Ubuntu Manpage: tap, vmnet — Ethernet tunnel software network interface

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NAME

       tap, vmnet — Ethernet tunnel software network interface

SYNOPSIS

       device tuntap

DESCRIPTION

The tap 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(4) driver does for terminals.

The tap driver, like the pty(4) 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(4)), and a
character-special device “control” interface. A client program transfers Ethernet frames to or from the
tap “control” interface. The tun(4) interface provides similar functionality at the network layer: a
client will transfer IP (by default) packets to or from a tun(4) “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_tuntap.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 tap, such as
VMware and ssh(1). It therefore defaults to being enabled until further notice.

Control devices (once successfully opened) persist until if_tuntap.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 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 and MTU). The type must be the same
as returned by TAPGIFINFO or set to IFT_ETHER else the ioctl(2) call will fail. 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 the MAC 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).

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, the interface is brought down (as if with “ifconfig tapN down”) and
has all of its configured addresses deleted unless the device is a VMnet device, or has IFF_LINK0 flag
set. All queued frames are thrown away. If the interface is up when the data device is not open, output
frames are thrown away rather than letting them pile up.

The tap device can also be used with the VMware port as a replacement for the old VMnet device driver.
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.

NAME

SYNOPSIS

DESCRIPTION

SEE ALSO