--- name: "tap" section: "4" platform: "linux" package: "freebsd-manpages" version: "6.0-5" release: "Debian 4.0 (Etch)" platforms: ["linux", "freebsd"] url: "https://new.carta.tech/tap/4" --- # tap(4) Ethernet tunnel software network interface ## NAME **tap** – Ethernet tunnel software network interface ## SYNOPSIS **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)](/pty/4.md), 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)](/devname/3.md) 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)](/ioctl/2.md)s, such as SIOCSIFADDR and SIOCSIFNETMASK, and thus can be used with [ifconfig(8)](/ifconfig/8.md) 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)](/sysctl/8.md) 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)](/write/2.md) 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)](/ioctl/2.md) calls are supported (defined in ⟨net/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. - FIONBIO 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). - 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. - SIOCGIFADDR 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. - SIOCSIFADDR 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)](/select/2.md) 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/vmnet***N**, and the network interface will be **vmnet***N*. Additionally, *VMnet* devices do not [ifconfig(8)](/ifconfig/8.md) themselves down when the control device is closed. Everything else is the same. In addition to the above mentioned [ioctl(2)](/ioctl/2.md) calls, there is an additional one for the VMware port. - VMIO\_SIOCSIFFLAGS VMware SIOCSIFFLAGS. ## SEE ALSO [inet(4)](/inet/4.md), [intro(4)](/intro/4.md)