<s>
The	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
is	O
a	O
hierarchical	O
and	O
distributed	O
naming	O
system	O
for	O
computers	O
,	O
services	O
,	O
and	O
other	O
resources	O
in	O
the	O
Internet	O
or	O
other	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
networks	O
.	O
</s>
<s>
Most	O
prominently	O
,	O
it	O
translates	O
readily	O
memorized	O
domain	O
names	O
to	O
the	O
numerical	O
IP	B-Protocol
addresses	I-Protocol
needed	O
for	O
locating	O
and	O
identifying	O
computer	O
services	O
and	O
devices	O
with	O
the	O
underlying	O
network	O
protocols	O
.	O
</s>
<s>
The	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
has	O
been	O
an	O
essential	O
component	O
of	O
the	O
functionality	O
of	O
the	O
Internet	O
since	O
1985	O
.	O
</s>
<s>
The	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
delegates	O
the	O
responsibility	O
of	O
assigning	O
domain	O
names	O
and	O
mapping	O
those	O
names	O
to	O
Internet	O
resources	O
by	O
designating	O
authoritative	O
name	B-Protocol
servers	I-Protocol
for	O
each	O
domain	O
.	O
</s>
<s>
Network	O
administrators	O
may	O
delegate	O
authority	O
over	O
sub-domains	B-Protocol
of	O
their	O
allocated	O
name	O
space	O
to	O
other	O
name	B-Protocol
servers	I-Protocol
.	O
</s>
<s>
This	O
mechanism	O
provides	O
distributed	O
and	O
fault-tolerant	B-General_Concept
service	O
and	O
was	O
designed	O
to	O
avoid	O
a	O
single	O
large	O
central	O
database	B-General_Concept
.	O
</s>
<s>
The	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
also	O
specifies	O
the	O
technical	O
functionality	O
of	O
the	O
database	B-General_Concept
service	O
that	O
is	O
at	O
its	O
core	O
.	O
</s>
<s>
It	O
defines	O
the	O
DNS	B-Protocol
protocol	I-Protocol
,	O
a	O
detailed	O
specification	O
of	O
the	O
data	O
structures	O
and	O
data	O
communication	O
exchanges	O
used	O
in	O
the	O
DNS	B-Protocol
,	O
as	O
part	O
of	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
.	O
</s>
<s>
The	O
Internet	O
maintains	O
two	O
principal	O
namespaces	O
,	O
the	O
domain	O
name	O
hierarchy	O
and	O
the	O
IP	B-Protocol
address	I-Protocol
spaces	O
.	O
</s>
<s>
The	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
maintains	O
the	O
domain	O
name	O
hierarchy	O
and	O
provides	O
translation	O
services	O
between	O
it	O
and	O
the	O
address	B-General_Concept
spaces	I-General_Concept
.	O
</s>
<s>
Internet	O
name	B-Protocol
servers	I-Protocol
and	O
a	O
communication	O
protocol	O
implement	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
.	O
</s>
<s>
A	O
DNS	B-Protocol
name	I-Protocol
server	O
is	O
a	O
server	O
that	O
stores	O
the	O
DNS	B-Protocol
records	O
for	O
a	O
domain	O
;	O
a	O
DNS	B-Protocol
name	I-Protocol
server	O
responds	O
with	O
answers	O
to	O
queries	O
against	O
its	O
database	B-General_Concept
.	O
</s>
<s>
The	O
most	O
common	O
types	O
of	O
records	O
stored	O
in	O
the	O
DNS	B-Protocol
database	B-General_Concept
are	O
for	O
start	B-Protocol
of	I-Protocol
authority	I-Protocol
(	O
SOA	B-Protocol
)	O
,	O
IP	B-Protocol
addresses	I-Protocol
(	O
A	O
and	O
AAAA	B-Protocol
)	O
,	O
SMTP	B-Protocol
mail	B-Protocol
exchangers	I-Protocol
(	O
MX	O
)	O
,	O
name	B-Protocol
servers	I-Protocol
(	O
NS	O
)	O
,	O
pointers	O
for	O
reverse	B-Protocol
DNS	I-Protocol
lookups	I-Protocol
(	O
PTR	O
)	O
,	O
and	O
domain	O
name	O
aliases	O
(	O
CNAME	B-Protocol
)	O
.	O
</s>
<s>
Although	O
not	O
intended	O
to	O
be	O
a	O
general	O
purpose	O
database	B-General_Concept
,	O
DNS	B-Protocol
has	O
been	O
expanded	O
over	O
time	O
to	O
store	O
records	O
for	O
other	O
types	O
of	O
data	O
for	O
either	O
automatic	O
lookups	O
,	O
such	O
as	O
DNSSEC	B-Protocol
records	O
,	O
or	O
for	O
human	O
queries	O
such	O
as	O
responsible	O
person	O
(	O
RP	O
)	O
records	O
.	O
</s>
<s>
As	O
a	O
general	O
purpose	O
database	B-General_Concept
,	O
the	O
DNS	B-Protocol
has	O
also	O
been	O
used	O
in	O
combating	O
unsolicited	O
email	O
(	O
spam	O
)	O
by	O
storing	O
a	O
real-time	O
blackhole	O
list	O
(	O
RBL	O
)	O
.	O
</s>
<s>
The	O
DNS	B-Protocol
database	B-General_Concept
is	O
traditionally	O
stored	O
in	O
a	O
structured	O
text	O
file	O
,	O
the	O
zone	B-Protocol
file	I-Protocol
,	O
but	O
other	O
database	B-General_Concept
systems	O
are	O
common	O
.	O
</s>
<s>
The	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
originally	O
used	O
the	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
as	O
transport	O
over	O
IP	O
.	O
</s>
<s>
Reliability	O
,	O
security	O
,	O
and	O
privacy	O
concerns	O
spawned	O
the	O
use	O
of	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
as	O
well	O
as	O
numerous	O
other	O
protocol	O
developments	O
.	O
</s>
<s>
An	O
often-used	O
analogy	O
to	O
explain	O
the	O
DNS	B-Protocol
is	O
that	O
it	O
serves	O
as	O
the	O
phone	O
book	O
for	O
the	O
Internet	O
by	O
translating	O
human-friendly	O
computer	O
hostnames	B-Protocol
into	O
IP	B-Protocol
addresses	I-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
the	O
hostname	B-Protocol
www.example.com	O
within	O
the	O
domain	O
name	O
example.com	O
translates	O
to	O
the	O
addresses	O
(	O
IPv4	B-Protocol
)	O
and	O
(	O
IPv6	B-Protocol
)	O
.	O
</s>
<s>
The	O
DNS	B-Protocol
can	O
be	O
quickly	O
and	O
transparently	O
updated	O
,	O
allowing	O
a	O
service	O
's	O
location	O
on	O
the	O
network	O
to	O
change	O
without	O
affecting	O
the	O
end	O
users	O
,	O
who	O
continue	O
to	O
use	O
the	O
same	O
hostname	B-Protocol
.	O
</s>
<s>
An	O
important	O
and	O
ubiquitous	B-Architecture
function	O
of	O
the	O
DNS	B-Protocol
is	O
its	O
central	O
role	O
in	O
distributed	O
Internet	B-Protocol
services	I-Protocol
such	O
as	O
cloud	B-Architecture
services	I-Architecture
and	O
content	B-Operating_System
delivery	I-Operating_System
networks	I-Operating_System
.	O
</s>
<s>
When	O
a	O
user	O
accesses	O
a	O
distributed	O
Internet	O
service	O
using	O
a	O
URL	O
,	O
the	O
domain	O
name	O
of	O
the	O
URL	O
is	O
translated	O
to	O
the	O
IP	B-Protocol
address	I-Protocol
of	O
a	O
server	O
that	O
is	O
proximal	O
to	O
the	O
user	O
.	O
</s>
<s>
The	O
key	O
functionality	O
of	O
the	O
DNS	B-Protocol
exploited	O
here	O
is	O
that	O
different	O
users	O
can	O
simultaneously	O
receive	O
different	O
translations	O
for	O
the	O
same	O
domain	O
name	O
,	O
a	O
key	O
point	O
of	O
divergence	O
from	O
a	O
traditional	O
phone-book	O
view	O
of	O
the	O
DNS	B-Protocol
.	O
</s>
<s>
This	O
process	O
of	O
using	O
the	O
DNS	B-Protocol
to	O
assign	O
proximal	O
servers	O
to	O
users	O
is	O
key	O
to	O
providing	O
faster	O
and	O
more	O
reliable	O
responses	O
on	O
the	O
Internet	O
and	O
is	O
widely	O
used	O
by	O
most	O
major	O
Internet	B-Protocol
services	I-Protocol
.	O
</s>
<s>
The	O
DNS	B-Protocol
reflects	O
the	O
structure	O
of	O
administrative	O
responsibility	O
on	O
the	O
Internet	O
.	O
</s>
<s>
Each	O
subdomain	B-Protocol
is	O
a	O
zone	B-Protocol
of	O
administrative	O
autonomy	O
delegated	O
to	O
a	O
manager	O
.	O
</s>
<s>
For	O
zones	O
operated	O
by	O
a	O
registry	B-Protocol
,	O
administrative	O
information	O
is	O
often	O
complemented	O
by	O
the	O
registry	B-Protocol
's	O
RDAP	O
and	O
WHOIS	B-Protocol
services	O
.	O
</s>
<s>
The	O
Stanford	O
Research	O
Institute	O
(	O
now	O
SRI	O
International	O
)	O
maintained	O
a	O
text	O
file	O
named	O
HOSTS.TXT	B-Protocol
that	O
mapped	O
host	B-Protocol
names	I-Protocol
to	O
the	O
numerical	O
addresses	O
of	O
computers	O
on	O
the	O
ARPANET	O
.	O
</s>
<s>
Computers	O
,	O
including	O
their	O
hostnames	B-Protocol
and	O
addresses	O
,	O
were	O
added	O
to	O
the	O
primary	O
file	O
by	O
contacting	O
the	O
SRI	O
Network	B-Protocol
Information	I-Protocol
Center	I-Protocol
(	O
NIC	O
)	O
,	O
directed	O
by	O
Feinler	O
,	O
via	O
telephone	O
during	O
business	O
hours	O
.	O
</s>
<s>
Later	O
,	O
Feinler	O
set	O
up	O
a	O
WHOIS	B-Protocol
directory	O
on	O
a	O
server	O
in	O
the	O
NIC	O
for	O
retrieval	O
of	O
information	O
about	O
resources	O
,	O
contacts	O
,	O
and	O
entities	O
.	O
</s>
<s>
She	O
and	O
her	O
team	O
managed	O
the	O
Host	O
Naming	O
Registry	B-Protocol
from	O
1972	O
to	O
1989	O
.	O
</s>
<s>
Mockapetris	O
instead	O
created	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
in	O
1983	O
while	O
at	O
the	O
University	O
of	O
Southern	O
California	O
.	O
</s>
<s>
In	O
1984	O
,	O
four	O
UC	O
Berkeley	O
students	O
,	O
Douglas	O
Terry	O
,	O
Mark	O
Painter	O
,	O
David	O
Riggle	O
,	O
and	O
Songnian	O
Zhou	O
,	O
wrote	O
the	O
first	O
Unix	B-Application
name	B-Protocol
server	I-Protocol
implementation	O
for	O
the	O
Berkeley	B-Protocol
Internet	I-Protocol
Name	I-Protocol
Domain	I-Protocol
,	O
commonly	O
referred	O
to	O
as	O
BIND	B-Protocol
.	O
</s>
<s>
In	O
1985	O
,	O
Kevin	O
Dunlap	O
of	O
DEC	O
substantially	O
revised	O
the	O
DNS	B-Protocol
implementation	O
.	O
</s>
<s>
Mike	O
Karels	O
,	O
Phil	O
Almquist	O
,	O
and	O
Paul	O
Vixie	O
then	O
took	O
over	O
BIND	B-Protocol
maintenance	O
.	O
</s>
<s>
Internet	O
Systems	O
Consortium	O
was	O
founded	O
in	O
1994	O
by	O
Rick	O
Adams	O
,	O
Paul	O
Vixie	O
,	O
and	O
Carl	O
Malamud	O
,	O
expressly	O
to	O
provide	O
a	O
home	O
for	O
BIND	B-Protocol
development	O
and	O
maintenance	O
.	O
</s>
<s>
BIND	B-Protocol
versions	O
from	O
4.9.3	O
onward	O
were	O
developed	O
and	O
maintained	O
by	O
ISC	O
,	O
with	O
support	O
provided	O
by	O
ISC	O
's	O
sponsors	O
.	O
</s>
<s>
As	O
co-architects/programmers	O
,	O
Bob	O
Halley	O
and	O
Paul	O
Vixie	O
released	O
the	O
first	O
production-ready	O
version	O
of	O
BIND	B-Protocol
version	O
8	O
in	O
May	O
1997	O
.	O
</s>
<s>
Since	O
2000	O
,	O
over	O
43	O
different	O
core	O
developers	O
have	O
worked	O
on	O
BIND	B-Protocol
.	O
</s>
<s>
In	O
November	O
1987	O
,	O
RFC	O
1034	O
and	O
RFC	O
1035	O
superseded	O
the	O
1983	O
DNS	B-Protocol
specifications	O
.	O
</s>
<s>
Several	O
additional	O
Request	O
for	O
Comments	O
have	O
proposed	O
extensions	O
to	O
the	O
core	O
DNS	B-Protocol
protocols	I-Protocol
.	O
</s>
<s>
The	O
domain	O
name	O
space	O
consists	O
of	O
a	O
tree	B-Application
data	I-Application
structure	I-Application
.	O
</s>
<s>
The	O
domain	O
name	O
itself	O
consists	O
of	O
the	O
label	O
,	O
concatenated	O
with	O
the	O
name	O
of	O
its	O
parent	B-Application
node	I-Application
on	O
the	O
right	O
,	O
separated	O
by	O
a	O
dot	O
.	O
</s>
<s>
The	O
tree	O
sub-divides	O
into	O
zones	O
beginning	O
at	O
the	O
root	B-Protocol
zone	I-Protocol
.	O
</s>
<s>
A	O
DNS	B-Protocol
zone	I-Protocol
may	O
consist	O
of	O
as	O
many	O
domains	O
and	O
sub	B-Protocol
domains	I-Protocol
as	O
the	O
zone	B-Protocol
manager	O
chooses	O
.	O
</s>
<s>
DNS	B-Protocol
can	O
also	O
be	O
partitioned	O
according	O
to	O
class	O
where	O
the	O
separate	O
classes	O
can	O
be	O
thought	O
of	O
as	O
an	O
array	O
of	O
parallel	O
namespace	O
trees	O
.	O
</s>
<s>
Administrative	O
responsibility	O
for	O
any	O
zone	B-Protocol
may	O
be	O
divided	O
by	O
creating	O
additional	O
zones	O
.	O
</s>
<s>
Authority	O
over	O
the	O
new	O
zone	B-Protocol
is	O
said	O
to	O
be	O
delegated	O
to	O
a	O
designated	O
name	B-Protocol
server	I-Protocol
.	O
</s>
<s>
The	O
parent	O
zone	B-Protocol
ceases	O
to	O
be	O
authoritative	O
for	O
the	O
new	O
zone	B-Protocol
.	O
</s>
<s>
The	O
hierarchy	O
of	O
domains	O
descends	O
from	O
right	O
to	O
left	O
;	O
each	O
label	O
to	O
the	O
left	O
specifies	O
a	O
subdivision	O
,	O
or	O
subdomain	B-Protocol
of	O
the	O
domain	O
to	O
the	O
right	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
label	O
example	O
specifies	O
a	O
subdomain	B-Protocol
of	O
the	O
com	O
domain	O
,	O
and	O
www	O
is	O
a	O
subdomain	B-Protocol
of	O
example.com	O
.	O
</s>
<s>
The	O
null	O
label	O
,	O
of	O
length	O
zero	O
,	O
is	O
reserved	O
for	O
the	O
root	B-Protocol
zone	I-Protocol
.	O
</s>
<s>
In	O
the	O
internal	O
binary	O
representation	O
of	O
the	O
DNS	B-Protocol
the	O
maximum	O
length	O
requires	O
255	O
octets	O
of	O
storage	O
,	O
as	O
it	O
also	O
stores	O
the	O
length	O
of	O
the	O
name	O
.	O
</s>
<s>
Although	O
no	O
technical	O
limitation	O
exists	O
to	O
prevent	O
domain	O
name	O
labels	O
using	O
any	O
character	O
which	O
is	O
representable	O
by	O
an	O
octet	O
,	O
hostnames	B-Protocol
use	O
a	O
preferred	O
format	O
and	O
character	O
set	O
.	O
</s>
<s>
The	O
characters	O
allowed	O
in	O
labels	O
are	O
a	O
subset	O
of	O
the	O
ASCII	B-Protocol
character	I-Protocol
set	I-Protocol
,	O
consisting	O
of	O
characters	O
a	O
through	O
z	O
,	O
A	O
through	O
Z	O
,	O
digits	O
0	O
through	O
9	O
,	O
and	O
hyphen	O
.	O
</s>
<s>
The	O
limited	O
set	O
of	O
ASCII	B-Protocol
characters	I-Protocol
permitted	O
in	O
the	O
DNS	B-Protocol
prevented	O
the	O
representation	O
of	O
names	O
and	O
words	O
of	O
many	O
languages	O
in	O
their	O
native	O
alphabets	O
or	O
scripts	O
.	O
</s>
<s>
To	O
make	O
this	O
possible	O
,	O
ICANN	B-Application
approved	O
the	O
Internationalizing	B-Language
Domain	I-Language
Names	I-Language
in	I-Language
Applications	I-Language
(	O
IDNA	B-Language
)	O
system	O
,	O
by	O
which	O
user	O
applications	O
,	O
such	O
as	O
web	B-Application
browsers	I-Application
,	O
map	O
Unicode	O
strings	O
into	O
the	O
valid	O
DNS	B-Protocol
character	O
set	O
using	O
Punycode	B-General_Concept
.	O
</s>
<s>
In	O
2009	O
ICANN	B-Application
approved	O
the	O
installation	O
of	O
internationalized	B-Language
domain	I-Language
name	I-Language
country	B-Application
code	I-Application
top-level	I-Application
domains	I-Application
(	O
ccTLDs	B-Application
)	O
.	O
</s>
<s>
In	O
addition	O
,	O
many	O
registries	B-Protocol
of	O
the	O
existing	O
top-level	O
domain	O
names	O
(	O
TLDs	O
)	O
have	O
adopted	O
the	O
IDNA	B-Language
system	O
,	O
guided	O
by	O
RFC	O
5890	O
,	O
RFC	O
5891	O
,	O
RFC	O
5892	O
,	O
RFC	O
5893	O
.	O
</s>
<s>
The	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
is	O
maintained	O
by	O
a	O
distributed	B-Operating_System
database	I-Operating_System
system	O
,	O
which	O
uses	O
the	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
model	I-Operating_System
.	O
</s>
<s>
The	O
nodes	O
of	O
this	O
database	B-General_Concept
are	O
the	O
name	B-Protocol
servers	I-Protocol
.	O
</s>
<s>
Each	O
domain	O
has	O
at	O
least	O
one	O
authoritative	O
DNS	B-Protocol
server	I-Protocol
that	O
publishes	O
information	O
about	O
that	O
domain	O
and	O
the	O
name	B-Protocol
servers	I-Protocol
of	O
any	O
domains	O
subordinate	O
to	O
it	O
.	O
</s>
<s>
The	O
top	O
of	O
the	O
hierarchy	O
is	O
served	O
by	O
the	O
root	B-Application
name	I-Application
servers	I-Application
,	O
the	O
servers	O
to	O
query	O
when	O
looking	O
up	O
(	O
resolving	O
)	O
a	O
TLD	O
.	O
</s>
<s>
An	O
authoritative	O
name	B-Protocol
server	I-Protocol
is	O
a	O
name	B-Protocol
server	I-Protocol
that	O
only	O
gives	O
answers	O
to	O
DNS	B-Protocol
queries	I-Protocol
from	O
data	O
that	O
have	O
been	O
configured	O
by	O
an	O
original	O
source	O
,	O
for	O
example	O
,	O
the	O
domain	O
administrator	O
or	O
by	O
dynamic	B-Protocol
DNS	I-Protocol
methods	O
,	O
in	O
contrast	O
to	O
answers	O
obtained	O
via	O
a	O
query	O
to	O
another	O
name	B-Protocol
server	I-Protocol
that	O
only	O
maintains	O
a	O
cache	O
of	O
data	O
.	O
</s>
<s>
An	O
authoritative	O
name	B-Protocol
server	I-Protocol
can	O
either	O
be	O
a	O
primary	O
server	O
or	O
a	O
secondary	O
server	O
.	O
</s>
<s>
Historically	O
the	O
terms	O
master/slave	B-Operating_System
and	O
primary/secondary	B-Operating_System
were	O
sometimes	O
used	O
interchangeably	O
but	O
the	O
current	O
practice	O
is	O
to	O
use	O
the	O
latter	O
form	O
.	O
</s>
<s>
A	O
primary	O
server	O
is	O
a	O
server	O
that	O
stores	O
the	O
original	O
copies	O
of	O
all	O
zone	B-Protocol
records	O
.	O
</s>
<s>
A	O
secondary	O
server	O
uses	O
a	O
special	O
automatic	B-Protocol
updating	I-Protocol
mechanism	I-Protocol
in	O
the	O
DNS	B-Protocol
protocol	I-Protocol
in	O
communication	O
with	O
its	O
primary	O
to	O
maintain	O
an	O
identical	O
copy	O
of	O
the	O
primary	O
records	O
.	O
</s>
<s>
Every	O
DNS	B-Protocol
zone	I-Protocol
must	O
be	O
assigned	O
a	O
set	O
of	O
authoritative	O
name	B-Protocol
servers	I-Protocol
.	O
</s>
<s>
This	O
set	O
of	O
servers	O
is	O
stored	O
in	O
the	O
parent	O
domain	O
zone	B-Protocol
with	O
name	B-Protocol
server	I-Protocol
(	O
NS	O
)	O
records	O
.	O
</s>
<s>
This	O
flag	O
is	O
usually	O
reproduced	O
prominently	O
in	O
the	O
output	O
of	O
DNS	B-Protocol
administration	O
query	O
tools	O
,	O
such	O
as	O
dig	B-Protocol
,	O
to	O
indicate	O
that	O
the	O
responding	O
name	B-Protocol
server	I-Protocol
is	O
an	O
authority	O
for	O
the	O
domain	O
name	O
in	O
question	O
.	O
</s>
<s>
When	O
a	O
name	B-Protocol
server	I-Protocol
is	O
designated	O
as	O
the	O
authoritative	O
server	O
for	O
a	O
domain	O
name	O
for	O
which	O
it	O
does	O
not	O
have	O
authoritative	O
data	O
,	O
it	O
presents	O
a	O
type	O
of	O
error	O
called	O
a	O
"	O
lame	O
delegation	O
"	O
or	O
"	O
lame	O
response	O
"	O
.	O
</s>
<s>
Domain	O
name	O
resolvers	O
determine	O
the	O
domain	B-Protocol
name	I-Protocol
servers	I-Protocol
responsible	O
for	O
the	O
domain	O
name	O
in	O
question	O
by	O
a	O
sequence	O
of	O
queries	O
starting	O
with	O
the	O
right-most	O
(	O
top-level	O
)	O
domain	O
label	O
.	O
</s>
<s>
For	O
proper	O
operation	O
of	O
its	O
domain	O
name	O
resolver	O
,	O
a	O
network	O
host	O
is	O
configured	O
with	O
an	O
initial	O
cache	O
(	O
hints	O
)	O
of	O
the	O
known	O
addresses	O
of	O
the	O
root	B-Application
name	I-Application
servers	I-Application
.	O
</s>
<s>
Assuming	O
the	O
resolver	O
has	O
no	O
cached	O
records	O
to	O
accelerate	O
the	O
process	O
,	O
the	O
resolution	O
process	O
starts	O
with	O
a	O
query	O
to	O
one	O
of	O
the	O
root	B-Application
servers	I-Application
.	O
</s>
<s>
In	O
typical	O
operation	O
,	O
the	O
root	B-Application
servers	I-Application
do	O
not	O
answer	O
directly	O
,	O
but	O
respond	O
with	O
a	O
referral	O
to	O
more	O
authoritative	O
servers	O
,	O
e.g.	O
,	O
a	O
query	O
for	O
"	O
www.wikipedia.org	O
"	O
is	O
referred	O
to	O
the	O
org	O
servers	O
.	O
</s>
<s>
The	O
diagram	O
illustrates	O
this	O
process	O
for	O
the	O
host	O
that	O
is	O
named	O
by	O
the	O
fully	B-Protocol
qualified	I-Protocol
domain	I-Protocol
name	I-Protocol
"	O
www.wikipedia.org	O
"	O
.	O
</s>
<s>
This	O
mechanism	O
would	O
place	O
a	O
large	O
traffic	O
burden	O
on	O
the	O
root	B-Application
servers	I-Application
,	O
if	O
every	O
resolution	O
on	O
the	O
Internet	O
required	O
starting	O
at	O
the	O
root	O
.	O
</s>
<s>
In	O
practice	O
caching	O
is	O
used	O
in	O
DNS	B-Protocol
servers	I-Protocol
to	O
off-load	O
the	O
root	B-Application
servers	I-Application
,	O
and	O
as	O
a	O
result	O
,	O
root	B-Application
name	I-Application
servers	I-Application
actually	O
are	O
involved	O
in	O
only	O
a	O
relatively	O
small	O
fraction	O
of	O
all	O
requests	O
.	O
</s>
<s>
In	O
theory	O
,	O
authoritative	O
name	B-Protocol
servers	I-Protocol
are	O
sufficient	O
for	O
the	O
operation	O
of	O
the	O
Internet	O
.	O
</s>
<s>
However	O
,	O
with	O
only	O
authoritative	O
name	B-Protocol
servers	I-Protocol
operating	O
,	O
every	O
DNS	B-Protocol
query	I-Protocol
must	O
start	O
with	O
recursive	O
queries	O
at	O
the	O
root	B-Protocol
zone	I-Protocol
of	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
and	O
each	O
user	O
system	O
would	O
have	O
to	O
implement	O
resolver	O
software	O
capable	O
of	O
recursive	O
operation	O
.	O
</s>
<s>
To	O
improve	O
efficiency	O
,	O
reduce	O
DNS	B-Protocol
traffic	O
across	O
the	O
Internet	O
,	O
and	O
increase	O
performance	O
in	O
end-user	O
applications	O
,	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
supports	O
DNS	B-Protocol
cache	O
servers	O
which	O
store	O
DNS	B-Protocol
query	I-Protocol
results	O
for	O
a	O
period	O
of	O
time	O
determined	O
in	O
the	O
configuration	O
(	O
time-to-live	B-Protocol
)	O
of	O
the	O
domain	O
name	O
record	O
in	O
question	O
.	O
</s>
<s>
Typically	O
,	O
such	O
caching	O
DNS	B-Protocol
servers	I-Protocol
also	O
implement	O
the	O
recursive	O
algorithm	O
necessary	O
to	O
resolve	O
a	O
given	O
name	O
starting	O
with	O
the	O
DNS	B-Protocol
root	I-Protocol
through	O
to	O
the	O
authoritative	O
name	B-Protocol
servers	I-Protocol
of	O
the	O
queried	O
domain	O
.	O
</s>
<s>
With	O
this	O
function	O
implemented	O
in	O
the	O
name	B-Protocol
server	I-Protocol
,	O
user	O
applications	O
gain	O
efficiency	O
in	O
design	O
and	O
operation	O
.	O
</s>
<s>
The	O
combination	O
of	O
DNS	B-Protocol
caching	O
and	O
recursive	O
functions	O
in	O
a	O
name	B-Protocol
server	I-Protocol
is	O
not	O
mandatory	O
;	O
the	O
functions	O
can	O
be	O
implemented	O
independently	O
in	O
servers	O
for	O
special	O
purposes	O
.	O
</s>
<s>
Internet	O
service	O
providers	O
typically	O
provide	O
recursive	O
and	O
caching	O
name	B-Protocol
servers	I-Protocol
for	O
their	O
customers	O
.	O
</s>
<s>
In	O
addition	O
,	O
many	O
home	O
networking	O
routers	O
implement	O
DNS	B-Protocol
caches	O
and	O
recursion	O
to	O
improve	O
efficiency	O
in	O
the	O
local	O
network	O
.	O
</s>
<s>
The	O
client	B-Operating_System
side	I-Operating_System
of	O
the	O
DNS	B-Protocol
is	O
called	O
a	O
DNS	B-Protocol
resolver	O
.	O
</s>
<s>
A	O
resolver	O
is	O
responsible	O
for	O
initiating	O
and	O
sequencing	O
the	O
queries	O
that	O
ultimately	O
lead	O
to	O
a	O
full	O
resolution	O
(	O
translation	O
)	O
of	O
the	O
resource	O
sought	O
,	O
e.g.	O
,	O
translation	O
of	O
a	O
domain	O
name	O
into	O
an	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
DNS	B-Protocol
resolvers	O
are	O
classified	O
by	O
a	O
variety	O
of	O
query	O
methods	O
,	O
such	O
as	O
recursive	O
,	O
non-recursive	O
,	O
and	O
iterative	O
.	O
</s>
<s>
In	O
a	O
non-recursive	O
query	O
,	O
a	O
DNS	B-Protocol
resolver	O
queries	O
a	O
DNS	B-Protocol
server	I-Protocol
that	O
provides	O
a	O
record	O
either	O
for	O
which	O
the	O
server	O
is	O
authoritative	O
,	O
or	O
it	O
provides	O
a	O
partial	O
result	O
without	O
querying	O
other	O
servers	O
.	O
</s>
<s>
In	O
case	O
of	O
a	O
caching	O
DNS	B-Protocol
resolver	O
,	O
the	O
non-recursive	O
query	O
of	O
its	O
local	O
DNS	B-Protocol
cache	O
delivers	O
a	O
result	O
and	O
reduces	O
the	O
load	O
on	O
upstream	O
DNS	B-Protocol
servers	I-Protocol
by	O
caching	O
DNS	B-Protocol
resource	O
records	O
for	O
a	O
period	O
of	O
time	O
after	O
an	O
initial	O
response	O
from	O
upstream	O
DNS	B-Protocol
servers	I-Protocol
.	O
</s>
<s>
In	O
a	O
recursive	O
query	O
,	O
a	O
DNS	B-Protocol
resolver	O
queries	O
a	O
single	O
DNS	B-Protocol
server	I-Protocol
,	O
which	O
may	O
in	O
turn	O
query	O
other	O
DNS	B-Protocol
servers	I-Protocol
on	O
behalf	O
of	O
the	O
requester	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
simple	O
stub	O
resolver	O
running	O
on	O
a	O
home	B-Application
router	I-Application
typically	O
makes	O
a	O
recursive	O
query	O
to	O
the	O
DNS	B-Protocol
server	I-Protocol
run	O
by	O
the	O
user	O
's	O
ISP	O
.	O
</s>
<s>
A	O
recursive	O
query	O
is	O
one	O
for	O
which	O
the	O
DNS	B-Protocol
server	I-Protocol
answers	O
the	O
query	O
completely	O
by	O
querying	O
other	O
name	B-Protocol
servers	I-Protocol
as	O
needed	O
.	O
</s>
<s>
In	O
typical	O
operation	O
,	O
a	O
client	O
issues	O
a	O
recursive	O
query	O
to	O
a	O
caching	O
recursive	O
DNS	B-Protocol
server	I-Protocol
,	O
which	O
subsequently	O
issues	O
non-recursive	O
queries	O
to	O
determine	O
the	O
answer	O
and	O
send	O
a	O
single	O
answer	O
back	O
to	O
the	O
client	O
.	O
</s>
<s>
The	O
resolver	O
,	O
or	O
another	O
DNS	B-Protocol
server	I-Protocol
acting	O
recursively	O
on	O
behalf	O
of	O
the	O
resolver	O
,	O
negotiates	O
use	O
of	O
recursive	O
service	O
using	O
bits	O
in	O
the	O
query	O
headers	O
.	O
</s>
<s>
DNS	B-Protocol
servers	I-Protocol
are	O
not	O
required	O
to	O
support	O
recursive	O
queries	O
.	O
</s>
<s>
The	O
iterative	O
query	O
procedure	O
is	O
a	O
process	O
in	O
which	O
a	O
DNS	B-Protocol
resolver	O
queries	O
a	O
chain	O
of	O
one	O
or	O
more	O
DNS	B-Protocol
servers	I-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
a	O
possible	O
resolution	O
of	O
www.example.com	O
would	O
query	O
a	O
global	O
root	B-Application
server	I-Application
,	O
then	O
a	O
"	O
com	O
"	O
server	O
,	O
and	O
finally	O
an	O
"	O
example.com	O
 "	O
server	O
.	O
</s>
<s>
Name	B-Protocol
servers	I-Protocol
in	O
delegations	O
are	O
identified	O
by	O
name	O
,	O
rather	O
than	O
by	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
This	O
means	O
that	O
a	O
resolving	O
name	B-Protocol
server	I-Protocol
must	O
issue	O
another	O
DNS	B-Protocol
request	O
to	O
find	O
out	O
the	O
IP	B-Protocol
address	I-Protocol
of	O
the	O
server	O
to	O
which	O
it	O
has	O
been	O
referred	O
.	O
</s>
<s>
If	O
the	O
name	O
given	O
in	O
the	O
delegation	O
is	O
a	O
subdomain	B-Protocol
of	O
the	O
domain	O
for	O
which	O
the	O
delegation	O
is	O
being	O
provided	O
,	O
there	O
is	O
a	O
circular	B-Language
dependency	I-Language
.	O
</s>
<s>
In	O
this	O
case	O
,	O
the	O
name	B-Protocol
server	I-Protocol
providing	O
the	O
delegation	O
must	O
also	O
provide	O
one	O
or	O
more	O
IP	B-Protocol
addresses	I-Protocol
for	O
the	O
authoritative	O
name	B-Protocol
server	I-Protocol
mentioned	O
in	O
the	O
delegation	O
.	O
</s>
<s>
The	O
delegating	O
name	B-Protocol
server	I-Protocol
provides	O
this	O
glue	O
in	O
the	O
form	O
of	O
records	O
in	O
the	O
additional	O
section	O
of	O
the	O
DNS	B-Protocol
response	O
,	O
and	O
provides	O
the	O
delegation	O
in	O
the	O
authority	O
section	O
of	O
the	O
response	O
.	O
</s>
<s>
A	O
glue	O
record	O
is	O
a	O
combination	O
of	O
the	O
name	B-Protocol
server	I-Protocol
and	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
if	O
the	O
authoritative	O
name	B-Protocol
server	I-Protocol
for	O
example.org	O
is	O
ns1.example.org,	O
a	O
computer	O
trying	O
to	O
resolve	O
www.example.org	O
first	O
resolves	O
ns1.example.org	O
.	O
</s>
<s>
As	O
ns1	O
is	O
contained	O
in	O
example.org,	O
this	O
requires	O
resolving	O
example.org	O
first	O
,	O
which	O
presents	O
a	O
circular	B-Language
dependency	I-Language
.	O
</s>
<s>
To	O
break	O
the	O
dependency	O
,	O
the	O
name	B-Protocol
server	I-Protocol
for	O
the	O
top	O
level	O
domain	O
org	O
includes	O
glue	O
along	O
with	O
the	O
delegation	O
for	O
example.org	O
.	O
</s>
<s>
The	O
glue	O
records	O
are	O
address	O
records	O
that	O
provide	O
IP	B-Protocol
addresses	I-Protocol
for	O
ns1.example.org	O
.	O
</s>
<s>
The	O
resolver	O
uses	O
one	O
or	O
more	O
of	O
these	O
IP	B-Protocol
addresses	I-Protocol
to	O
query	O
one	O
of	O
the	O
domain	O
's	O
authoritative	O
servers	O
,	O
which	O
allows	O
it	O
to	O
complete	O
the	O
DNS	B-Protocol
query	I-Protocol
.	O
</s>
<s>
A	O
standard	O
practice	O
in	O
implementing	O
name	O
resolution	O
in	O
applications	O
is	O
to	O
reduce	O
the	O
load	O
on	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
servers	O
by	O
caching	O
results	O
locally	O
,	O
or	O
in	O
intermediate	O
resolver	O
hosts	B-Protocol
.	O
</s>
<s>
Results	O
obtained	O
from	O
a	O
DNS	B-Protocol
request	O
are	O
always	O
associated	O
with	O
the	O
time	B-Protocol
to	I-Protocol
live	I-Protocol
(	O
TTL	B-Protocol
)	O
,	O
an	O
expiration	O
time	O
after	O
which	O
the	O
results	O
must	O
be	O
discarded	O
or	O
refreshed	O
.	O
</s>
<s>
The	O
TTL	B-Protocol
is	O
set	O
by	O
the	O
administrator	O
of	O
the	O
authoritative	O
DNS	B-Protocol
server	I-Protocol
.	O
</s>
<s>
As	O
a	O
result	O
of	O
this	O
distributed	O
caching	O
architecture	O
,	O
changes	O
to	O
DNS	B-Protocol
records	O
do	O
not	O
propagate	O
throughout	O
the	O
network	O
immediately	O
,	O
but	O
require	O
all	O
caches	O
to	O
expire	O
and	O
to	O
be	O
refreshed	O
after	O
the	O
TTL	B-Protocol
.	O
</s>
<s>
RFC	O
1912	O
conveys	O
basic	O
rules	O
for	O
determining	O
appropriate	O
TTL	B-Protocol
values	O
.	O
</s>
<s>
Some	O
resolvers	O
may	O
override	O
TTL	B-Protocol
values	O
,	O
as	O
the	O
protocol	O
supports	O
caching	O
for	O
up	O
to	O
sixty-eight	O
years	O
or	O
no	O
caching	O
at	O
all	O
.	O
</s>
<s>
Negative	B-Error_Name
caching	I-Error_Name
,	O
i.e.	O
</s>
<s>
the	O
caching	O
of	O
the	O
fact	O
of	O
non-existence	O
of	O
a	O
record	O
,	O
is	O
determined	O
by	O
name	B-Protocol
servers	I-Protocol
authoritative	O
for	O
a	O
zone	B-Protocol
which	O
must	O
include	O
the	O
Start	B-Protocol
of	I-Protocol
Authority	I-Protocol
(	O
SOA	B-Protocol
)	O
record	O
when	O
reporting	O
no	O
data	O
of	O
the	O
requested	O
type	O
exists	O
.	O
</s>
<s>
The	O
value	O
of	O
the	O
minimum	O
field	O
of	O
the	O
SOA	B-Protocol
record	I-Protocol
and	O
the	O
TTL	B-Protocol
of	O
the	O
SOA	B-Protocol
itself	O
is	O
used	O
to	O
establish	O
the	O
TTL	B-Protocol
for	O
the	O
negative	O
answer	O
.	O
</s>
<s>
A	O
reverse	B-Protocol
DNS	I-Protocol
lookup	I-Protocol
is	O
a	O
query	O
of	O
the	O
DNS	B-Protocol
for	O
domain	O
names	O
when	O
the	O
IP	B-Protocol
address	I-Protocol
is	O
known	O
.	O
</s>
<s>
Multiple	O
domain	O
names	O
may	O
be	O
associated	O
with	O
an	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
The	O
DNS	B-Protocol
stores	O
IP	B-Protocol
addresses	I-Protocol
in	O
the	O
form	O
of	O
domain	O
names	O
as	O
specially	O
formatted	O
names	O
in	O
pointer	O
(	O
PTR	O
)	O
records	O
within	O
the	O
infrastructure	O
top-level	O
domain	O
arpa	O
.	O
</s>
<s>
For	O
IPv4	B-Protocol
,	O
the	O
domain	O
is	O
in-addr.arpa	B-Protocol
.	O
</s>
<s>
For	O
IPv6	B-Protocol
,	O
the	O
reverse	O
lookup	O
domain	O
is	O
ip6.arpa	O
.	O
</s>
<s>
The	O
IP	B-Protocol
address	I-Protocol
is	O
represented	O
as	O
a	O
name	O
in	O
reverse-ordered	O
octet	O
representation	O
for	O
IPv4	B-Protocol
,	O
and	O
reverse-ordered	O
nibble	O
representation	O
for	O
IPv6	B-Protocol
.	O
</s>
<s>
When	O
performing	O
a	O
reverse	O
lookup	O
,	O
the	O
DNS	B-Protocol
client	O
converts	O
the	O
address	O
into	O
these	O
formats	O
before	O
querying	O
the	O
name	O
for	O
a	O
PTR	O
record	O
following	O
the	O
delegation	O
chain	O
as	O
for	O
any	O
DNS	B-Protocol
query	I-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
assuming	O
the	O
IPv4	B-Protocol
address	I-Protocol
208.80.152.2	O
is	O
assigned	O
to	O
Wikimedia	O
,	O
it	O
is	O
represented	O
as	O
a	O
DNS	B-Protocol
name	I-Protocol
in	O
reverse	O
order	O
:	O
2.152.80.208.in-addr.arpa	O
.	O
</s>
<s>
When	O
the	O
DNS	B-Protocol
resolver	O
gets	O
a	O
pointer	O
(	O
PTR	O
)	O
request	O
,	O
it	O
begins	O
by	O
querying	O
the	O
root	B-Application
servers	I-Application
,	O
which	O
point	O
to	O
the	O
servers	O
of	O
American	O
Registry	B-Protocol
for	O
Internet	O
Numbers	O
(	O
ARIN	O
)	O
for	O
the	O
208.in-addr.arpa	O
zone	B-Protocol
.	O
</s>
<s>
ARIN	O
's	O
servers	O
delegate	O
152.80.208.in-addr.arpa	O
to	O
Wikimedia	O
to	O
which	O
the	O
resolver	O
sends	O
another	O
query	O
for	O
2.152.80.208.in-addr.arpa	O
,	O
which	O
results	O
in	O
an	O
authoritative	O
response	O
.	O
</s>
<s>
Users	O
generally	O
do	O
not	O
communicate	O
directly	O
with	O
a	O
DNS	B-Protocol
resolver	O
.	O
</s>
<s>
Instead	O
DNS	B-Protocol
resolution	O
takes	O
place	O
transparently	O
in	O
applications	O
such	O
as	O
web	B-Application
browsers	I-Application
,	O
e-mail	B-Protocol
clients	I-Protocol
,	O
and	O
other	O
Internet	O
applications	O
.	O
</s>
<s>
When	O
an	O
application	O
makes	O
a	O
request	O
that	O
requires	O
a	O
domain	O
name	O
lookup	O
,	O
such	O
programs	O
send	O
a	O
resolution	O
request	O
to	O
the	O
DNS	B-Protocol
resolver	O
in	O
the	O
local	O
operating	O
system	O
,	O
which	O
in	O
turn	O
handles	O
the	O
communications	O
required	O
.	O
</s>
<s>
The	O
DNS	B-Protocol
resolver	O
will	O
almost	O
invariably	O
have	O
a	O
cache	O
(	O
see	O
above	O
)	O
containing	O
recent	O
lookups	O
.	O
</s>
<s>
If	O
the	O
cache	O
does	O
not	O
contain	O
the	O
answer	O
,	O
the	O
resolver	O
will	O
send	O
the	O
request	O
to	O
one	O
or	O
more	O
designated	O
DNS	B-Protocol
servers	I-Protocol
.	O
</s>
<s>
In	O
the	O
case	O
of	O
most	O
home	O
users	O
,	O
the	O
Internet	O
service	O
provider	O
to	O
which	O
the	O
machine	O
connects	O
will	O
usually	O
supply	O
this	O
DNS	B-Protocol
server	I-Protocol
:	O
such	O
a	O
user	O
will	O
either	O
have	O
configured	O
that	O
server	O
's	O
address	O
manually	O
or	O
allowed	O
DHCP	B-Protocol
to	O
set	O
it	O
;	O
however	O
,	O
where	O
systems	O
administrators	O
have	O
configured	O
systems	O
to	O
use	O
their	O
own	O
DNS	B-Protocol
servers	I-Protocol
,	O
their	O
DNS	B-Protocol
resolvers	O
point	O
to	O
separately	O
maintained	O
name	B-Protocol
servers	I-Protocol
of	O
the	O
organization	O
.	O
</s>
<s>
In	O
any	O
event	O
,	O
the	O
name	B-Protocol
server	I-Protocol
thus	O
queried	O
will	O
follow	O
the	O
process	O
outlined	O
above	O
,	O
until	O
it	O
either	O
successfully	O
finds	O
a	O
result	O
or	O
does	O
not	O
.	O
</s>
<s>
It	O
then	O
returns	O
its	O
results	O
to	O
the	O
DNS	B-Protocol
resolver	O
;	O
assuming	O
it	O
has	O
found	O
a	O
result	O
,	O
the	O
resolver	O
duly	O
caches	O
that	O
result	O
for	O
future	O
use	O
,	O
and	O
hands	O
the	O
result	O
back	O
to	O
the	O
software	O
which	O
initiated	O
the	O
request	O
.	O
</s>
<s>
Some	O
large	O
ISPs	O
have	O
configured	O
their	O
DNS	B-Protocol
servers	I-Protocol
to	O
violate	O
rules	O
,	O
such	O
as	O
by	O
disobeying	O
TTLs	O
,	O
or	O
by	O
indicating	O
that	O
a	O
domain	O
name	O
does	O
not	O
exist	O
just	O
because	O
one	O
of	O
its	O
name	B-Protocol
servers	I-Protocol
does	O
not	O
respond	O
.	O
</s>
<s>
Some	O
applications	O
such	O
as	O
web	B-Application
browsers	I-Application
maintain	O
an	O
internal	O
DNS	B-Protocol
cache	O
to	O
avoid	O
repeated	O
lookups	O
via	O
the	O
network	O
.	O
</s>
<s>
This	O
practice	O
can	O
add	O
extra	O
difficulty	O
when	O
debugging	O
DNS	B-Protocol
issues	O
as	O
it	O
obscures	O
the	O
history	O
of	O
such	O
data	O
.	O
</s>
<s>
Internet	B-Application
Explorer	I-Application
represents	O
a	O
notable	O
exception	O
:	O
versions	O
up	O
to	O
IE	O
3.x	O
cache	O
DNS	B-Protocol
records	O
for	O
24	O
hours	O
by	O
default	O
.	O
</s>
<s>
Internet	B-Application
Explorer	I-Application
4.x	O
and	O
later	O
versions	O
(	O
up	O
to	O
IE	O
8	O
)	O
decrease	O
the	O
default	O
timeout	O
value	O
to	O
half	O
an	O
hour	O
,	O
which	O
may	O
be	O
changed	O
by	O
modifying	O
the	O
default	O
configuration	O
.	O
</s>
<s>
When	O
Google	B-Application
Chrome	I-Application
detects	O
issues	O
with	O
the	O
DNS	B-Protocol
server	I-Protocol
it	O
displays	O
a	O
specific	O
error	O
message	O
.	O
</s>
<s>
The	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
includes	O
several	O
other	O
functions	O
and	O
features	O
.	O
</s>
<s>
Hostnames	B-Protocol
and	O
IP	B-Protocol
addresses	I-Protocol
are	O
not	O
required	O
to	O
match	O
in	O
a	O
one-to-one	O
relationship	O
.	O
</s>
<s>
Multiple	O
hostnames	B-Protocol
may	O
correspond	O
to	O
a	O
single	O
IP	B-Protocol
address	I-Protocol
,	O
which	O
is	O
useful	O
in	O
virtual	O
hosting	O
,	O
in	O
which	O
many	O
web	O
sites	O
are	O
served	O
from	O
a	O
single	O
host	O
.	O
</s>
<s>
Alternatively	O
,	O
a	O
single	O
hostname	B-Protocol
may	O
resolve	O
to	O
many	O
IP	B-Protocol
addresses	I-Protocol
to	O
facilitate	O
fault	B-General_Concept
tolerance	I-General_Concept
and	O
load	B-Application
distribution	I-Application
to	O
multiple	O
server	O
instances	O
across	O
an	O
enterprise	O
or	O
the	O
global	O
Internet	O
.	O
</s>
<s>
DNS	B-Protocol
serves	O
other	O
purposes	O
in	O
addition	O
to	O
translating	O
names	O
to	O
IP	B-Protocol
addresses	I-Protocol
.	O
</s>
<s>
For	O
instance	O
,	O
mail	B-Protocol
transfer	I-Protocol
agents	I-Protocol
use	O
DNS	B-Protocol
to	O
find	O
the	O
best	O
mail	B-Protocol
server	I-Protocol
to	O
deliver	O
e-mail	O
:	O
An	O
MX	B-Protocol
record	I-Protocol
provides	O
a	O
mapping	O
between	O
a	O
domain	O
and	O
a	O
mail	B-Protocol
exchanger	I-Protocol
;	O
this	O
can	O
provide	O
an	O
additional	O
layer	O
of	O
fault	B-General_Concept
tolerance	I-General_Concept
and	O
load	B-Application
distribution	I-Application
.	O
</s>
<s>
The	O
DNS	B-Protocol
is	O
used	O
for	O
efficient	O
storage	O
and	O
distribution	O
of	O
IP	B-Protocol
addresses	I-Protocol
of	O
blacklisted	O
email	O
hosts	B-Protocol
.	O
</s>
<s>
A	O
common	O
method	O
is	O
to	O
place	O
the	O
IP	B-Protocol
address	I-Protocol
of	O
the	O
subject	O
host	O
into	O
the	O
sub-domain	B-Protocol
of	O
a	O
higher	O
level	O
domain	O
name	O
,	O
and	O
to	O
resolve	O
that	O
name	O
to	O
a	O
record	O
that	O
indicates	O
a	O
positive	O
or	O
a	O
negative	O
indication	O
.	O
</s>
<s>
This	O
hostname	B-Protocol
is	O
either	O
not	O
configured	O
,	O
or	O
resolves	O
to	O
127.0.0.2	O
.	O
</s>
<s>
E-mail	B-Protocol
servers	I-Protocol
can	O
query	O
blacklist.example	O
to	O
find	O
out	O
if	O
a	O
specific	O
host	O
connecting	O
to	O
them	O
is	O
in	O
the	O
blacklist	O
.	O
</s>
<s>
To	O
provide	O
resilience	O
in	O
the	O
event	O
of	O
computer	O
or	O
network	O
failure	O
,	O
multiple	O
DNS	B-Protocol
servers	I-Protocol
are	O
usually	O
provided	O
for	O
coverage	O
of	O
each	O
domain	O
.	O
</s>
<s>
At	O
the	O
top	O
level	O
of	O
global	O
DNS	B-Protocol
,	O
thirteen	O
groups	O
of	O
root	B-Application
name	I-Application
servers	I-Application
exist	O
,	O
with	O
additional	O
"	O
copies	O
"	O
of	O
them	O
distributed	O
worldwide	O
via	O
anycast	B-Protocol
addressing	O
.	O
</s>
<s>
Dynamic	B-Protocol
DNS	I-Protocol
(	O
DDNS	B-Protocol
)	O
updates	O
a	O
DNS	B-Protocol
server	I-Protocol
with	O
a	O
client	O
IP	B-Protocol
address	I-Protocol
on-the-fly	O
,	O
for	O
example	O
,	O
when	O
moving	O
between	O
ISPs	O
or	O
mobile	O
hot	B-Application
spots	I-Application
,	O
or	O
when	O
the	O
IP	B-Protocol
address	I-Protocol
changes	O
administratively	O
.	O
</s>
<s>
The	O
DNS	B-Protocol
protocol	I-Protocol
uses	O
two	O
types	O
of	O
DNS	B-Protocol
messages	O
,	O
queries	O
and	O
replies	O
;	O
both	O
have	O
the	O
same	O
format	O
.	O
</s>
<s>
+Header	O
flags	O
format	O
Field	O
Description	O
Length	O
(	O
bits	O
)	O
QR	O
Indicates	O
if	O
the	O
message	O
is	O
a	O
query	O
(	O
0	O
)	O
or	O
a	O
reply	O
(	O
1	O
)	O
1	O
OPCODE	O
The	O
type	O
can	O
be	O
QUERY	O
(	O
standard	O
query	O
,	O
0	O
)	O
,	O
IQUERY	O
(	O
inverse	O
query	O
,	O
1	O
)	O
,	O
or	O
STATUS	O
(	O
server	O
status	O
request	O
,	O
2	O
)	O
4	O
AA	O
Authoritative	O
Answer	O
,	O
in	O
a	O
response	O
,	O
indicates	O
if	O
the	O
DNS	B-Protocol
server	I-Protocol
is	O
authoritative	O
for	O
the	O
queried	O
hostname	B-Protocol
1	O
TC	O
TrunCation	O
,	O
indicates	O
that	O
this	O
message	O
was	O
truncated	O
due	O
to	O
excessive	O
length	O
1	O
RD	O
Recursion	O
Desired	O
,	O
indicates	O
if	O
the	O
client	O
means	O
a	O
recursive	O
query	O
1	O
RA	O
Recursion	O
Available	O
,	O
in	O
a	O
response	O
,	O
indicates	O
if	O
the	O
replying	O
DNS	B-Protocol
server	I-Protocol
supports	O
recursion	O
1	O
Z	O
Zero	O
,	O
reserved	O
for	O
future	O
use	O
3	O
RCODE	O
Response	O
code	O
,	O
can	O
be	O
NOERROR	O
(	O
0	O
)	O
,	O
FORMERR	O
(	O
1	O
,	O
Format	O
error	O
)	O
,	O
SERVFAIL	O
(	O
2	O
)	O
,	O
NXDOMAIN	O
(	O
3	O
,	O
Nonexistent	O
domain	O
)	O
,	O
etc	O
.	O
</s>
<s>
+	O
Resource	O
record	O
(	O
RR	O
)	O
fields	O
Field	O
Description	O
Length	O
(	O
octets	O
)	O
NAME	O
Name	O
of	O
the	O
requested	O
resource	O
Variable	O
TYPE	O
Type	O
of	O
RR	O
(	O
A	O
,	O
AAAA	B-Protocol
,	O
MX	O
,	O
TXT	O
,	O
etc	O
.	O
)	O
</s>
<s>
The	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
specifies	O
a	O
database	B-General_Concept
of	O
information	O
elements	O
for	O
network	O
resources	O
.	O
</s>
<s>
The	O
types	O
of	O
information	O
elements	O
are	O
categorized	O
and	O
organized	O
with	O
a	O
list	B-Protocol
of	I-Protocol
DNS	I-Protocol
record	I-Protocol
types	I-Protocol
,	O
the	O
resource	O
records	O
(	O
RRs	O
)	O
.	O
</s>
<s>
Each	O
record	O
has	O
a	O
type	O
(	O
name	O
and	O
number	O
)	O
,	O
an	O
expiration	O
time	O
(	O
time	B-Protocol
to	I-Protocol
live	I-Protocol
)	O
,	O
a	O
class	O
,	O
and	O
type-specific	O
data	O
.	O
</s>
<s>
DNS	B-Protocol
resolvers	O
return	O
the	O
entire	O
set	O
upon	O
query	O
,	O
but	O
servers	O
may	O
implement	O
round-robin	B-Protocol
ordering	I-Protocol
to	O
achieve	O
load	B-Application
balancing	I-Application
.	O
</s>
<s>
In	O
contrast	O
,	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
Security	I-Protocol
Extensions	I-Protocol
(	O
DNSSEC	B-Protocol
)	O
work	O
on	O
the	O
complete	O
set	O
of	O
resource	O
record	O
in	O
canonical	O
order	O
.	O
</s>
<s>
When	O
sent	O
over	O
an	O
Internet	B-Protocol
Protocol	I-Protocol
network	O
,	O
all	O
records	O
use	O
the	O
common	O
format	O
specified	O
in	O
RFC	O
1035	O
:	O
</s>
<s>
NAME	O
is	O
the	O
fully	B-Protocol
qualified	I-Protocol
domain	I-Protocol
name	I-Protocol
of	O
the	O
node	O
in	O
the	O
tree	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
A	O
record	O
is	O
used	O
to	O
translate	O
from	O
a	O
domain	O
name	O
to	O
an	O
IPv4	B-Protocol
address	I-Protocol
,	O
the	O
NS	O
record	O
lists	O
which	O
name	B-Protocol
servers	I-Protocol
can	O
answer	O
lookups	O
on	O
a	O
DNS	B-Protocol
zone	I-Protocol
,	O
and	O
the	O
MX	B-Protocol
record	I-Protocol
specifies	O
the	O
mail	B-Protocol
server	I-Protocol
used	O
to	O
handle	O
mail	O
for	O
a	O
domain	O
specified	O
in	O
an	O
e-mail	O
address	O
.	O
</s>
<s>
RDATA	O
is	O
data	O
of	O
type-specific	O
relevance	O
,	O
such	O
as	O
the	O
IP	B-Protocol
address	I-Protocol
for	O
address	O
records	O
,	O
or	O
the	O
priority	O
and	O
hostname	B-Protocol
for	O
MX	B-Protocol
records	I-Protocol
.	O
</s>
<s>
The	O
CLASS	O
of	O
a	O
record	O
is	O
set	O
to	O
IN	O
(	O
for	O
Internet	O
)	O
for	O
common	O
DNS	B-Protocol
records	O
involving	O
Internet	O
hostnames	B-Protocol
,	O
servers	O
,	O
or	O
IP	B-Protocol
addresses	I-Protocol
.	O
</s>
<s>
In	O
addition	O
,	O
the	O
classes	O
Chaos	B-General_Concept
(	O
CH	O
)	O
and	O
Hesiod	B-Protocol
(	O
HS	O
)	O
exist	O
.	O
</s>
<s>
Each	O
class	O
is	O
an	O
independent	O
name	O
space	O
with	O
potentially	O
different	O
delegations	O
of	O
DNS	B-Protocol
zones	I-Protocol
.	O
</s>
<s>
In	O
addition	O
to	O
resource	O
records	O
defined	O
in	O
a	O
zone	B-Protocol
file	I-Protocol
,	O
the	O
domain	B-Protocol
name	I-Protocol
system	I-Protocol
also	O
defines	O
several	O
request	O
types	O
that	O
are	O
used	O
only	O
in	O
communication	O
with	O
other	O
DNS	B-Protocol
nodes	O
(	O
on	O
the	O
wire	O
)	O
,	O
such	O
as	O
when	O
performing	O
zone	B-Protocol
transfers	I-Protocol
(	O
AXFR/IXFR	O
)	O
or	O
for	O
EDNS	B-Protocol
(	O
OPT	O
)	O
.	O
</s>
<s>
The	O
domain	B-Protocol
name	I-Protocol
system	I-Protocol
supports	O
wildcard	B-Protocol
DNS	I-Protocol
records	I-Protocol
which	O
specify	O
names	O
that	O
start	O
with	O
the	O
asterisk	O
label	O
,	O
'	O
*	O
 '	O
,	O
e.g.	O
,	O
*	O
.example	O
.	O
</s>
<s>
DNS	B-Protocol
records	O
belonging	O
to	O
wildcard	O
domain	O
names	O
specify	O
rules	O
for	O
generating	O
resource	O
records	O
within	O
a	O
single	O
DNS	B-Protocol
zone	I-Protocol
by	O
substituting	O
whole	O
labels	O
with	O
matching	O
components	O
of	O
the	O
query	O
name	O
,	O
including	O
any	O
specified	O
descendants	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
the	O
following	O
configuration	O
,	O
the	O
DNS	B-Protocol
zone	I-Protocol
x.example	O
specifies	O
that	O
all	O
subdomains	B-Protocol
,	O
including	O
subdomains	B-Protocol
of	O
subdomains	B-Protocol
,	O
of	O
x.example	O
use	O
the	O
mail	B-Protocol
exchanger	I-Protocol
(	O
MX	O
)	O
a.x.example	O
.	O
</s>
<s>
The	O
A	O
record	O
for	O
a.x.example	O
is	O
needed	O
to	O
specify	O
the	O
mail	B-Protocol
exchanger	I-Protocol
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
As	O
this	O
has	O
the	O
result	O
of	O
excluding	O
this	O
domain	O
name	O
and	O
its	O
subdomains	B-Protocol
from	O
the	O
wildcard	O
matches	O
,	O
an	O
additional	O
MX	B-Protocol
record	I-Protocol
for	O
the	O
subdomain	B-Protocol
a.x.example	O
,	O
as	O
well	O
as	O
a	O
wildcarded	O
MX	B-Protocol
record	I-Protocol
for	O
all	O
of	O
its	O
subdomains	B-Protocol
,	O
must	O
also	O
be	O
defined	O
in	O
the	O
DNS	B-Protocol
zone	I-Protocol
.	O
</s>
<s>
The	O
original	O
DNS	B-Protocol
protocol	I-Protocol
had	O
limited	O
provisions	O
for	O
extension	O
with	O
new	O
features	O
.	O
</s>
<s>
In	O
1999	O
,	O
Paul	O
Vixie	O
published	O
in	O
RFC	B-Protocol
2671	I-Protocol
(	O
superseded	O
by	O
RFC	O
6891	O
)	O
an	O
extension	O
mechanism	O
,	O
called	O
Extension	B-Protocol
Mechanisms	I-Protocol
for	I-Protocol
DNS	I-Protocol
(	O
EDNS	B-Protocol
)	O
that	O
introduced	O
optional	O
protocol	O
elements	O
without	O
increasing	O
overhead	O
when	O
not	O
in	O
use	O
.	O
</s>
<s>
This	O
was	O
accomplished	O
through	O
the	O
OPT	O
pseudo-resource	O
record	O
that	O
only	O
exists	O
in	O
wire	O
transmissions	O
of	O
the	O
protocol	O
,	O
but	O
not	O
in	O
any	O
zone	B-Protocol
files	I-Protocol
.	O
</s>
<s>
Initial	O
extensions	O
were	O
also	O
suggested	O
(	O
EDNS0	B-Protocol
)	O
,	O
such	O
as	O
increasing	O
the	O
DNS	B-Protocol
message	O
size	O
in	O
UDP	O
datagrams	O
.	O
</s>
<s>
Dynamic	B-Protocol
DNS	I-Protocol
updates	I-Protocol
use	O
the	O
UPDATE	O
DNS	B-Protocol
opcode	O
to	O
add	O
or	O
remove	O
resource	O
records	O
dynamically	O
from	O
a	O
zone	B-Protocol
database	B-General_Concept
maintained	O
on	O
an	O
authoritative	O
DNS	B-Protocol
server	I-Protocol
.	O
</s>
<s>
This	O
facility	O
is	O
useful	O
to	O
register	O
network	O
clients	O
into	O
the	O
DNS	B-Protocol
when	O
they	O
boot	O
or	O
become	O
otherwise	O
available	O
on	O
the	O
network	O
.	O
</s>
<s>
As	O
a	O
booting	O
client	O
may	O
be	O
assigned	O
a	O
different	O
IP	B-Protocol
address	I-Protocol
each	O
time	O
from	O
a	O
DHCP	B-Protocol
server	I-Protocol
,	O
it	O
is	O
not	O
possible	O
to	O
provide	O
static	O
DNS	B-Protocol
assignments	O
for	O
such	O
clients	O
.	O
</s>
<s>
From	O
the	O
time	O
of	O
its	O
origin	O
in	O
1983	O
the	O
DNS	B-Protocol
has	O
used	O
the	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
for	O
transport	O
over	O
IP	O
.	O
</s>
<s>
DNS	B-Protocol
over	O
UDP/53	O
(	O
Do53	O
)	O
UDP	O
reserves	O
port	B-Protocol
number	I-Protocol
53	O
for	O
servers	O
listening	O
to	O
queries	O
.	O
</s>
<s>
Such	O
queries	O
consist	O
of	O
a	O
clear-text	O
request	O
sent	O
in	O
a	O
single	O
UDP	B-Protocol
packet	I-Protocol
from	O
the	O
client	O
,	O
responded	O
to	O
with	O
a	O
clear-text	O
reply	O
sent	O
in	O
a	O
single	O
UDP	B-Protocol
packet	I-Protocol
from	O
the	O
server	O
.	O
</s>
<s>
When	O
the	O
length	O
of	O
the	O
answer	O
exceeds	O
512	O
bytes	O
and	O
both	O
client	O
and	O
server	O
support	O
Extension	B-Protocol
Mechanisms	I-Protocol
for	I-Protocol
DNS	I-Protocol
(	O
EDNS	B-Protocol
)	O
,	O
larger	O
UDP	B-Protocol
packets	I-Protocol
may	O
be	O
used	O
.	O
</s>
<s>
Use	O
of	O
DNS	B-Protocol
over	O
UDP	O
is	O
limited	O
by	O
,	O
among	O
other	O
things	O
,	O
its	O
lack	O
of	O
transport-layer	O
encryption	O
,	O
authentication	O
,	O
reliable	O
delivery	O
,	O
and	O
message	O
length	O
.	O
</s>
<s>
DNS	B-Protocol
over	O
TCP/53	O
(	O
Do53/TCP	O
)	O
In	O
1989	O
,	O
RFC	O
1123	O
specified	O
optional	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
transport	O
for	O
DNS	B-Protocol
queries	I-Protocol
,	O
replies	O
and	O
,	O
particularly	O
,	O
zone	B-Protocol
transfers	I-Protocol
.	O
</s>
<s>
DNS	B-Protocol
over	I-Protocol
TLS	I-Protocol
(	O
DoT	O
)	O
DNS	B-Protocol
over	I-Protocol
TLS	I-Protocol
emerged	O
as	O
an	O
IETF	O
standard	O
for	O
encrypted	O
DNS	B-Protocol
in	O
2016	O
,	O
utilizing	O
Transport	O
Layer	O
Security	O
(	O
TLS	O
)	O
to	O
protect	O
the	O
entire	O
connection	O
,	O
rather	O
than	O
just	O
the	O
DNS	B-Protocol
payload	O
.	O
</s>
<s>
DoT	O
servers	O
listen	O
on	O
TCP	B-Protocol
port	I-Protocol
853	O
.	O
</s>
<s>
DNS	B-Protocol
over	I-Protocol
HTTPS	I-Protocol
(	O
DoH	O
)	O
DNS	B-Protocol
over	I-Protocol
HTTPS	I-Protocol
was	O
developed	O
as	O
a	O
competing	O
standard	O
for	O
DNS	B-Protocol
query	I-Protocol
transport	O
in	O
2018	O
,	O
tunneling	O
DNS	B-Protocol
query	I-Protocol
data	O
over	O
HTTPS	O
,	O
which	O
transports	O
HTTP	O
over	O
TLS	O
.	O
</s>
<s>
DoH	O
was	O
promoted	O
as	O
a	O
more	O
web-friendly	O
alternative	O
to	O
DNS	B-Protocol
since	O
,	O
like	O
DNSCrypt	B-Protocol
,	O
it	O
uses	O
TCP	B-Protocol
port	I-Protocol
443	O
,	O
and	O
thus	O
looks	O
similar	O
to	O
web	O
traffic	O
,	O
though	O
they	O
are	O
easily	O
differentiable	O
in	O
practice	O
.	O
</s>
<s>
Oblivious	O
DNS	B-Protocol
(	O
ODNS	O
)	O
and	O
Oblivious	O
DoH	O
(	O
ODoH	O
)	O
Oblivious	O
DNS	B-Protocol
(	O
ODNS	O
)	O
was	O
invented	O
and	O
implemented	O
by	O
researchers	O
at	O
Princeton	O
University	O
and	O
the	O
University	O
of	O
Chicago	O
as	O
an	O
extension	O
to	O
unencrypted	O
DNS	B-Protocol
,	O
before	O
DoH	O
was	O
standardized	O
and	O
widely	O
deployed	O
.	O
</s>
<s>
DNS	B-Protocol
over	O
TOR	B-Application
DNS	B-Protocol
may	O
be	O
run	O
over	O
virtual	B-Application
private	I-Application
networks	I-Application
(	O
VPNs	B-Application
)	O
and	O
tunneling	B-Protocol
protocols	I-Protocol
.	O
</s>
<s>
A	O
use	O
which	O
has	O
become	O
common	O
since	O
2019	O
to	O
warrant	O
its	O
own	O
frequently	O
used	O
acronym	O
is	O
DNS	B-Protocol
over	O
Tor	B-Application
.	O
</s>
<s>
The	O
privacy	O
gains	O
of	O
Oblivious	O
DNS	B-Protocol
can	O
be	O
garnered	O
through	O
the	O
use	O
of	O
the	O
preexisting	O
Tor	B-Application
network	I-Application
of	O
ingress	O
and	O
egress	O
nodes	O
,	O
paired	O
with	O
the	O
transport-layer	O
encryption	O
provided	O
by	O
TLS	O
.	O
</s>
<s>
DNS	B-Protocol
over	O
QUIC	B-Protocol
(	O
DoQ	O
)	O
by	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
describes	O
DNS	B-Protocol
over	O
QUIC	B-Protocol
.	O
</s>
<s>
DNSCrypt	B-Protocol
The	O
DNSCrypt	B-Protocol
protocol	O
,	O
which	O
was	O
developed	O
in	O
2011	O
outside	O
the	O
IETF	O
standards	O
framework	O
,	O
introduced	O
DNS	B-Protocol
encryption	O
on	O
the	O
downstream	O
side	O
of	O
recursive	O
resolvers	O
,	O
wherein	O
clients	O
encrypt	O
query	O
payloads	O
using	O
servers	O
 '	O
public	O
keys	O
,	O
which	O
are	O
published	O
in	O
the	O
DNS	B-Protocol
(	O
rather	O
than	O
relying	O
upon	O
third-party	O
certificate	O
authorities	O
)	O
and	O
which	O
may	O
in	O
turn	O
be	O
protected	O
by	O
DNSSEC	B-Protocol
signatures	O
.	O
</s>
<s>
DNSCrypt	B-Protocol
uses	O
either	O
TCP	O
or	O
UDP	O
port	O
443	O
,	O
the	O
same	O
port	O
as	O
HTTPS	O
encrypted	O
web	O
traffic	O
.	O
</s>
<s>
In	O
2019	O
,	O
DNSCrypt	B-Protocol
was	O
further	O
extended	O
to	O
support	O
an	O
"	O
anonymized	O
"	O
mode	O
,	O
similar	O
to	O
the	O
proposed	O
"	O
Oblivious	O
DNS	B-Protocol
,	O
"	O
in	O
which	O
an	O
ingress	O
node	O
receives	O
a	O
query	O
which	O
has	O
been	O
encrypted	O
with	O
the	O
public	O
key	O
of	O
a	O
different	O
server	O
,	O
and	O
relays	O
it	O
to	O
that	O
server	O
,	O
which	O
acts	O
as	O
an	O
egress	O
node	O
,	O
performing	O
the	O
recursive	O
resolution	O
.	O
</s>
<s>
DNSCrypt	B-Protocol
was	O
first	O
implemented	O
in	O
production	O
by	O
OpenDNS	B-Protocol
in	O
December	O
2011	O
.	O
</s>
<s>
It	O
is	O
available	O
for	O
a	O
variety	O
of	O
operating	O
systems	O
,	O
including	O
Unix	B-Application
,	O
Apple	O
iOS	B-Application
,	O
Linux	O
,	O
Android	B-Application
,	O
and	O
MS	O
Windows	O
.	O
</s>
<s>
Originally	O
,	O
security	O
concerns	O
were	O
not	O
major	O
design	O
considerations	O
for	O
DNS	B-Protocol
software	O
or	O
any	O
software	O
for	O
deployment	O
on	O
the	O
early	O
Internet	O
,	O
as	O
the	O
network	O
was	O
not	O
open	O
for	O
participation	O
by	O
the	O
general	O
public	O
.	O
</s>
<s>
One	O
such	O
issue	O
is	O
DNS	B-Protocol
cache	O
poisoning	O
,	O
in	O
which	O
data	O
is	O
distributed	O
to	O
caching	O
resolvers	O
under	O
the	O
pretense	O
of	O
being	O
an	O
authoritative	O
origin	O
server	O
,	O
thereby	O
polluting	O
the	O
data	O
store	O
with	O
potentially	O
false	O
information	O
and	O
long	O
expiration	O
times	O
(	O
time-to-live	B-Protocol
)	O
.	O
</s>
<s>
Subsequently	O
,	O
legitimate	O
application	O
requests	O
may	O
be	O
redirected	O
to	O
network	O
hosts	B-Protocol
operated	O
with	O
malicious	O
intent	O
.	O
</s>
<s>
DNS	B-Protocol
responses	O
traditionally	O
do	O
not	O
have	O
a	O
cryptographic	O
signature	O
,	O
leading	O
to	O
many	O
attack	O
possibilities	O
;	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
Security	I-Protocol
Extensions	I-Protocol
(	O
DNSSEC	B-Protocol
)	O
modify	O
DNS	B-Protocol
to	O
add	O
support	O
for	O
cryptographically	O
signed	O
responses	O
.	O
</s>
<s>
DNSCurve	B-Protocol
has	O
been	O
proposed	O
as	O
an	O
alternative	O
to	O
DNSSEC	B-Protocol
.	O
</s>
<s>
Other	O
extensions	O
,	O
such	O
as	O
TSIG	B-Protocol
,	O
add	O
support	O
for	O
cryptographic	O
authentication	O
between	O
trusted	O
peers	O
and	O
are	O
commonly	O
used	O
to	O
authorize	O
zone	B-Protocol
transfer	I-Protocol
or	O
dynamic	O
update	O
operations	O
.	O
</s>
<s>
This	O
problem	O
,	O
known	O
as	O
the	O
IDN	B-General_Concept
homograph	I-General_Concept
attack	I-General_Concept
,	O
is	O
acute	O
in	O
systems	O
that	O
support	O
internationalized	B-Language
domain	I-Language
names	I-Language
,	O
as	O
many	O
character	O
codes	O
in	O
ISO	O
10646	O
may	O
appear	O
identical	O
on	O
typical	O
computer	O
screens	O
.	O
</s>
<s>
Techniques	O
such	O
as	O
forward-confirmed	B-Protocol
reverse	I-Protocol
DNS	I-Protocol
can	O
also	O
be	O
used	O
to	O
help	O
validate	O
DNS	B-Protocol
results	O
.	O
</s>
<s>
DNS	B-Protocol
can	O
also	O
"	O
leak	O
"	O
from	O
otherwise	O
secure	O
or	O
private	O
connections	O
,	O
if	O
attention	O
is	O
not	O
paid	O
to	O
their	O
configuration	O
,	O
and	O
at	O
times	O
DNS	B-Protocol
has	O
been	O
used	O
to	O
bypass	O
firewalls	O
by	O
malicious	O
persons	O
,	O
and	O
exfiltrate	O
data	O
,	O
since	O
it	O
is	O
often	O
seen	O
as	O
innocuous	O
.	O
</s>
<s>
Originally	O
designed	O
as	O
a	O
public	O
,	O
hierarchical	O
,	O
distributed	O
and	O
heavily	O
cached	O
database	B-General_Concept
,	O
DNS	B-Protocol
protocol	I-Protocol
has	O
no	O
confidentiality	O
controls	O
.	O
</s>
<s>
User	O
queries	O
and	O
nameserver	B-Protocol
responses	O
are	O
being	O
sent	O
unencrypted	O
which	O
enables	O
network	O
packet	O
sniffing	O
,	O
DNS	B-Protocol
hijacking	I-Protocol
,	O
DNS	B-Protocol
cache	O
poisoning	O
and	O
man-in-the-middle	O
attacks	O
.	O
</s>
<s>
This	O
deficiency	O
is	O
commonly	O
used	O
by	O
cybercriminals	O
and	O
network	O
operators	O
for	O
marketing	O
purposes	O
,	O
user	O
authentication	O
on	O
captive	O
portals	O
and	O
censorship	B-Application
.	O
</s>
<s>
User	O
privacy	O
is	O
further	O
exposed	O
by	O
proposals	O
for	O
increasing	O
the	O
level	O
of	O
client	O
IP	O
information	O
in	O
DNS	B-Protocol
queries	I-Protocol
(	O
RFC	O
7871	O
)	O
for	O
the	O
benefit	O
of	O
Content	B-Operating_System
Delivery	I-Operating_System
Networks	I-Operating_System
.	O
</s>
<s>
The	O
main	O
approaches	O
that	O
are	O
in	O
use	O
to	O
counter	O
privacy	O
issues	O
with	O
DNS	B-Protocol
:	O
</s>
<s>
VPNs	B-Application
,	O
which	O
move	O
DNS	B-Protocol
resolution	O
to	O
the	O
VPN	B-Application
operator	O
and	O
hide	O
user	O
traffic	O
from	O
local	O
ISP	O
,	O
</s>
<s>
Tor	B-Application
,	O
which	O
replaces	O
traditional	O
DNS	B-Protocol
resolution	O
with	O
anonymous	O
.onion	O
domains	O
,	O
hiding	O
both	O
name	O
resolution	O
and	O
user	O
traffic	O
behind	O
onion	B-Protocol
routing	I-Protocol
counter-surveillance	O
,	O
</s>
<s>
Proxies	B-Application
and	O
public	O
DNS	B-Protocol
servers	I-Protocol
,	O
which	O
move	O
the	O
actual	O
DNS	B-Protocol
resolution	O
to	O
a	O
third-party	O
provider	O
,	O
who	O
usually	O
promises	O
little	O
or	O
no	O
request	O
logging	O
and	O
optional	O
added	O
features	O
,	O
such	O
as	O
DNS-level	O
advertisement	O
or	O
pornography	O
blocking	O
.	O
</s>
<s>
Solutions	O
preventing	O
DNS	B-Protocol
inspection	O
by	O
local	O
network	O
operator	O
are	O
criticized	O
for	O
thwarting	O
corporate	O
network	O
security	O
policies	O
and	O
Internet	B-Application
censorship	I-Application
.	O
</s>
<s>
They	O
are	O
also	O
criticized	O
from	O
a	O
privacy	O
point	O
of	O
view	O
,	O
as	O
giving	O
away	O
the	O
DNS	B-Protocol
resolution	O
to	O
the	O
hands	O
of	O
a	O
small	O
number	O
of	O
companies	O
known	O
for	O
monetizing	O
user	O
traffic	O
and	O
for	O
centralizing	O
DNS	B-Protocol
name	I-Protocol
resolution	O
,	O
which	O
is	O
generally	O
perceived	O
as	O
harmful	O
for	O
the	O
Internet	O
.	O
</s>
<s>
The	O
right	O
to	O
use	O
a	O
domain	O
name	O
is	O
delegated	O
by	O
domain	O
name	O
registrars	O
which	O
are	O
accredited	O
by	O
the	O
Internet	B-Application
Corporation	I-Application
for	I-Application
Assigned	I-Application
Names	I-Application
and	I-Application
Numbers	I-Application
(	O
ICANN	B-Application
)	O
or	O
other	O
organizations	O
such	O
as	O
OpenNIC	B-Protocol
,	O
that	O
are	O
charged	O
with	O
overseeing	O
the	O
name	O
and	O
number	O
systems	O
of	O
the	O
Internet	O
.	O
</s>
<s>
In	O
addition	O
to	O
ICANN	B-Application
,	O
each	O
top-level	O
domain	O
(	O
TLD	O
)	O
is	O
maintained	O
and	O
serviced	O
technically	O
by	O
an	O
administrative	O
organization	O
,	O
operating	O
a	O
registry	B-Protocol
.	O
</s>
<s>
A	O
registry	B-Protocol
is	O
responsible	O
for	O
operating	O
the	O
database	B-General_Concept
of	O
names	O
within	O
its	O
authoritative	O
zone	B-Protocol
,	O
although	O
the	O
term	O
is	O
most	O
often	O
used	O
for	O
TLDs	O
.	O
</s>
<s>
The	O
registry	B-Protocol
receives	O
registration	O
information	O
from	O
each	O
domain	O
name	O
registrar	O
,	O
which	O
is	O
authorized	O
(	O
accredited	O
)	O
to	O
assign	O
names	O
in	O
the	O
corresponding	O
zone	B-Protocol
and	O
publishes	O
the	O
information	O
using	O
the	O
WHOIS	B-Protocol
protocol	O
.	O
</s>
<s>
ICANN	B-Application
publishes	O
the	O
complete	O
list	O
of	O
TLDs	O
,	O
TLD	O
registries	B-Protocol
,	O
and	O
domain	O
name	O
registrars	O
.	O
</s>
<s>
Registrant	O
information	O
associated	O
with	O
domain	O
names	O
is	O
maintained	O
in	O
an	O
online	O
database	B-General_Concept
accessible	O
with	O
the	O
WHOIS	B-Protocol
service	O
.	O
</s>
<s>
For	O
most	O
of	O
the	O
more	O
than	O
290	O
country	B-Application
code	I-Application
top-level	I-Application
domains	I-Application
(	O
ccTLDs	B-Application
)	O
,	O
the	O
domain	B-Protocol
registries	I-Protocol
maintain	O
the	O
WHOIS	B-Protocol
(	O
Registrant	O
,	O
name	B-Protocol
servers	I-Protocol
,	O
expiration	O
dates	O
,	O
etc	O
.	O
)	O
</s>
<s>
From	O
about	O
2001	O
,	O
most	O
Generic	O
top-level	O
domain	O
(	O
gTLD	O
)	O
registries	B-Protocol
have	O
adopted	O
this	O
so-called	O
thick	O
registry	B-Protocol
approach	O
,	O
i.e.	O
</s>
<s>
keeping	O
the	O
WHOIS	B-Protocol
data	O
in	O
central	O
registries	B-Protocol
instead	O
of	O
registrar	O
databases	O
.	O
</s>
<s>
For	O
top-level	O
domains	O
on	O
COM	O
and	O
NET	O
,	O
a	O
thin	O
registry	B-Protocol
model	O
is	O
used	O
.	O
</s>
<s>
The	O
domain	B-Protocol
registry	I-Protocol
(	O
e.g.	O
,	O
GoDaddy	O
,	O
BigRock	O
and	O
PDR	O
,	O
VeriSign	B-Application
,	O
etc.	O
,	O
etc	O
.	O
)	O
</s>
<s>
holds	O
basic	O
WHOIS	B-Protocol
data	O
(	O
i.e.	O
,	O
registrar	O
and	O
name	B-Protocol
servers	I-Protocol
,	O
etc	O
.	O
)	O
.	O
</s>
<s>
Organizations	O
,	O
or	O
registrants	O
using	O
ORG	O
on	O
the	O
other	O
hand	O
,	O
are	O
on	O
the	O
Public	O
Interest	O
Registry	B-Protocol
exclusively	O
.	O
</s>
<s>
Some	O
domain	B-Protocol
name	I-Protocol
registries	I-Protocol
,	O
often	O
called	O
network	B-Protocol
information	I-Protocol
centers	I-Protocol
(	O
NIC	O
)	O
,	O
also	O
function	O
as	O
registrars	O
to	O
end-users	O
,	O
in	O
addition	O
to	O
providing	O
access	O
to	O
the	O
WHOIS	B-Protocol
datasets	O
.	O
</s>
<s>
The	O
top-level	O
domain	B-Protocol
registries	I-Protocol
,	O
such	O
as	O
for	O
the	O
domains	O
COM	O
,	O
NET	O
,	O
and	O
ORG	O
use	O
a	O
registry-registrar	O
model	O
consisting	O
of	O
many	O
domain	O
name	O
registrars	O
.	O
</s>
<s>
In	O
this	O
method	O
of	O
management	O
,	O
the	O
registry	B-Protocol
only	O
manages	O
the	O
domain	O
name	O
database	B-General_Concept
and	O
the	O
relationship	O
with	O
the	O
registrars	O
.	O
</s>
<s>
The	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
is	O
defined	O
by	O
Request	O
for	O
Comments	O
(	O
RFC	O
)	O
documents	O
published	O
by	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
Internet	O
standards	O
)	O
.	O
</s>
<s>
The	O
following	O
is	O
a	O
list	O
of	O
RFCs	O
that	O
define	O
the	O
DNS	B-Protocol
protocol	I-Protocol
.	O
</s>
