TAP(4)

NAME
tap - Ethernet tunnel software network interface

SYNOPSIS
pseudo-device tap

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 the if_tap.ko module is unloaded, or until removed with “ifconfig destroy” (see below).

The 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 by using the “ifconfig tapN destroy” command.

The tap interface permits opens on the special control device /dev/tap. When this special device is opened, tap will return a handle for the lowest unused tap device (use devname(3) to determine which).

Control devices (once successfully opened) persist until the if_tap.ko module 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/tap/if_tap.h>):

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.

TAPSIFINFO
    Set network interface information (line speed and MTU). The type must be the same as returned by TAPGIFINFO or set to IFT_ETHER, otherwise 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.

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.

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.

SIOCGIFFLAGS
    Retrieve the flags of the network interface associated with the control device into the argument int value.

SIOCGIFADDR
    Retrieve the MAC address of the associated network interface. The argument should be a pointer to a uint8_t array of length ETHER_ADDR_LEN. This command is used by the vke(4) device.

SIOCSIFADDR
    Set the MAC address of the associated network interface. The argument should be a pointer to a uint8_t array of length ETHER_ADDR_LEN.

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”). 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.

SEE ALSO
inet(4), tun(4), vke(4), ifconfig(8)