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 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 (if tap is built into your kernel, the network interfaces cannot be removed).
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). Control devices (once successfully opened) persist until if_tap.ko is unloaded in the same way that network interfaces persist (see above).
Each interface supports the usual Ethernet network interface ioctl 2 s, such as SIOCSIFADDR and SIOCSIFNETMASK, 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/tap N, 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. 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
.In net/if_tap.h ) :
