1.\" $NetBSD: tun.4,v 1.1 1996/06/25 22:17:37 pk Exp $ 2.\" $FreeBSD$ 3.\" Based on PR#2411 4.\" 5.Dd March 10, 1996 6.Dt TUN 4 7.Os 8.Sh NAME 9.Nm tun 10.Nd tunnel software network interface 11.Sh SYNOPSIS 12.Cd pseudo-device tun 13.Sh DESCRIPTION 14The 15.Nm 16interface is a software loopback mechanism that can be loosely 17described as the network interface analog of the 18.Xr pty 4 , 19that is, 20.Nm 21does for network interfaces what the 22.Nm pty 23driver does for terminals. 24.Pp 25The 26.Nm 27driver, like the 28.Nm pty 29driver, provides two interfaces: an interface like the usual facility 30it is simulating 31.Po 32a network interface in the case of 33.Nm , 34or a terminal for 35.Nm pty 36.Pc , 37and a character-special device 38.Dq control 39interface. 40.Pp 41The network interfaces are named 42.Sy tun Ns Ar 0 , 43.Sy tun Ns Ar 1 , 44etc, as many as were made by 45.Xr MAKEDEV 8 . 46Each one supports the usual network-interface 47.Xr ioctl 2 Ns s , 48such as 49.Dv SIOCSIFADDR 50and 51.Dv SIOCSIFNETMASK , 52and thus can be used with 53.Xr ifconfig 8 54like any other interface. At boot time, they are 55.Dv POINTOPOINT 56interfaces, but this can be changed; see the description of the control 57device, below. When the system chooses to transmit a packet on the 58network interface, the packet can be read from the control device 59.Po 60it appears as 61.Dq input 62there 63.Pc ; 64writing a packet to the control device generates an input 65packet on the network interface, as if the 66.Pq non-existent 67hardware had just received it. 68.Pp 69The tunnel device, normally 70.Pa /dev/tun Ns Sy N , 71is exclusive-open 72.Po 73it cannot be opened if it is already open 74.Pc 75and is restricted to the super-user. 76A 77.Fn read 78call will return an error 79.Pq Er EHOSTDOWN 80if the interface is not 81.Dq ready 82.Po 83which means that the control device is open and the interface's 84address has been set 85.Pc . 86Once the interface is ready, 87.Fn read 88will return a packet if one is available; if not, it will either block 89until one is or return 90.Er EWOULDBLOCK , 91depending on whether non-blocking I/O has been enabled. If the packet 92is longer than is allowed for in the buffer passed to 93.Fn read , 94the extra data will be silently dropped. 95.Pp 96Packets can be optionally prepended with the destination address as presented 97to the network interface output routine 98.Pq Sq Li tunoutput . 99The destination address is in 100.Sq Li struct sockaddr 101format. 102The actual length of the prepended address is in the member 103.Sq Li sa_len . 104The packet data follows immediately. 105A 106.Xr write 2 107call passes a packet in to be 108.Dq received 109on the pseudo-interface. Each 110.Fn write 111call supplies exactly one packet; the packet length is taken from the 112amount of data provided to 113.Fn write . 114Writes will not block; if the packet cannot be accepted for a 115transient reason 116.Pq e.g., no buffer space available , 117it is silently dropped; if the reason is not transient 118.Pq e.g., packet too large , 119an error is returned. 120If 121.Dq link-layer mode 122is on 123.Pq see Dv TUNSLMODE No below , 124the actual packet data must be preceded by a 125.Sq Li struct sockaddr . 126The driver currently only inspects the 127.Sq Li sa_family 128field. 129The following 130.Xr ioctl 2 131calls are supported 132.Pq defined in Aq Pa net/if_tun.h Ns : 133.Bl -tag -width TUNSIFMODE 134.It Dv TUNSDEBUG 135The argument should be a pointer to an 136.Va int ; 137this sets the internal debugging variable to that value. What, if 138anything, this variable controls is not documented here; see the source 139code. 140.It Dv TUNGDEBUG 141The argument should be a pointer to an 142.Va int ; 143this stores the internal debugging variable's value into it. 144.It Dv TUNSIFMODE 145The argument should be a pointer to an 146.Va int ; 147its value must be either 148.Dv IFF_POINTOPOINT 149or 150.Dv IFF_BROADCAST . 151The type of the corresponding 152.Em tun Ns Sy n 153interface is set to the supplied type. If the value is anything else, 154an 155.Er EINVAL 156error occurs. The interface must be down at the time; if it is up, an 157.Er EBUSY 158error occurs. 159.It Dv TUNSLMODE 160The argument should be a pointer to an 161.Va int ; 162a non-zero value turns on 163.Dq link-layer 164mode, causing packets read from the tunnel device to be prepended with 165network destination address. 166.It Dv FIONBIO 167Turn non-blocking I/O for reads off or on, according as the argument 168.Va int Ns 's 169value is or isn't zero 170.Pq Writes are always nonblocking . 171.It Dv FIOASYNC 172Turn asynchronous I/O for reads 173.Po 174i.e., generation of 175.Dv SIGIO 176when data is available to be read 177.Pc off or on, according as the argument 178.Va int Ns 's 179value is or isn't zero. 180.It Dv FIONREAD 181If any packets are queued to be read, store the size of the first one 182into the argument 183.Va int ; 184otherwise, store zero. 185.It Dv TIOCSPGRP 186Set the process group to receive 187.Dv SIGIO 188signals, when asynchronous I/O is enabled, to the argument 189.Va int 190value. 191.It Dv TIOCGPGRP 192Retrieve the process group value for 193.Dv SIGIO 194signals into the argument 195.Va int 196value. 197.El 198.Pp 199The control device also supports 200.Xr select 2 201for read; selecting for write is pointless, and always succeeds, since 202writes are always non-blocking. 203.Pp 204On the last close of the data device, by default, the interface is 205brought down 206.Po as if with 207.Dq ifconfig tun Ns Sy n No down 208.Pc . 209All queued packets are thrown away. 210If the interface is up when the data device is not open 211output packets are always thrown away rather than letting 212them pile up. 213.Sh SEE ALSO 214.Xr inet 4 , 215.Xr intro 4 216.Sh BUGS 217Currently is IP-only. 218.Sh AUTHORS 219This man page has been obtained from 220.Bx Net . 221