path: root/share/man/man4/tun.4

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.\" Copyright (c) 2003 Marcus D. Watts  All rights reserved.
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.Dd $Mdocdate: November 18 2024 $
.Dt TUN 4
.Os
.Sh NAME
.Nm tun ,
.Nm tap
.Nd network tunnel pseudo-device
.Sh SYNOPSIS
.Cd "pseudo-device tun"
.Pp
.In sys/types.h
.In net/if_tun.h
.Sh DESCRIPTION
The
.Nm tun
pseudo-device driver provides character special devices for
communicating with the kernel network stack via the
.Nm tun
and
.Nm tap
network interfaces.
Packets sent to these interfaces can be read from the device special
file by a userland process and processed as desired.
Packets written to the device special file by the userland process
are injected back into the kernel networking subsystem.
.Pp
.Nm tun
and
.Nm tap
interfaces can be created at runtime using the
.Ic ifconfig iface Ns Ar N Ic create
command, or by opening the character special devices
.Pa /dev/tunN
or
.Pa /dev/tapN
respectively.
.Pp
The minor number of the device special files are associated with
the unit number of the network interfaces.
.Pp
Each device has an exclusive open property: it cannot be opened
if it is already open and in use by another process.
On the last close of the device all queued packets are discarded.
If the device was created by opening a device special file
it will be automatically destroyed.
The last close of a device special file associated with an interface
created via
.Xr ifconfig 8
will be marked as not running and traffic sent out the will be dropped.
.Pp
Each read returns at most one packet; if insufficient
buffer space is provided, the packet is truncated.
Each write supplies exactly one packet.
Writes never block.
If the protocol queue is full, the packet is dropped, a
.Dq collision
is counted, and
.Er ENOBUFS
is returned.
.Pp
The following
.Xr ioctl 2
special commands defined in
.In net/if_tun.h
are supported:
.Pp
.Bl -tag -width indent -compact
.It Dv TUNGIFINFO Fa "struct tuninfo *"
.It Dv TUNSIFINFO Fa "struct tuninfo *"
Get or set the interface characteristics.
.Bd -literal
/* iface info */
struct tuninfo {
        u_int   mtu;
        u_short type;
        u_short flags;
        u_int   baudrate;
};
.Ed
.Pp
.Va flags
and
.Va type
are set by the kernel when an interface is created,
and must be set to the same values that the kernel provided.
.Pp
.It Dv TUNSIFMODE Fa int *
Provided for backwards compatibility.
The flags set must match what the kernel initialised them to.
.Pp
.It Dv TUNSCAP Fa struct tun_capabilities *
Enable the prepending of network packets with a
.Vt struct tun_hdr
offload header,
and set which interface offload capabilities userland can
handle on behalf of the kernel.
.Bd -literal
struct tun_capabilities {
	uint32_t	tun_if_capabilities;
};
.Ed
.Pp
The
.Fa tun_if_capabilities
field is set with IFCAP values from
.In net/if.h .
It is acceptable to enable the offload header without enabling any
interface offload capabilities.
.Pp
The prepending of network packets with a
.Vt struct tun_hdr
offload header is disabled,
and the interface offload capabilities are reset when the device
special file is closed.
.Pp
.It Dv TUNGCAP Fa struct tun_capabilities *
Get which interface offload capabilities are currently enabled.
If the interface has not been configured with
.Dv TUNSCAP
this command will fail with
.Er ENODEV .
.Pp
.It Dv TUNDCAP
Disable the prepending of network packets with the
.Vt struct tun_hdr
offload header,
and clear interface offload capabilities.
.El
.Pp
The generic ioctls
.Dv FIONREAD ,
.Dv FIONBIO ,
.Dv FIOASYNC ,
.Dv FIOSETOWN ,
.Dv FIOGETOWN
are also supported.
.Ss Point-to-Point Layer 3 tunnel interface (tun)
Each packet read from or written to a
.Nm tun
interface is prefixed with a tunnel header consisting of
a 4-byte network byte order integer containing the address family of
the packet.
.Nm tun
supports
.Dv AF_INET ,
.Dv AF_INET6 ,
and
.Dv AF_MPLS
packets.
.Ss Ethernet tunnel interface (tap)
Each packet read from or written to a
.Nm tap
interface is an Ethernet packet.
The Ethernet CRC at the end of the frame is not required.
.Pp
The device special files for
.Nm tap
interfaces support the following additional
.Xr ioctl 2
commands:
.Pp
.Bl -tag -width indent -compact
.It Dv SIOCGIFADDR Fa uint8_t Ns [ETHER_ADDR_LEN]
.It Dv SIOCSIFADDR Fa uint8_t Ns [ETHER_ADDR_LEN]
Get or set the link layer address (MAC address) of the interface.
.El
.Ss Network offload support
When network offload support has been enabled with the
.Dv TUNSCAP
.Xr ioctl 2
command,
.Va struct tun_hdr
is prepended to packets read from and written to the device special
file.
.Pp
The kernel will populate the offload header for reads from the
device special file depending on which interface offload capabilities
are enabled.
A program may use any offload feature when writing to the device
special file regardless of which interface offload capabilities are
enabled.
The offload header can be read from one device special file and
written directly to another without interpretation or modification.
.Bd -literal
struct tun_hdr {
	uint16_t	th_flags;
	uint16_t	th_pad;
	uint16_t	th_vtag;
	uint16_t	th_mss;
};
.Ed
.Pp
The fields in the
.Va struct tun_hdr
are in host native byte order.
.Pp
The following flags can be set in
.Fa th_flags
to specify which offloads are requested for the current packet:
.Pp
.Bl -tag -width "TUN_H_ICMP_CSUM" -compact
.It Dv TUN_H_VTAG
.Fa th_vtag
is set.
.It Dv TUN_H_TCP_MSS
TCP segmentation offload is requested for the current packet.
The maximum segment size is specified in
.Fa th_mss .
.It Dv TUN_H_IPV4_CSUM
IPv4 header checksum calculation requested.
.It Dv TUN_H_TCP_CSUM
TCP checksum calculation requested.
.It Dv TUN_H_UDP_CSUM
UDP checksum calculation requested.
.It Dv TUN_H_ICMP_CSUM
ICMP checksum calculation requested.
.El
.Pp
Only one of
.Dv TUN_H_TCP_CSUM ,
.Dv TUN_H_UDP_CSUM ,
and
.Dv TUN_H_ICMP_CSUM
can be specified at a time.
.Dv TUN_H_VTAG
can only be used with
.Nm tap
interfaces.
.Pp
.Fa th_pad
should be set to 0 when writing to the device special file, and
ignored when reading from the device special file.
.Sh FILES
.Bl -tag -width /dev/tun* -compact
.It Pa /dev/tun*
.It Pa /dev/tap*
.El
.Sh ERRORS
If open fails,
.Xr errno 2
may be set to one of:
.Bl -tag -width Er
.It Bq Er ENXIO
Not that many devices configured.
.\" The associated interface existed, but is being destroyed.
.It Bq Er EBUSY
Device was already open.
.El
.Pp
If a
.Xr write 2
call fails,
.Xr errno 2
is set to one of:
.Bl -tag -width Er
.It Bq Er EMSGSIZE
The packet supplied was too small or too large.
The maximum sized packet allowed is currently 16384 bytes.
.It Bq Er ENOMEM
There were no mbufs.
.\" .It Bq Er ENOBUFS
.\" The queue for the outgoing protocol is full.
.El
.Pp
.Xr ioctl 2
commands may fail with:
.Bl -tag -width Er
.It Bq Er EINVAL
Invalid parameters were specified.
.It Bq Er ENOTTY
Unrecognized ioctl command.
.El
.Pp
A
.Xr read 2
call may fail because of:
.Bl -tag -width Er
.It Bq Er EIO
The associated interface has been destroyed.
.It Bq Er EWOULDBLOCK
Non-blocking I/O was selected and no packets were available.
.El
.Sh SEE ALSO
.Xr ioctl 2 ,
.Xr inet 4 ,
.Xr intro 4 ,
.Xr netintro 4 ,
.Xr hostname.if 5 ,
.Xr ifconfig 8 ,
.Xr netstart 8
.Sh HISTORY
.Nm tun
interfaces originally supported both Layer 3 and Ethernet packets
by reconfiguring the type of interface with
.Dv TUNSIFINFO .
Ethernet packet support was split into the separate
.Nm tap
interface in
.Ox 5.9 .
.Sh AUTHORS
.Nm tun
was written by
.An Julian Onions Aq Mt Julian.Onions@nexor.co.uk
at Nottingham University.
.Pp
The
.Nm tap
interface functionality was written by
.An Claudio Jeker Aq Mt claudio@openbsd.org .