned. The interface must be down at the time; if it is up, an
EBUSYerror is returned.
- 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).
- Will set the pid owning the tunnel device to the current process's pid.
- 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.
- 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.
- 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.)
- Turn asynchronous I/O for reads (i.e., generation of
SIGIOwhen data is available to be read) off or on, according as the argument int's value is or is not zero.
- If any packets are queued to be read, store the size of the first one into the argument int; otherwise, store zero.
- Set the process group to receive
SIGIOsignals, when asynchronous I/O is enabled, to the argument int value.
- Retrieve the process group value for
SIGIOsignals 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
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.
This manual page was originally obtained from NetBSD.
|April 29, 2019||Debian|