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STF(4)                     Kernel Interfaces Manual                     STF(4)

     stf -- 6to4 tunnel interface

     pseudo-device stf

     The stf interface supports ``6to4'' IPv6 in IPv4 encapsulation.  It can
     tunnel IPv6 traffic over IPv4, as specified in RFC3056.  stf interfaces
     are dynamically created and destroyed with the ifconfig(8) create and
     destroy subcommands.  Only one stf interface may be created.

     For ordinary nodes in 6to4 sites, you do not need a stf interface.  The
     stf interface is only necessary on the site border router (called the
     ``6to4 router'' in the specification).

     Due to the way the 6to4 protocol is specified, stf interfaces require
     certain configuration to work properly.  A single (no more than one)
     valid 6to4 address needs to be configured on the interface.  ``A valid
     6to4 address'' is an address which has the following properties.  If any
     of the following properties are not satisfied, stf raises a runtime error
     on packet transmission.  Read the specification for more details.

     o   matches 2002:xxyy:zzuu::/48, where xxyy:zzuu is the hexadecimal
         notation of an IPv4 address for the node.  The IPv4 address used can
         be taken from any interface your node has.  Since the specification
         forbids the use of IPv4 private address, the address needs to be a
         global IPv4 address.

     o   Subnet identifier portion (48th to 63rd bit) and interface identifier
         portion (lower 64 bits) are properly filled to avoid address

     If you would like the node to behave as a relay router, the prefix length
     for the IPv6 interface address needs to be 16 so that the node would
     consider any 6to4 destination as ``on-link''.  If you would like to
     restrict 6to4 peers to be inside a certain IPv4 prefix, you may want to
     configure the IPv6 prefix length to be ``16 + IPv4 prefix length''.  The
     stf interface will check the IPv4 source address on packets if the IPv6
     prefix length is larger than 16.

     stf can be configured to be ECN (Explicit Congestion Notification)
     friendly.  This can be configured by IFF_LINK1.  See gif(4) for details.

     Please note that the 6to4 specification is written as an ``accept
     tunneled packet from everyone'' tunneling device.  By enabling the stf
     device, you are making it much easier for malicious parties to inject
     fabricated IPv6 packets to your node.  Also, malicious parties can inject
     IPv6 packets with fabricated source addresses to make your node generate
     improper tunneled packets.  Administrators must be cautious when enabling
     the interface.  To prevent possible attacks, the stf interface filters
     out the following packets (note that the checks are in no way complete):

     o   Packets with IPv4 unspecified addresses as outer IPv4
         source/destination (

     o   Packets with the loopback address as outer IPv4 source/destination

     o   Packets with IPv4 multicast addresses as outer IPv4
         source/destination (

     o   Packets with limited broadcast addresses as outer IPv4
         source/destination (

     o   Packets with private addresses as outer IPv4 source/destination

     o   Packets with IPv4 link-local addresses as outer IPv4
         source/destination (

     o   Packets with subnet broadcast addresses as outer IPv4
         source/destination.  The check is made against subnet broadcast
         addresses for all of the directly connected subnets.

     o   Packets that do not pass ingress filtering.  Outer IPv4 source
         addresses must meet the IPv4 topology on the routing table.  Ingress
         filtering can be turned off by IFF_LINK2 bit.

     o   The same set of rules are applied against the IPv4 address embedded
         into the inner IPv6 address, if the IPv6 address matches the 6to4

     o   Packets with site-local or link-local unicast addresses as inner IPv6

     o   Packets with node-local or link-local multicast addresses as inner
         IPv6 source/destination

     It is recommended to filter/audit incoming IPv4 packets with IP protocol
     number 41, as necessary.  It is also recommended to filter/audit
     encapsulated IPv6 packets as well.  You may also want to run normal
     ingress filtering against inner IPv6 addresses to avoid spoofing.

     By setting the IFF_LINK0 flag on the stf interface, it is possible to
     disable the input path, making direct attacks from the outside
     impossible.  Note, however, that other security risks exist.  If you wish
     to use the configuration, you must not advertise your 6to4 addresses to

     Note that 8504:0506 is equal to, written in hexadecimal.

     # ifconfig ne0 inet netmask 0xffffff00
     # ifconfig stf0 create inet6 2002:8504:0506:0000:a00:5aff:fe38:6f86 \
             prefixlen 16 alias

     The following configuration accepts packets from IPv4 source address only.  It emits 6to4 packets only for IPv6 destination
     2002:0901::/32 (IPv4 destination will match

     # ifconfig ne0 inet netmask 0xffff0000
     # ifconfig stf0 create inet6 2002:0901:0203:0000:a00:5aff:fe38:6f86 \
             prefixlen 32 alias

     The following configuration uses the stf interface as an output-only
     device.  You need to have alternative IPv6 connectivity (other than 6to4)
     to use this configuration.  For outbound traffic, you can reach other
     6to4 networks efficiently via stf.  For inbound traffic, you will not
     receive any 6to4-tunneled packets (less security drawbacks).  Be careful
     not to advertise your 6to4 prefix to others (2002:8504:0506::/48), and
     not to use your 6to4 prefix as a source address.

     # ifconfig ne0 inet netmask 0xffffff00
     # ifconfig stf0 create inet6 2002:8504:0506:0000:a00:5aff:fe38:6f86 \
             prefixlen 16 alias deprecated link0
     # route add -inet6 2002:: -prefixlen 16 ::1 -ifp stf0

     gif(4), inet(4), inet6(4)

     Brian Carpenter and Keith Moore, Connection of IPv6 Domains via IPv4
     Clouds, RFC, 3056, February 2001.

     C. Huitema, An Anycast Prefix for 6to4 Relay Routers, RFC, 3068, June

     F. Baker and P. Savola, Ingress Filtering for Multihomed Networks, RFC,
     3704, March 2004.

     P. Savola and C. Patel, Security Considerations for 6to4, RFC, 3964,
     December 2004.

     Jun-ichiro itojun Hagino, Possible abuse against IPv6 transition
     technologies, draft-itojun-ipv6-transition-abuse-01.txt, July 2000,
     expired, work in progress.

     The stf device first appeared in WIDE/KAME IPv6 stack.

     No more than one stf interface is allowed for a node, and no more than
     one IPv6 interface address is allowed for an stf interface.  This is to
     avoid source address selection conflicts between the IPv6 layer and the
     IPv4 layer, and to cope with ingress filtering rules on the other side.
     This is a feature to make stf work right for all occasions.

NetBSD 6.1.5                    January 2, 2011                   NetBSD 6.1.5