tun - tunnel software network interface
The driver, like the pty(4) driver, provides two interfaces: an interface like the usual facility it is simulating (a network interface in the case of , or a terminal for pty(4)), and a character-special device ``control'' interface.
The network interfaces are named ``tun0 '' ``tun1 '' etc., one for each control device that has been opened. These network interfaces persist until the if_tun.ko module is unloaded, or until removed with the ifconfig(8) command.
devices are created using interface cloning. This is done using the ``ifconfig tun N create '' command. This is the preferred method of creating devices. The same method allows removal of interfaces. For this, use the ``ifconfig tun N destroy '' command.
If the sysctl(8) variable net.link.tun.devfs_cloning is non-zero, the interface permits opens on the special control device /dev/tun When this device is opened, will return a handle for the lowest unused device (use devname(3) to determine which).
Bf Em Disabling the legacy devfs cloning functionality may break existing applications which use , such as ppp(8) and ssh(1). It therefore defaults to being enabled until further notice. Ef
Control devices (once successfully opened) persist until if_tun.ko is unloaded in the same way that network interfaces persist (see above).
Each interface supports the usual network-interface ioctl(2)Nss, such as SIOCAIFADDR and thus can be used with ifconfig(8) like any other interface. At boot time, they are POINTOPOINT interfaces, but this can be changed; see the description of the control device, below. When the system chooses to transmit a packet on the network interface, the packet can be read from the control device (it appears as ``input'' there); writing a packet to the control device generates an input packet on the network interface, as if the (non-existent) hardware had just received it.
The tunnel device (/dev/tun N ) is exclusive-open (it cannot be opened if it is already open). A read(2) call will return an error (Er EHOSTDOWN ) if the interface is not ``ready'' (which means that the control device is open and the interface's address has been set).
Once the interface is ready, read(2) will return a packet 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 packet is longer than is allowed for in the buffer passed to read(2), the extra data will be silently dropped.
ioctl has been set, packets read from the control device will be prepended
with the destination address as presented to the network interface output
The destination address is in Vt struct sockaddr format. The actual length of the prepended address is in the member sa_len If the TUNSIFHEAD ioctl has been set, packets will be prepended with a four byte address family in network byte order. TUNSLMODE and TUNSIFHEAD are mutually exclusive. In any case, the packet data follows immediately.
A write(2) call passes a packet in to be ``received'' on the pseudo-interface. If the TUNSIFHEAD ioctl has been set, the address family must be prepended, otherwise the packet is assumed to be of type AF_INET Each write(2) call supplies exactly one packet; the packet length is taken from the amount of data provided to write(2) (minus any supplied address family). Writes will not block; if the packet 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., packet too large), an error is returned.
The following ioctl(2) calls are supported (defined in In net/if_tun.h ) :
The use of this ioctl is restricted to the super-user.
The control device also supports select(2) for read; selecting for write is pointless, and always succeeds, since writes are always non-blocking.
On the last close of the data device, by default, the interface is brought down (as if with ifconfig tunN down All queued packets are thrown away. If the interface is up when the data device is not open output packets are always thrown away rather than letting them pile up.
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Created 1996-2021 by Maxim Chirkov
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