table of contents
|TAP(4)||Device Drivers Manual||TAP(4)|
— Ethernet tunnel software network
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
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
tap, or a terminal for
pty(4)), and a character-special device
“control” interface. A client program transfers Ethernet
frames to or from the
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”
The network interfaces are named
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
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
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
net.link.tap.up_on_open is non-zero, the tunnel device
will be marked “up” when the control device is opened. A
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
read(), the extra data will be silently
A write(2) call passes an Ethernet
frame in to be “received” on the pseudo-interface. Each
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
- Set network interface information (line speed and MTU). The type must be
the same as returned by
TAPGIFINFOor set to
IFT_ETHERelse the ioctl(2) call will fail. The argument should be a pointer to a struct tapinfo.
- Retrieve network interface information (line speed, MTU and type). The argument should be a pointer to a struct tapinfo.
- 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.
- 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.
- 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).
- 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 frames 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.
- 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
MACaddress. At the open time both “local” and “remote”
MACaddresses are the same, so this command could be used to retrieve the “local”
- 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
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.
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
In addition to the above mentioned ioctl(2) calls, there is an additional one for the VMware port.
|January 13, 2020||Debian|