<s>
In	O
computer	B-Architecture
networking	I-Architecture
,	O
Teredo	O
is	O
a	O
transition	B-Protocol
technology	I-Protocol
that	O
gives	O
full	O
IPv6	B-Protocol
connectivity	O
for	O
IPv6-capable	O
hosts	O
that	O
are	O
on	O
the	O
IPv4	B-Protocol
Internet	I-Protocol
but	O
have	O
no	O
native	O
connection	O
to	O
an	O
IPv6	B-Protocol
network	O
.	O
</s>
<s>
Unlike	O
similar	O
protocols	O
such	O
as	O
6to4	B-Protocol
,	O
it	O
can	O
perform	O
its	O
function	O
even	O
from	O
behind	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
devices	O
such	O
as	O
home	O
routers	O
.	O
</s>
<s>
Teredo	O
operates	O
using	O
a	O
platform	B-Operating_System
independent	I-Operating_System
tunneling	B-Protocol
protocol	I-Protocol
that	O
provides	O
IPv6	B-Protocol
(	O
Internet	B-Protocol
Protocol	I-Protocol
version	I-Protocol
6	I-Protocol
)	O
connectivity	O
by	O
encapsulating	B-Protocol
IPv6	B-Protocol
datagram	B-Protocol
packets	O
within	O
IPv4	B-Protocol
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	B-Protocol
)	O
packets	O
.	O
</s>
<s>
Teredo	O
routes	O
these	O
datagrams	B-Protocol
on	O
the	O
IPv4	B-Protocol
Internet	I-Protocol
and	O
through	O
NAT	B-Protocol
devices	I-Protocol
.	O
</s>
<s>
Teredo	O
nodes	O
elsewhere	O
on	O
the	O
IPv6	B-Protocol
network	O
(	O
called	O
Teredo	O
relays	O
)	O
receive	O
the	O
packets	O
,	O
un-encapsulate	O
them	O
,	O
and	O
pass	O
them	O
on	O
.	O
</s>
<s>
In	O
the	O
long	O
term	O
,	O
all	O
IPv6	B-Protocol
hosts	O
should	O
use	O
native	O
IPv6	B-Protocol
connectivity	O
.	O
</s>
<s>
Teredo	O
should	O
be	O
disabled	O
when	O
native	O
IPv6	B-Protocol
connectivity	O
becomes	O
available	O
.	O
</s>
<s>
The	O
Teredo	O
server	O
listens	O
on	O
UDP	B-Protocol
port	O
3544	O
.	O
</s>
<s>
For	O
6to4	B-Protocol
,	O
the	O
most	O
common	O
IPv6	B-Protocol
over	O
IPv4	B-Protocol
tunneling	B-Protocol
protocol	I-Protocol
,	O
requires	O
that	O
the	O
tunnel	O
endpoint	O
have	O
a	O
public	O
IPv4	B-Protocol
address	O
.	O
</s>
<s>
However	O
,	O
many	O
hosts	O
currently	O
attach	O
to	O
the	O
IPv4	B-Protocol
Internet	I-Protocol
through	O
one	O
or	O
several	O
NAT	B-Protocol
devices	I-Protocol
,	O
usually	O
because	O
of	O
IPv4	B-Protocol
address	I-Protocol
shortage	I-Protocol
.	O
</s>
<s>
In	O
such	O
a	O
situation	O
,	O
the	O
only	O
available	O
public	O
IPv4	B-Protocol
address	O
is	O
assigned	O
to	O
the	O
NAT	B-Protocol
device	I-Protocol
,	O
and	O
the	O
6to4	B-Protocol
tunnel	O
endpoint	O
must	O
be	O
implemented	O
on	O
the	O
NAT	B-Protocol
device	I-Protocol
itself	O
.	O
</s>
<s>
The	O
problem	O
is	O
that	O
many	O
NAT	B-Protocol
devices	I-Protocol
currently	O
deployed	O
cannot	O
be	O
upgraded	O
to	O
implement	O
6to4	B-Protocol
,	O
for	O
technical	O
or	O
economic	O
reasons	O
.	O
</s>
<s>
Teredo	O
alleviates	O
this	O
problem	O
by	O
encapsulating	B-Protocol
IPv6	B-Protocol
packets	O
within	O
UDP/IPv4	O
datagrams	B-Protocol
,	O
which	O
most	O
NATs	O
can	O
forward	O
properly	O
.	O
</s>
<s>
Thus	O
,	O
IPv6-aware	O
hosts	O
behind	O
NATs	O
can	O
serve	O
as	O
Teredo	B-Protocol
tunnel	I-Protocol
endpoints	O
even	O
when	O
they	O
do	O
n't	O
have	O
a	O
dedicated	O
public	O
IPv4	B-Protocol
address	O
.	O
</s>
<s>
In	O
effect	O
,	O
a	O
host	O
that	O
implements	O
Teredo	O
can	O
gain	O
IPv6	B-Protocol
connectivity	O
with	O
no	O
cooperation	O
from	O
the	O
local	O
network	O
environment	O
.	O
</s>
<s>
In	O
the	O
long	O
term	O
,	O
all	O
IPv6	B-Protocol
hosts	O
should	O
use	O
native	O
IPv6	B-Protocol
connectivity	O
.	O
</s>
<s>
The	O
temporary	O
Teredo	O
protocol	O
includes	O
provisions	O
for	O
a	O
sunset	O
procedure	O
:	O
Teredo	O
implementation	O
should	O
provide	O
a	O
way	O
to	O
stop	O
using	O
Teredo	O
connectivity	O
when	O
IPv6	B-Protocol
matures	O
and	O
connectivity	O
becomes	O
available	O
using	O
a	O
less	O
brittle	O
mechanism	O
.	O
</s>
<s>
Teredo	O
client	O
A	O
host	O
that	O
has	O
IPv4	B-Protocol
connectivity	O
to	O
the	O
Internet	O
from	O
behind	O
a	O
NAT	O
and	O
uses	O
the	O
Teredo	B-Protocol
tunneling	I-Protocol
protocol	O
to	O
access	O
the	O
IPv6	B-Protocol
Internet	I-Protocol
.	O
</s>
<s>
Teredo	O
clients	O
are	O
assigned	O
an	O
IPv6	B-Protocol
address	I-Protocol
that	O
starts	O
with	O
the	O
Teredo	O
prefix	O
(	O
2001::/32	O
)	O
.	O
</s>
<s>
Teredo	O
server	O
A	O
well-known	O
host	O
used	O
for	O
initial	O
configuration	O
of	O
a	O
Teredo	B-Protocol
tunnel	I-Protocol
.	O
</s>
<s>
A	O
Teredo	O
server	O
never	O
forwards	O
any	O
traffic	O
for	O
the	O
client	O
(	O
apart	O
from	O
IPv6	B-Protocol
pings	O
)	O
,	O
and	O
has	O
therefore	O
modest	O
bandwidth	O
requirements	O
(	O
a	O
few	O
hundred	O
bits	O
per	O
second	O
per	O
client	O
at	O
most	O
)	O
,	O
which	O
means	O
a	O
single	O
server	O
can	O
support	O
many	O
clients	O
.	O
</s>
<s>
Additionally	O
,	O
a	O
Teredo	O
server	O
can	O
be	O
implemented	O
in	O
a	O
fully	O
stateless	B-Protocol
manner	O
,	O
thus	O
using	O
the	O
same	O
amount	O
of	O
memory	O
regardless	O
of	O
how	O
many	O
clients	O
it	O
supports	O
.	O
</s>
<s>
Teredo	O
relay	O
The	O
remote	O
end	O
of	O
a	O
Teredo	B-Protocol
tunnel	I-Protocol
.	O
</s>
<s>
Each	O
Teredo	O
relay	O
serves	O
a	O
range	O
of	O
IPv6	B-Protocol
hosts	O
(	O
e.g.	O
</s>
<s>
a	O
single	O
campus	O
or	O
company	O
,	O
an	O
ISP	O
or	O
a	O
whole	O
operator	O
network	O
,	O
or	O
even	O
the	O
whole	O
IPv6	B-Protocol
Internet	I-Protocol
)	O
;	O
it	O
forwards	O
traffic	O
between	O
any	O
Teredo	O
clients	O
and	O
any	O
host	O
within	O
said	O
range	O
.	O
</s>
<s>
A	O
computer	O
with	O
a	O
host-specific	O
relay	O
uses	O
Teredo	O
to	O
communicate	O
with	O
Teredo	O
clients	O
,	O
but	O
sticks	O
to	O
its	O
main	O
IPv6	B-Protocol
connectivity	O
provider	O
to	O
reach	O
the	O
rest	O
of	O
the	O
IPv6	B-Protocol
Internet	I-Protocol
.	O
</s>
<s>
Each	O
Teredo	O
client	O
is	O
assigned	O
a	O
public	O
IPv6	B-Protocol
address	I-Protocol
,	O
which	O
is	O
constructed	O
as	O
follows	O
(	O
the	O
higher	O
order	O
bit	O
is	O
numbered	O
0	O
)	O
:	O
</s>
<s>
Bits	O
32	O
to	O
63	O
embed	O
the	O
primary	O
IPv4	B-Protocol
address	O
of	O
the	O
Teredo	O
server	O
that	O
is	O
used	O
.	O
</s>
<s>
The	O
12	O
"	O
A	O
"	O
bits	O
were	O
0	O
in	O
the	O
original	O
RFC	O
4380	O
specification	O
,	O
but	O
were	O
changed	O
to	O
random	O
bits	O
chosen	O
by	O
the	O
Teredo	O
client	O
in	O
RFC	O
5991	O
to	O
provide	O
the	O
Teredo	O
node	O
with	O
additional	O
protection	O
against	O
IPv6-based	O
scanning	O
attacks	O
.	O
</s>
<s>
Bits	O
80	O
to	O
95	O
contain	O
the	O
obfuscated	O
UDP	B-Protocol
port	O
number	O
.	O
</s>
<s>
Bits	O
96	O
to	O
127	O
contain	O
the	O
obfuscated	O
IPv4	B-Protocol
address	O
.	O
</s>
<s>
This	O
is	O
the	O
public	O
IPv4	B-Protocol
address	O
of	O
the	O
NAT	O
with	O
all	O
bits	O
inverted	O
.	O
</s>
<s>
As	O
an	O
example	O
,	O
the	O
IPv6	B-Protocol
address	I-Protocol
2001:0000:4136:e378:8000:63bf:3fff:fdd2	O
refers	O
to	O
a	O
Teredo	O
client	O
that	O
:	O
</s>
<s>
Teredo	O
clients	O
use	O
Teredo	O
servers	O
to	O
autodetect	O
the	O
kind	O
of	O
NAT	O
they	O
are	O
behind	O
(	O
if	O
any	O
)	O
,	O
through	O
a	O
simplified	O
STUN-like	O
qualification	O
procedure	O
.	O
</s>
<s>
Teredo	O
clients	O
also	O
maintain	O
a	O
binding	O
on	O
their	O
NAT	O
toward	O
their	O
Teredo	O
server	O
by	O
sending	O
a	O
UDP	B-Protocol
packet	I-Protocol
at	O
regular	O
intervals	O
.	O
</s>
<s>
If	O
a	O
Teredo	O
relay	O
(	O
or	O
another	O
Teredo	O
client	O
)	O
must	O
send	O
an	O
IPv6	B-Protocol
packet	O
to	O
a	O
Teredo	O
client	O
,	O
it	O
first	O
sends	O
a	O
Teredo	O
bubble	O
packet	O
to	O
the	O
client	O
's	O
Teredo	O
server	O
,	O
whose	O
IP	O
address	O
it	O
infers	O
from	O
the	O
Teredo	O
IPv6	B-Protocol
address	I-Protocol
of	O
the	O
Teredo	O
client	O
.	O
</s>
<s>
Teredo	O
servers	O
can	O
also	O
transmit	O
ICMPv6	O
packet	O
from	O
Teredo	O
clients	O
toward	O
the	O
IPv6	B-Protocol
Internet	I-Protocol
.	O
</s>
<s>
In	O
practice	O
,	O
when	O
a	O
Teredo	O
client	O
wants	O
to	O
contact	O
a	O
native	O
IPv6	B-Protocol
node	O
,	O
it	O
must	O
locate	O
the	O
corresponding	O
Teredo	O
relay	O
,	O
i.e.	O
,	O
to	O
which	O
public	O
IPv4	B-Protocol
and	O
UDP	B-Protocol
port	O
number	O
to	O
send	O
encapsulated	O
IPv6	B-Protocol
packets	O
.	O
</s>
<s>
To	O
do	O
that	O
,	O
the	O
client	O
crafts	O
an	O
ICMPv6	O
Echo	O
Request	O
(	O
ping	O
)	O
toward	O
the	O
IPv6	B-Protocol
node	O
,	O
and	O
sends	O
it	O
through	O
its	O
configured	O
Teredo	O
server	O
.	O
</s>
<s>
The	O
Teredo	O
server	O
de-capsulates	O
the	O
ping	O
onto	O
the	O
IPv6	B-Protocol
Internet	I-Protocol
,	O
so	O
that	O
the	O
ping	O
should	O
eventually	O
reach	O
the	O
IPv6	B-Protocol
node	O
.	O
</s>
<s>
The	O
IPv6	B-Protocol
node	O
should	O
then	O
reply	O
with	O
an	O
ICMPv6	O
Echo	O
Reply	O
,	O
as	O
mandated	O
by	O
RFC	O
2460	O
.	O
</s>
<s>
Maintaining	O
a	O
Teredo	O
server	O
requires	O
little	O
bandwidth	O
,	O
because	O
they	O
are	O
not	O
involved	O
in	O
actual	O
transmission	O
and	O
reception	O
of	O
IPv6	B-Protocol
traffic	O
packets	O
.	O
</s>
<s>
Two	O
distinct	O
public	O
IPv4	B-Protocol
addresses	O
.	O
</s>
<s>
Also	O
,	O
it	O
must	O
export	O
(	O
advertise	O
)	O
a	O
route	O
toward	O
the	O
Teredo	O
IPv6	B-Protocol
prefix	O
(	O
2001::/32	O
)	O
to	O
other	O
IPv6	B-Protocol
hosts	O
.	O
</s>
<s>
That	O
way	O
,	O
the	O
Teredo	O
relay	O
receives	O
traffic	O
from	O
the	O
IPv6	B-Protocol
hosts	O
addressed	O
to	O
any	O
Teredo	O
client	O
,	O
and	O
forwards	O
it	O
over	O
UDP/IPv4	O
.	O
</s>
<s>
Symmetrically	O
,	O
it	O
receives	O
packets	O
from	O
Teredo	O
clients	O
addressed	O
to	O
native	O
IPv6	B-Protocol
hosts	O
over	O
UDP/IPv4	O
and	O
injects	O
those	O
into	O
the	O
native	O
IPv6	B-Protocol
network	O
.	O
</s>
<s>
This	O
provides	O
a	O
short	O
path	O
between	O
their	O
IPv6	B-Protocol
network	O
and	O
any	O
Teredo	O
client	O
.	O
</s>
<s>
However	O
,	O
setting	O
up	O
a	O
Teredo	O
relay	O
on	O
a	O
scale	O
beyond	O
that	O
of	O
a	O
single	O
network	O
requires	O
the	O
ability	O
to	O
export	O
BGP	B-Protocol
IPv6	B-Protocol
routes	O
to	O
the	O
other	O
autonomous	B-Architecture
systems	I-Architecture
(	O
AS	O
's	O
)	O
.	O
</s>
<s>
Unlike	O
6to4	B-Protocol
,	O
where	O
the	O
two	O
halves	O
of	O
a	O
connection	O
can	O
use	O
different	O
relays	O
,	O
traffic	O
between	O
a	O
native	O
IPv6	B-Protocol
host	O
and	O
a	O
Teredo	O
client	O
uses	O
the	O
same	O
Teredo	O
relay	O
,	O
namely	O
the	O
one	O
closest	O
to	O
the	O
native	O
IPv6	B-Protocol
host	O
network-wise	O
.	O
</s>
<s>
The	O
Teredo	O
client	O
cannot	O
localize	O
a	O
relay	O
by	O
itself	O
(	O
since	O
it	O
cannot	O
send	O
IPv6	B-Protocol
packets	O
by	O
itself	O
)	O
.	O
</s>
<s>
If	O
it	O
needs	O
to	O
initiate	O
a	O
connection	O
to	O
a	O
native	O
IPv6	B-Protocol
host	O
,	O
it	O
sends	O
the	O
first	O
packet	O
through	O
the	O
Teredo	O
server	O
,	O
which	O
sends	O
a	O
packet	O
to	O
the	O
native	O
IPv6	B-Protocol
host	O
using	O
the	O
client	O
's	O
Teredo	O
IPv6	B-Protocol
address	I-Protocol
.	O
</s>
<s>
The	O
native	O
IPv6	B-Protocol
host	O
then	O
responds	O
as	O
usual	O
to	O
the	O
client	O
's	O
Teredo	O
IPv6	B-Protocol
address	I-Protocol
,	O
which	O
eventually	O
causes	O
the	O
packet	O
to	O
find	O
a	O
Teredo	O
relay	O
,	O
which	O
initiates	O
a	O
connection	O
to	O
the	O
client	O
(	O
possibly	O
using	O
the	O
Teredo	O
server	O
for	O
NAT	B-Protocol
piercing	I-Protocol
)	O
.	O
</s>
<s>
The	O
Teredo	O
Client	O
and	O
native	O
IPv6	B-Protocol
host	O
then	O
use	O
the	O
relay	O
for	O
communication	O
as	O
long	O
as	O
they	O
need	O
to	O
.	O
</s>
<s>
This	O
design	O
means	O
that	O
neither	O
the	O
Teredo	O
server	O
nor	O
client	O
needs	O
to	O
know	O
the	O
IPv4	B-Protocol
address	O
of	O
any	O
Teredo	O
relays	O
.	O
</s>
<s>
They	O
find	O
a	O
suitable	O
one	O
automatically	O
via	O
the	O
global	O
IPv6	B-Protocol
routing	O
table	O
,	O
since	O
all	O
Teredo	O
relays	O
advertise	O
the	O
network	O
2001::/32	O
.	O
</s>
<s>
On	O
March	O
30	O
,	O
2006	O
,	O
Italian	O
ISP	O
ITGate	O
was	O
the	O
first	O
AS	O
to	O
start	O
advertising	O
a	O
route	O
toward	O
2001::/32	O
on	O
the	O
IPv6	B-Protocol
Internet	I-Protocol
,	O
so	O
that	O
RFC	O
4380-compliant	O
Teredo	O
implementations	O
would	O
be	O
fully	O
usable	O
.	O
</s>
<s>
In	O
Q1	O
2009	O
,	O
IPv6	B-Protocol
backbone	O
Hurricane	O
Electric	O
enabled	O
14	O
Teredo	O
relays	O
in	O
an	O
anycast	B-Protocol
implementation	O
and	O
advertising	O
2001::/32	O
globally	O
.	O
</s>
<s>
As	O
with	O
6to4	B-Protocol
,	O
it	O
remains	O
unclear	O
how	O
well	O
the	O
Teredo	O
service	O
will	O
scale	O
up	O
if	O
a	O
large	O
proportion	O
of	O
Internet	O
hosts	O
start	O
using	O
IPv6	B-Protocol
through	O
Teredo	O
in	O
addition	O
to	O
IPv4	B-Protocol
.	O
</s>
<s>
While	O
Microsoft	O
has	O
operated	O
a	O
set	O
of	O
Teredo	O
servers	O
since	O
they	O
released	O
the	O
first	O
Teredo	O
pseudo-tunnel	O
for	O
Windows	O
XP	O
,	O
they	O
have	O
never	O
provided	O
a	O
Teredo	O
relay	O
service	O
for	O
the	O
IPv6	B-Protocol
Internet	I-Protocol
as	O
a	O
whole	O
.	O
</s>
<s>
Teredo	O
is	O
not	O
compatible	O
with	O
all	O
NAT	B-Protocol
devices	I-Protocol
.	O
</s>
<s>
Using	O
the	O
terminology	O
of	O
RFC	O
3489	O
,	O
it	O
supports	O
full	O
cone	O
,	O
restricted	O
,	O
and	O
port-restricted	O
NAT	B-Protocol
devices	I-Protocol
,	O
but	O
does	O
not	O
support	O
symmetric	O
NATs	O
.	O
</s>
<s>
People	O
at	O
the	O
National	O
Chiao	O
Tung	O
University	O
in	O
Taiwan	O
later	O
proposed	O
SymTeredo	O
,	O
which	O
enhanced	O
the	O
original	O
Teredo	O
protocol	O
to	O
support	O
symmetric	O
NATs	O
,	O
and	O
the	O
Microsoft	O
and	O
Miredo	B-Protocol
implementations	O
implement	O
certain	O
unspecified	O
non-standard	O
extensions	O
to	O
improve	O
support	O
for	O
symmetric	O
NATs	O
.	O
</s>
<s>
Indeed	O
,	O
Teredo	O
assumes	O
that	O
when	O
two	O
clients	O
exchange	O
encapsulated	O
IPv6	B-Protocol
packets	O
,	O
the	O
mapped/external	O
UDP	B-Protocol
port	O
numbers	O
used	O
will	O
be	O
the	O
same	O
as	O
those	O
that	O
were	O
used	O
to	O
contact	O
the	O
Teredo	O
server	O
(	O
and	O
building	O
the	O
Teredo	O
IPv6	B-Protocol
address	I-Protocol
)	O
.	O
</s>
<s>
Without	O
this	O
assumption	O
,	O
it	O
would	O
not	O
be	O
possible	O
to	O
establish	O
a	O
direct	O
communication	O
between	O
the	O
two	O
clients	O
,	O
and	O
a	O
costly	O
relay	O
would	O
have	O
to	O
be	O
used	O
to	O
perform	O
triangle	B-Protocol
routing	I-Protocol
.	O
</s>
<s>
Teredo	O
can	O
only	O
provide	O
a	O
single	O
IPv6	B-Protocol
address	I-Protocol
per	O
tunnel	O
endpoint	O
.	O
</s>
<s>
As	O
such	O
,	O
it	O
is	O
not	O
possible	O
to	O
use	O
a	O
single	O
Teredo	B-Protocol
tunnel	I-Protocol
to	O
connect	O
multiple	O
hosts	O
,	O
unlike	O
6to4	B-Protocol
and	O
some	O
point-to-point	O
IPv6	B-Protocol
tunnels	O
.	O
</s>
<s>
The	O
bandwidth	O
available	O
to	O
all	O
Teredo	O
clients	O
toward	O
the	O
IPv6	B-Protocol
Internet	I-Protocol
is	O
limited	O
by	O
the	O
availability	O
of	O
Teredo	O
relays	O
,	O
which	O
are	O
no	O
different	O
than	O
6to4	B-Protocol
relays	O
in	O
that	O
respect	O
.	O
</s>
<s>
6to4	B-Protocol
requires	O
a	O
public	O
IPv4	B-Protocol
address	O
,	O
but	O
provides	O
a	O
large	O
48-bit	O
IPv6	B-Protocol
prefix	O
for	O
each	O
tunnel	O
endpoint	O
,	O
and	O
has	O
a	O
lower	O
encapsulation	B-Protocol
overhead	O
.	O
</s>
<s>
Point-to-point	O
tunnels	O
can	O
be	O
more	O
reliable	O
and	O
are	O
more	O
accountable	O
than	O
Teredo	O
,	O
and	O
typically	O
provide	O
permanent	O
IPv6	B-Protocol
addresses	I-Protocol
that	O
do	O
not	O
depend	O
on	O
the	O
IPv4	B-Protocol
address	O
of	O
the	O
tunnel	O
endpoint	O
.	O
</s>
<s>
Some	O
point-to-point	O
tunnel	B-Protocol
brokers	I-Protocol
also	O
support	O
UDP	B-Protocol
encapsulation	B-Protocol
to	O
traverse	O
NATs	O
(	O
for	O
instance	O
,	O
the	O
AYIYA	B-Protocol
protocol	O
can	O
do	O
this	O
)	O
.	O
</s>
<s>
Automated	O
tools	O
(	O
for	O
instance	O
AICCU	B-Language
)	O
make	O
it	O
easy	O
to	O
use	O
Point-to-Point	O
tunnels	O
.	O
</s>
<s>
Teredo	O
increases	O
the	O
attack	O
surface	O
by	O
assigning	O
globally	O
routable	O
IPv6	B-Protocol
addresses	I-Protocol
to	O
network	O
hosts	O
behind	O
NAT	B-Protocol
devices	I-Protocol
,	O
which	O
would	O
otherwise	O
be	O
unreachable	O
from	O
the	O
Internet	O
.	O
</s>
<s>
By	O
doing	O
so	O
,	O
Teredo	O
potentially	O
exposes	O
any	O
IPv6-enabled	O
application	O
with	O
an	O
open	O
port	O
to	O
the	O
outside	O
.	O
</s>
<s>
Teredo	B-Protocol
tunnel	I-Protocol
encapsulation	B-Protocol
can	O
also	O
cause	O
the	O
contents	O
of	O
the	O
IPv6	B-Protocol
data	O
traffic	O
to	O
become	O
invisible	O
to	O
packet	O
inspection	O
software	O
,	O
facilitating	O
the	O
spread	O
of	O
malware	O
.	O
</s>
<s>
Finally	O
,	O
Teredo	O
exposes	O
the	O
IPv6	B-Protocol
stack	O
and	O
the	O
tunneling	B-Protocol
software	I-Protocol
to	O
attacks	O
should	O
they	O
have	O
any	O
remotely	O
exploitable	O
vulnerability	O
.	O
</s>
<s>
In	O
order	O
to	O
reduce	O
the	O
attack	O
surface	O
,	O
the	O
Microsoft	O
IPv6	B-Protocol
stack	O
has	O
a	O
"	O
protection	O
level	O
"	O
socket	B-Protocol
option	O
.	O
</s>
<s>
This	O
allows	O
applications	O
to	O
specify	O
from	O
which	O
sources	O
they	O
are	O
willing	O
to	O
accept	O
IPv6	B-Protocol
traffic	O
:	O
from	O
the	O
Teredo	B-Protocol
tunnel	I-Protocol
,	O
from	O
anywhere	O
except	O
Teredo	O
(	O
the	O
default	O
)	O
,	O
or	O
only	O
from	O
the	O
local	O
intranet	B-General_Concept
.	O
</s>
<s>
For	O
a	O
Teredo	O
pseudo-tunnel	O
to	O
operate	O
properly	O
,	O
outgoing	O
UDP	B-Protocol
packets	I-Protocol
to	O
port	O
3544	O
must	O
be	O
unfiltered	O
.	O
</s>
<s>
Teredo	B-Protocol
tunneling	I-Protocol
software	O
reports	O
a	O
fatal	O
error	O
and	O
stops	O
if	O
outgoing	O
IPv4	B-Protocol
UDP	B-Protocol
traffic	O
is	O
blocked	O
.	O
</s>
<s>
In	O
2010	O
,	O
new	O
methods	O
to	O
create	O
denial	O
of	O
service	O
attacks	O
via	O
routing	O
loops	O
that	O
use	O
Teredo	B-Protocol
tunnels	I-Protocol
were	O
uncovered	O
.	O
</s>
<s>
Microsoft	B-Application
Windows	I-Application
as	O
of	O
Windows	B-Operating_System
10	I-Operating_System
,	O
version	O
1803	O
and	O
later	O
disable	O
Teredo	O
by	O
default	O
.	O
</s>
<s>
If	O
needed	O
,	O
this	O
transitional	O
technology	O
can	O
be	O
enabled	O
via	O
a	O
CLI	B-Application
command	O
or	O
Group	O
Policy	O
.	O
</s>
<s>
Windows	B-Application
Server	I-Application
2003	I-Application
has	O
a	O
relay	O
and	O
server	O
provided	O
under	O
the	O
Microsoft	O
Beta	O
program	O
.	O
</s>
<s>
Windows	B-Application
Vista	I-Application
and	O
Windows	B-Device
7	I-Device
have	O
built-in	O
support	O
for	O
Teredo	O
with	O
an	O
unspecified	O
extension	O
for	O
symmetric	O
NAT	B-Protocol
traversal	I-Protocol
.	O
</s>
<s>
However	O
,	O
if	O
only	O
a	O
link-local	O
and	O
Teredo	O
address	O
are	O
present	O
,	O
these	O
operating	O
systems	O
do	O
n't	O
try	O
to	O
resolve	O
IPv6	B-Protocol
DNS	O
AAAA	O
records	O
if	O
a	O
DNS	O
A	O
record	O
is	O
present	O
,	O
in	O
which	O
case	O
they	O
use	O
IPv4	B-Protocol
.	O
</s>
<s>
Therefore	O
,	O
only	O
literal	O
IPv6	B-Protocol
URLs	O
typically	O
use	O
Teredo	O
.	O
</s>
<s>
This	O
behavior	O
can	O
be	O
modified	O
in	O
the	O
registry	B-Protocol
.	O
</s>
<s>
Windows	B-Operating_System
10	I-Operating_System
version	O
1803	O
and	O
later	O
disable	O
Teredo	O
by	O
default	O
.	O
</s>
<s>
If	O
needed	O
,	O
this	O
transitional	O
technology	O
can	O
be	O
enabled	O
via	O
a	O
CLI	B-Application
command	O
or	O
Group	O
Policy	O
.	O
</s>
<s>
Miredo	B-Protocol
is	O
a	O
client	O
,	O
relay	O
,	O
and	O
server	O
for	O
Linux	B-Operating_System
,	O
*	O
BSD	O
,	O
and	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
,	O
</s>
<s>
ng_teredo	O
is	O
a	O
relay	O
and	O
server	O
based	O
on	O
netgraph	B-Device
for	O
FreeBSD	B-Operating_System
from	O
the	O
LIP6	O
University	O
and	O
6WIND	O
.	O
</s>
<s>
NICI-Teredo	O
is	O
a	O
relay	O
for	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
and	O
a	O
userland	O
Teredo	O
server	O
,	O
developed	O
at	O
the	O
National	O
Chiao	O
Tung	O
University	O
.	O
</s>
<s>
The	O
initial	O
nickname	O
of	O
the	O
Teredo	B-Protocol
tunneling	I-Protocol
protocol	O
was	O
Shipworm	O
.	O
</s>
<s>
The	O
idea	O
was	O
that	O
the	O
protocol	O
would	O
pierce	O
through	O
NAT	B-Protocol
devices	I-Protocol
,	O
much	O
as	O
the	O
shipworm	O
(	O
a	O
kind	O
of	O
marine	O
wood-boring	O
clam	O
)	O
bores	O
tunnels	O
through	O
wood	O
.	O
</s>
