TAP(4) FreeBSD Kernel Interfaces Manual TAP(4)


tapEthernet tunnel software network interface


device tap


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 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 tap N create” command. This is the preferred method of creating tap devices. The same method allows removal of interfaces. For this, use the “ifconfig tap N 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_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 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>):

Set network interface information (line speed, MTU and type). 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 SIGIO when 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 SIGIO signals, when asynchronous I/O is enabled, to the argument int value.
Retrieve the process group value for SIGIO signals 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/vmnet N or /dev/tap N). 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.
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/vmnet N).

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 tap N down”) unless the device is a VMnet device. 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. 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.



inet(4), intro(4)
January 26, 2012 FreeBSD