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TAP(4) | Device Drivers Manual | TAP(4) |
NAME¶
tap
—
Ethernet tunnel software network interface
SYNOPSIS¶
device tap
DESCRIPTION¶
Thetap
interface is a software loopback
mechanism that can be loosely described as the network interface analog of the
pty(4), that is,
tap
does for network interfaces what the
pty
driver does for terminals.
The tap
driver, like the
pty
driver, provides two interfaces: an
interface like the usual facility it is simulating (an Ethernet network
interface in the case of tap
, or a terminal
for pty
), and a character-special device
“control” interface.
The network interfaces are named “tap0
”,
“tap1
”, etc., one for each control
device that has been opened. These Ethernet network interfaces persist until
if_tap.ko module is unloaded, or until
removed with "ifconfig destroy" (see below).
tap
devices are created using interface
cloning. This is done using the “ifconfig
tapN create”
command. This is the preferred method of creating
tap
devices. The same method allows removal
of interfaces. For this, use the “ifconfig
tapN destroy”
command.
If the sysctl(8) variable
net.link.tap.devfs_cloning is non-zero, the
tap
interface permits opens on the special
control device /dev/tap. When this device
is opened, tap
will return a handle for the
lowest unused tap
device (use
devname(3) to determine which).
Disabling the legacy devfs cloning functionality may break
existing applications which use
Control devices (once successfully opened) persist until
if_tap.ko is unloaded or the interface is
destroyed.
Each interface supports the usual Ethernet network interface
ioctl(2)s and thus can be used with
ifconfig(8) like any other Ethernet interface.
When the system chooses to transmit an Ethernet frame on the network
interface, the frame can be read from the control device (it appears as
“input” there); writing an Ethernet frame to the control device
generates an input frame on the network interface, as if the (non-existent)
hardware had just received it.
The Ethernet tunnel device, normally
/dev/tapN, is
exclusive-open (it cannot be opened if it is already open) and is restricted
to the super-user, unless the sysctl(8) variable
net.link.tap.user_open is non-zero. If the
sysctl(8) variable
net.link.tap.up_on_open is non-zero, the
tunnel device will be marked “up” when the control device is
opened. A tap
, such
as VMware and ssh(1). It therefore defaults to
being enabled until further notice.read
() call will return an error
(EHOSTDOWN
) if the interface is not
“ready”. Once the interface is ready,
read
() will return an Ethernet frame 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 frame is longer than is allowed for
in the buffer passed to read
(), the extra
data will be silently dropped.
A write(2) call passes an Ethernet frame in to be
“received” on the pseudo-interface. Each
write
() call supplies exactly one frame;
the frame length is taken from the amount of data provided to
write
(). Writes will not block; if the
frame 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.,
frame too large), an error is returned. The following
ioctl(2) calls are supported (defined in
<net/if_tap.h>
):
TAPSIFINFO
- Set network interface information (line speed, MTU and type). The argument should be a pointer to a struct tapinfo.
TAPGIFINFO
- Retrieve network interface information (line speed, MTU and type). The argument should be a pointer to a struct tapinfo.
TAPSDEBUG
- 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.
TAPGDEBUG
- The argument should be a pointer to an int; this stores the internal debugging variable's value into it.
TAPGIFNAME
- Retrieve network interface name. The argument should be a pointer to a struct ifreq. The interface name will be returned in the ifr_name field.
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 nonblocking).
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 frames 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. SIOCGIFADDR
- Retrieve the Media Access Control (
MAC
) address of the “remote” side. This command is used by the VMware port and expected to be executed on descriptor, associated with control device (usually /dev/vmnetN or /dev/tapN). The buffer, which is passed as the argument, is expected to have enough space to store theMAC
address. At the open time both “local” and “remote”MAC
addresses are the same, so this command could be used to retrieve the “local”MAC
address. SIOCSIFADDR
- Set the Media Access Control (
MAC
) address of the “remote” side. This command is used by VMware port and expected to be executed on a descriptor, associated with control device (usually /dev/vmnetN).
tap
device can also be used with the
VMware port as a replacement for the old VMnet
device driver. The driver uses the minor number to select between
tap
and
vmnet
devices.
VMnet minor numbers begin at
0x800000 +
N; where N
is a VMnet unit number. In this case the control
device is expected to be
/dev/vmnetN,
and the network interface will be
vmnetN.
Additionally, VMnet devices do not
ifconfig(8) themselves down when the control
device is closed. Everything else is the same.
In addition to the above mentioned ioctl(2) calls,
there is an additional one for the VMware port.
VMIO_SIOCSIFFLAGS
- VMware
SIOCSIFFLAGS
.
SEE ALSO¶
inet(4), intro(4)January 26, 2012 | Debian |