tun, TUN − tunneling STREAMS module





tun and atun are STREAMS modules that implement an IP-in-IP tunneling mechanism. IPv6-in-IPv4 and IPv4-in-IPv4 tunnels are supported.

Tunnels are configured as point-to-point interfaces. Ipv4-in-Ipv4 allows IPv4 packets to be encapsulated within IPv4 packets. IPv6-in-IPv4 tunnels allow IPv6 packets to be encapsulated within IPv4 packets. Both the tunnel source and the tunnel destination are required to configure these type of tunnels. Configured tunnels support encapsulated multicast packets. See ifconfig(1M) for examples of these tunnel configurations.

The atun module is used to configure automatic tunnels. It supports IPv6 packets encapsulated within IPv4 packets. An IPv4 address is required for the tunnel source of these interfaces and the IPv4 compatible IPv6 source address must match this address. IPv6 packets using this interface must have IPv4 compatible source and destination addresses. Automatic tunnels are not point-to-point, and they do not allow multicast packets to be sent. If the destination of an automatic tunnel is a router, the packets will not be forwarded.

Network startup scripts look at /etc/hostname.ip.* to find the available tunneling interfaces.

The same tunnel source address (tsrc) and destination address (tdst) is be used for all instances (luns) of a specific interface.

Tunnels do not support snooping. Instead, a filter made up of the combination of addresses can be used on the physical interface to capture relevant packets.

If there is a tunnel set up between two multicast routers, then multicast routing should be configured to use the tunnel, rather than a special multicast routing virtual interface.


The tunnel module is architected to be plumbed between two instances of IP.

The following ioctl() calls may be used to configure a tunneling interface. The ioctl()s are defined in <sys/sockio.h>. This structure is defined in <net/if.h>.

/* currently tunnels only support IPv4 or IPv6 */
enum ifta_proto {


/* tunnel configuration structure */

struct iftun_req {
    char        ifta_lifr_name[LIFNAMSIZ];  /* if name */
    struct sockaddr_storage ifta_saddr;     /* source address */
    struct sockaddr_storage ifta_daddr;     /* destination address */
    uint_t      ifta_flags;                 /* See below */
                                /* IP version information is read only */
    enum ifta_proto ifta_upper;             /* IP version above tunnel */
    enum ifta_proto ifta_lower;             /* IP versin below tunnel */
    uint_t      ifta_vers;                  /* Version number */
    uint32_t    ifta_secinfo[IFTUN_SECINFOLEN]; /* Security prefs. */
           /* These flags are set to indicate which members are valid */

#define    IFTUN_SRC             0x01
#define    IFTUN_DST             0x02
#define    IFTUN_SECURITY        0x04

The ifta_vers field indicates what IPsec request structure is overlayed on top of ifta_secinfo. The current value of IFTUN_VERSION implies an overlay of ipsec_req_t. See ipsec(7P).

Set tunnel parameters. This ioctl() allows the tunnel’s source or destination address to be set. The IFTUN_SRC bit set in ta_flags indicates that the tunnel should bound to the source
address supplied in ta_saddr. The source must be a valid configured interface IP address. The IFTUN_DST bit set in ta_flags indicates that the tunnel should bound to the destination address supplied in ta_daddr. The destination address must be reachable.


Get tunnel parameters. Valid fields are indicated by the returned value of ta_flags bitmask. The version of IP plumbed above or below the tunnel may be determined by
inspecting ta_upper and ta_lower by comparing the members against the mutually exclusive defined values IFTAP_INVALID, IFTAP_IPV4, and IFTAP_IPV6. Currently, only IFTAP_IPV4 is supported, as IP is currently version 4.

Tunnels and DLPI
The tunnel module is a DLPI style 2 service provider. All M_PROTO and M_PCPROTO type messages are interpreted as DLPIprimitives. Valid DLPI primitives are defined in <sys/dlpi.h>. Refer to dlpi(7P) for more information. An explicit DL_ATTACH_REQ message by the user is required to associate the opened stream with a particular device (ppa). The ppa indicates the corresponding device instance (unit) number. The device is initialized on first attach and deinitialized (stopped) on last detach.

The values returned by the module in the DL_INFO_ACK primitive in response to the DL_INFO_REQ from the user are as follows:

The maximum SDU is usually 4196 ("ip_max_mtu - size of IP header").

The minimum SDU is 1.

The dlsap address length is 0 for configured tunnels and non-zero for automatic tunnels.

The MAC type is DL_OTHER.

The sap length value is 0.

The service mode is DL_CLDLS.

No optional quality of service (QOS) support is included at present so the QOS fields are 0.

The provider style is DL_STYLE2.

The version is DL_VERSION_2.

The broadcast address value is 0

Once in the DL_ATTACHED state, the user must send a DL_BIND_REQ to associate a particular SAP (Service Access Pointer) with the stream. The tunneling module interprets the sap field within the DL_BIND_REQ as an IP "type" therefore the valid value for the sap field is IP_DL_SAP.

Once in the DL_BOUND state, the user may transmit packets through the tunnel by sending DL_UNITDATA_REQ messages to the tunnel module. Configured tunnels will encapsulate the packet with the appropriate IP header using the source and destination specified by tsrc and tdst parameters of ifconfig(1M). The tunnel module will decapsulate received
packets and route them to the first open and bound stream having a sap, tsrc and tdst which matches the the configured information. Packets are routed to exactly one open stream and not duplicated.

The module does not support additional primitives. DL_ERROR_ACK with the dl_error set to DL_UNSUPPORTED will be returned in the case that an unsupported DLPI primitive is encountered.


A tunnel creates what appears to be a physical interface to IP. It can be "trusted" as a physical link only so far as the underlying security protocols, if used, can be trusted. If the security associations (see ipsec(7P) are securely set up then the tunnel can be trusted in that packets that come off the tunnel came from the peer specified in the tunnel destination. If this trust exists, per-interface IP forwarding can be used to create a Virtual Private Network ("VPN"). See ip(7P).


See attributes(5) for descriptions of the following attributes:



System Administration Guide: IP Services

Gilligan, R. and Nordmark, E., RFC 1933, Transition Mechanisms for IPv6 Hosts and Routers, The Internet Society, 1996.