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STF(4)                   BSD Kernel Interfaces Manual                   STF(4)

NAME
     stf -- 6to4 tunnel interface

SYNOPSIS
     pseudo-device stf

DESCRIPTION
     The stf interface supports ``6to4'' IPv6 in IPv4 encapsulation.  It can tunnel IPv6 traffic over IPv4,
     as specified in RFC3056.

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

     Due to the way 6to4 protocol is specified, stf interface requires certain configuration to work prop-erly. properly.
     erly.  Single (no more than 1) valid 6to4 address needs to be configured to 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 runtime error on packet transmission.  Read the specification for more
     details.

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

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

     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 certain IPv4 prefix, you may want to configure IPv6 pre-fix prefix
     fix length as ``16 + IPv4 prefix length''.  stf interface will check the IPv4 source address on pack-ets, packets,
     ets, if the IPv6 prefix length is larger than 16.

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

     Please note that 6to4 specification is written as ``accept tunnelled packet from everyone'' tunnelling
     device.  By enabling stf device, you are making it much easier for malicious parties to inject fabri-cated fabricated
     cated IPv6 packet to your node.  Also, malicious party can inject an IPv6 packet with fabricated source
     address to make your node generate improper tunnelled packet.  Administrators must take caution when
     enabling the interface.  To prevent possible attacks, stf interface filters out the following packets.
     Note that the checks are no way complete:

     •   Packets with IPv4 unspecified addrss as outer IPv4 source/destination (0.0.0.0/8)

     •   Packets with loopback address as outer IPv4 source/destination (127.0.0.0/8)

     •   Packets with IPv4 multicast address as outer IPv4 source/destination (224.0.0.0/4)

     •   Packets with limited broadcast address as outer IPv4 source/destination (255.0.0.0/8)

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

     •   Packets that does not pass ingress filtering.  Outer IPv4 source address must meet the IPv4 topol-ogy topology
         ogy on the routing table.  Ingress filter can be turned off by IFF_LINK2 bit.

     •   The same set of rules are appplied against the IPv4 address embedded into inner IPv6 address, if
         the IPv6 address matches 6to4 prefix.

     It is recommended to filter/audit incoming IPv4 packet 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 filter against inner IPv6 address to avoid spoofing.

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

EXAMPLES
     Note that 8504:0506 is equal to 133.4.5.6, written in hexadecimals.

     # ifconfig ne0 inet 133.4.5.6 netmask 0xffffff00
     # ifconfig stf0 inet6 2002:8504:0506:0000:a00:5aff:fe38:6f86 \
             prefixlen 16 alias

     The following configuration accepts packets from IPv4 source 9.1.0.0/16 only.  It emits 6to4 packet
     only for IPv6 destination 2002:0901::/32 (IPv4 destination will match 9.1.0.0/16).

     # ifconfig ne0 inet 9.1.2.3 netmask 0xffff0000
     # ifconfig stf0 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 alterna-tive alternative
     tive 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-tun-neled 6to4-tunneled
     neled 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.

     # ifconfig ne0 inet 133.4.5.6 netmask 0xffffff00
     # ifconfig stf0 inet6 2002:8504:0506:0000:a00:5aff:fe38:6f86 \
             prefixlen 16 alias deprecated link0
     # route add -inet6 2002:: -prefixlen 16 ::1
     # route change -inet6 2002:: -prefixlen 16 ::1 -ifp stf0

SEE ALSO
     gif(4), inet(4), inet6 (4)

     http://www .6bone.net/6bone_6to4.html

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

     Jun-ichiro itojun Hagino, Possible abuse against IPv6 transition technologies, draft-itojun-ipv6-transition-abuse-01.txt, draft-itojunipv6-transition-abuse-01.txt,
     ipv6-transition-abuse-01.txt, July 2000, work in progress.

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

BUGS
     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.  It is to avoid source address selection conflicts between IPv6 layer and
     IPv4 layer, and to cope with ingress filtering rule on the other side.  This is a feature to make stf
     work right for all occasions.

BSD                             April 27, 2001                             BSD