.\" $FreeBSD: src/share/man/man4/tap.4,v 1.1.2.7 2002/04/16 23:59:28 trhodes Exp $ .\" Based on PR#2411 .\" .Dd July 19, 2018 .Dt TAP 4 .Os .Sh NAME .Nm tap .Nd Ethernet tunnel software network interface .Sh SYNOPSIS .Cd pseudo-device tap .Sh DESCRIPTION The .Nm interface is a software loopback mechanism that can be loosely described as the network interface analog of the .Xr pty 4 , that is, .Nm does for network interfaces what the .Xr pty 4 driver does for terminals. .Pp The .Nm driver, like the .Xr pty 4 driver, provides two interfaces: an interface like the usual facility it is simulating (an Ethernet network interface in the case of .Nm , or a terminal for .Xr pty 4 ) , and a character-special device .Dq control interface. A client program transfers Ethernet frames to or from the .Nm .Dq control interface. The .Xr tun 4 interface provides similar functionality at the network layer: a client will transfer IP (by default) packets to or from a .Xr tun 4 .Dq control interface. .Pp The network interfaces are named .Dq Li tap0 , .Dq Li tap1 , etc., one for each control device that has been opened. These Ethernet network interfaces persist until the .Pa if_tap.ko module is unloaded, or until removed with .Dq ifconfig destroy (see below). .Pp The .Nm devices are created using interface cloning. This is done using the .Dq ifconfig tap Ns Sy N No create command. This is the preferred method of creating .Nm devices. The same method allows removal of interfaces by using the .Dq ifconfig tap Ns Sy N No destroy command. .Pp The .Nm interface permits opens on the special control device .Pa /dev/tap . When this special device is opened, .Nm will return a handle for the lowest unused .Nm device (use .Xr devname 3 to determine which). .Pp Control devices (once successfully opened) persist until the .Pa if_tap.ko module is unloaded or the interface is destroyed. .Pp Each interface supports the usual Ethernet network interface .Xr ioctl 2 Ns s and thus can be used with .Xr 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 .Dq 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. .Pp The Ethernet tunnel device, normally .Pa /dev/tap Ns Sy N , is exclusive-open (it cannot be opened if it is already open) and is restricted to the super-user, unless the .Xr sysctl 8 variable .Va net.link.tap.user_open is non-zero. If the .Xr sysctl 8 variable .Va net.link.tap.up_on_open is non-zero, the tunnel device will be marked .Dq up when the control device is opened. A .Fn read call will return an error .Pq Er EHOSTDOWN if the interface is not .Dq ready . Once the interface is ready, .Fn read will return an Ethernet frame if one is available; if not, it will either block until one is or return .Er 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 .Fn read , the extra data will be silently dropped. .Pp A .Xr write 2 call passes an Ethernet frame in to be .Dq received on the pseudo-interface. Each .Fn write call supplies exactly one frame; the frame length is taken from the amount of data provided to .Fn 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 .Xr ioctl 2 calls are supported (defined in .In net/tap/if_tap.h ) : .Bl -tag -width ".Dv TAPSIFINFO" .It Dv TAPSDEBUG The argument should be a pointer to an .Va int ; this sets the internal debugging variable to that value. What, if anything, this variable controls is not documented here, see the source code. .It Dv TAPGDEBUG The argument should be a pointer to an .Va int ; this stores the internal debugging variable's value into it. .It Dv TAPSIFINFO Set network interface information (line speed and MTU). The type must be the same as returned by .Dv TAPGIFINFO or set to .Dv IFT_ETHER , otherwise the .Xr ioctl 2 call will fail. The argument should be a pointer to a .Va struct tapinfo . .It Dv TAPGIFINFO Retrieve network interface information (line speed, MTU and type). The argument should be a pointer to a .Va struct tapinfo . .It Dv TAPGIFNAME Retrieve network interface name. The argument should be a pointer to a .Va struct ifreq . The interface name will be returned in the .Va ifr_name field. .It Dv FIOASYNC Turn asynchronous I/O for reads (i.e., generation of .Dv SIGIO when data is available to be read) off or on, according as the argument .Va int Ns 's value is or is not zero. .It Dv FIONREAD If any frames are queued to be read, store the size of the first one into the argument .Va int ; otherwise, store zero. .It Dv TIOCSPGRP Set the process group to receive .Dv SIGIO signals, when asynchronous I/O is enabled, to the argument .Va int value. .It Dv TIOCGPGRP Retrieve the process group value for .Dv SIGIO signals into the argument .Va int value. .It Dv SIOCGIFFLAGS Retrieve the flags of the network interface associated with the control device into the argument .Va int value. .It Dv SIOCGIFADDR Retrieve the MAC address of the associated network interface. The argument should be a pointer to a .Va uint8_t array of length .Dv ETHER_ADDR_LEN . This command is used by the .Xr vke 4 device. .It Dv SIOCSIFADDR Set the MAC address of the associated network interface. The argument should be a pointer to a .Va uint8_t array of length .Dv ETHER_ADDR_LEN . .El .Pp The control device also supports .Xr select 2 for read; selecting for write is pointless, and always succeeds, since writes are always non-blocking. .Pp On the last close of the data device, the interface is brought down (as if with .Dq ifconfig tap Ns Sy N No 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. .Sh SEE ALSO .Xr inet 4 , .Xr tun 4 , .Xr vke 4 , .Xr ifconfig 8