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.Dd November 16, 2021
.Dt STF 4
.Os
.Sh NAME
.Nm stf
.Nd
.Tn 6to4
tunnel interface
.Sh SYNOPSIS
.Cd "device stf"
.Sh DESCRIPTION
The
.Nm
interface supports
.Dq 6to4
and
.Dq 6rd
IPv6 in IPv4 encapsulation.
It can tunnel IPv6 traffic over IPv4, as specified in
.Li RFC3056
or
.Li RFC5969 .
.Pp
For ordinary nodes in a 6to4 or 6RD site, you do not need
.Nm
interface.
The
.Nm
interface is necessary for site border routers
(called
.Dq 6to4 routers
or
.Dq 6rd Customer Edge (CE)
in the specification).
.Pp
Each
.Nm
interface is created at runtime using interface cloning.
This is
most easily done with the
.Xr ifconfig 8
.Cm create
command or using the
.Va cloned_interfaces
variable in
.Xr rc.conf 5 .
.Sh 6to4
Due to the way 6to4 protocol is specified,
.Nm
interface requires certain configuration to work properly.
Single
(no more than 1)
valid 6to4 address needs to be configured to the interface.
.Dq A valid 6to4 address
is an address which has the following properties.
If any of the following properties are not satisfied,
.Nm
raises runtime error on packet transmission.
Read the specification for more details.
.Bl -bullet
.It
matches
.Li 2002:xxyy:zzuu::/48
where
.Li 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.
.It
Subnet identifier portion
(48th to 63rd bit)
and interface identifier portion
(lower 64 bits)
are properly filled to avoid address collisions.
.El
.Pp
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
.Dq on-link .
If you would like to restrict 6to4 peers to be inside certain IPv4 prefix,
you may want to configure IPv6 prefix length as
.Dq 16 + IPv4 prefix length .
.Nm
interface will check the IPv4 source address on packets,
if the IPv6 prefix length is larger than 16.
.Pp
.Nm
can be configured to be ECN friendly.
This can be configured by
.Dv IFF_LINK1 .
See
.Xr gif 4
for details.
.Pp
Please note that 6to4 specification is written as
.Dq accept tunnelled packet from everyone
tunnelling device.
By enabling
.Nm
device, you are making it much easier for malicious parties to inject
fabricated 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,
.Nm
interface filters out the following packets.
Note that the checks are no way complete:
.Bl -bullet
.It
Packets with IPv4 unspecified address as outer IPv4 source/destination
.Pq Li 0.0.0.0/8
.It
Packets with loopback address as outer IPv4 source/destination
.Pq Li 127.0.0.0/8
.It
Packets with IPv4 multicast address as outer IPv4 source/destination
.Pq Li 224.0.0.0/4
.It
Packets with limited broadcast address as outer IPv4 source/destination
.Pq Li 255.0.0.0/8
.It
Packets with private address as outer IPv4 source/destination
.Pq Li 10.0.0.0/8 , 172.16.0.0/12 , 192.168.0.0/16
.It
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.
.It
Packets that does not pass ingress filtering.
Outer IPv4 source address must meet the IPv4 topology on the routing table.
Ingress filter can be turned off by
.Dv IFF_LINK2
bit.
.It
The same set of rules are applied against the IPv4 address embedded into
inner IPv6 address, if the IPv6 address matches 6to4 prefix.
.El
.Pp
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.
.Pp
By setting the
.Dv IFF_LINK0
flag on the
.Nm
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.
.\"
.Sh 6rd
Like
.Dq 6to4
.Dq 6rd
also requires configuration before it can be used.
The required configuration parameters are:
.Bl -bullet
.It
The IPv6 address and prefix length.
.It
The border router IPv4 address.
.It
The IPv4 WAN address.
.It
The prefix length of the IPv4 WAN address.
.El
.Pp
These can parameters are all configured through
.Xr ifconfig 8 .
.Pp
The IPv6 address and prefix length can be configured like any other IPv6 address.
Note that the prefix length is the IPv6 prefix length excluding the embedded
IPv4 address bits.
The prefix length of the delegated network is the sum of the IPv6 prefix length
and the IPv4 prefix length.
.Pp
The border router IPv4 address is configured with the
.Xr ifconfig 8
.Cm stfv4br
command.
.Pp
The IPv4 WAN address and IPv4 prefix length are configured using the
.Xr ifconfig 8
.Cm stfv4net
command.
.Sh SYSCTL VARIABLES
The following
.Xr sysctl 8
variables can be used to control the behavior of the
.Nm stf .
The default value is shown next to each variable.
.Bl -tag -width indent
.It Va net.link.stf.permit_rfc1918 : No 0
The RFC3056 requires the use of globally unique 32-bit IPv4
addresses.
This sysctl variable controls the behaviour of this requirement.
When it set to not 0,
.Nm stf
allows the use of private IPv4 addresses described in the RFC1918.
This may be useful for an Intranet environment or when some mechanisms
of network address translation (NAT) are used.
.El
.Sh EXAMPLES
Note that
.Li 8504:0506
is equal to
.Li 133.4.5.6 ,
written in hexadecimals.
.Bd -literal
# ifconfig ne0 inet 133.4.5.6 netmask 0xffffff00
# ifconfig stf0 inet6 2002:8504:0506:0000:a00:5aff:fe38:6f86 \\
	prefixlen 16 alias
.Ed
.Pp
The following configuration accepts packets from IPv4 source
.Li 9.1.0.0/16
only.
It emits 6to4 packet only for IPv6 destination 2002:0901::/32
(IPv4 destination will match
.Li 9.1.0.0/16 ) .
.Bd -literal
# ifconfig ne0 inet 9.1.2.3 netmask 0xffff0000
# ifconfig stf0 inet6 2002:0901:0203:0000:a00:5aff:fe38:6f86 \\
	prefixlen 32 alias
.Ed
.Pp
The following configuration uses the
.Nm
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
.Nm 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
.Pq Li 2002:8504:0506::/48 ,
and not to use your 6to4 prefix as a source.
.Bd -literal
# 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
.Ed
.Pp
The following example configures a
.Dq 6rd
tunnel on a
.Dq 6rd CE
where the ISP's
.Dq 6rd
IPv6 prefix is 2001:db8::/32.
The border router is 192.0.2.1.
The
.Dq 6rd CE
has a WAN address of 192.0.2.2 and the full IPv4 address is embedded in the
.Dq 6rd IPv6 address:
.Bd -literal
# ifconfig stf0 inet6 2001:db8:c000:0202:: prefixlen 32 up
# ifconfig stf0 stfv4br 192.0.2.1
# ifconfig stf0 stfv4net 192.0.2.2/32
.Ed
.\"
.Sh SEE ALSO
.Xr gif 4 ,
.Xr inet 4 ,
.Xr inet6 4
.Rs
.%A Brian Carpenter
.%A Keith Moore
.%T "Connection of IPv6 Domains via IPv4 Clouds"
.%D February 2001
.%R RFC
.%N 3056
.Re
.Rs
.%A Jun-ichiro itojun Hagino
.%T "Possible abuse against IPv6 transition technologies"
.%D July 2000
.%N draft-itojun-ipv6-transition-abuse-01.txt
.%O work in progress
.Re
.\"
.Sh HISTORY
The
.Nm
device first appeared in WIDE/KAME IPv6 stack.
.\"
.Sh BUGS
No more than one
.Nm
interface is allowed for a node,
and no more than one IPv6 interface address is allowed for an
.Nm
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
.Nm
work right for all occasions.