Based on PR#2411
.Dd July 9, 2000 .Os .Dt TAP 4 .Sh NAME .Nm tap .Nd Ethernet tunnel software network interface .Sh SYNOPSIS .Cd pseudo-device tap .Sh DESCRIPTION The .Nm tap interface is a software loopback mechanism that can be loosely described as the network interface analog of the .Xr pty 4 , that is, .Nm tap does for network interfaces what the .Nm pty driver does for terminals.
p The .Nm tap driver, like the .Nm pty driver, provides two interfaces: an interface like the usual facility it is simulating
o an Ethernet network interface in the case of .Nm tap , or a terminal for .Nm pty
c , and a character-special device .Dq control interface.
p The network interfaces are named .Sy tap Ns Ar 0 , .Sy tap Ns Ar 1 , etc, as many as were made by .Xr MAKEDEV 8 . Each one supports the usual Ethernet network-interface .Xr ioctl 2 Ns s , such as .Dv SIOCSIFADDR and .Dv SIOCSIFNETMASK , 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
o it appears as .Dq input there
c ; writing an Ethernet frame to the control device generates an input frame on the network interface, as if the
q non-existent hardware had just received it.
p The Ethernet tunnel device, normally
a /dev/tap Ns Sy N , is exclusive-open
o it cannot be opened if it is already open
c and is restricted to the super-user. A .Fn read call will return an error
q 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.
p 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
q e.g., no buffer space available , it is silently dropped; if the reason is not transient
q e.g., frame too large , an error is returned. The following .Xr ioctl 2 calls are supported
q defined in Aq Pa net/if_tap.h Ns : l -tag -width VMIO_SIOCSETMACADDR t 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. t Dv TAPGDEBUG The argument should be a pointer to an .Va int ; this stores the internal debugging variable's value into it. t Dv FIONBIO Turn non-blocking I/O for reads off or on, according as the argument .Va int Ns 's value is or isn't zero
q Writes are always nonblocking . t Dv FIOASYNC Turn asynchronous I/O for reads
o i.e., generation of .Dv SIGIO when data is available to be read
c off or on, according as the argument .Va int Ns 's value is or isn't zero. t 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. t Dv TIOCSPGRP Set the process group to receive .Dv SIGIO signals, when asynchronous I/O is enabled, to the argument .Va int value. t Dv TIOCGPGRP Retrieve the process group value for .Dv SIGIO signals into the argument .Va int value. t SIOCGIFADDR Retrieve the Media Access Control
q MAC address. This command should be executed on descriptor, associated with control device
q Pa /dev/tap Ns Sy N . The .Va buffer , which is passed as argument, is expected to have enought space to store
q MAC address. t SIOCSIFADDR Set the Media Access Control
q MAC address. This command should be executed on a descriptor, associated with control device
p The control device also supports .Xr select 2 for read; selecting for write is pointless, and always succeeds, since writes are always non-blocking.
p On the last close of the data device, by default, the interface is brought down
o as if with .Dq ifconfig tap Ns Sy N No down
c . All queued frames are thrown away. If the interface is up when the data device is not open output frames are always thrown away rather than letting them pile up.
p The .Nm tap device is also can be used with VMware port as a replacement of VMnet device driver. The driver uses minor number to select between .Nm tap and .Nm vmnet devices. VMnet minor numbering is .Va 0x10000 + .Va N . Where .Va N is a VMnet unit number. In this case control device is expected to be
a /dev/vmnet Ns Sy N , and network interface will be .Sy vmnet Ns Ar N . Everything else is the same.
p
In addition to mentioned above
.Xr ioctl 2
there are additional one for VMware port.
l -tag -width VMIO_SIOCSETMACADDR t VMIO_SIOCSIFFLAGS VMware
.Dv SIOCSIFFLAGS .
.El
.Sh SEE ALSO
.Xr inet 4 ,
.Xr intro 4
.Sh BUGS
.Sh AUTHORS
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