<s>
The	O
most	O
important	O
function	O
of	O
DNS	B-Protocol
servers	I-Protocol
is	O
the	O
translation	O
(	O
resolution	O
)	O
of	O
human-memorable	O
domain	O
names	O
(	O
example.com	O
)	O
into	O
the	O
corresponding	O
numeric	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
addresses	O
(	O
159.89.194.43	O
)	O
,	O
the	O
second	O
principal	O
name	O
space	O
of	O
the	O
Internet	O
,	O
which	O
is	O
used	O
to	O
identify	O
and	O
locate	O
computer	O
systems	O
and	O
resources	O
on	O
the	O
Internet	O
.	O
</s>
<s>
Although	O
it	O
is	O
typically	O
used	O
in	O
reference	O
to	O
DNS	B-Protocol
(	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
)	O
,	O
the	O
term	O
name	B-Protocol
server	I-Protocol
may	O
also	O
be	O
used	O
for	O
any	O
computer	O
application	O
that	O
implements	O
a	O
network	B-General_Concept
service	I-General_Concept
for	O
providing	O
responses	O
to	O
queries	O
against	O
a	O
directory	O
service	O
which	O
translates	O
an	O
often	O
humanly	O
meaningful	O
,	O
text-based	O
identifier	O
to	O
a	O
system-internal	O
,	O
often	O
numeric	O
identification	O
or	O
addressing	O
component	O
.	O
</s>
<s>
The	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
maintains	O
the	O
domain	B-Protocol
namespace	I-Protocol
and	O
provides	O
translation	O
services	O
between	O
these	O
two	O
namespaces	O
.	O
</s>
<s>
Internet	O
name	B-Protocol
servers	I-Protocol
implement	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
.	O
</s>
<s>
The	O
top	O
hierarchy	O
of	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
is	O
served	O
by	O
the	O
root	B-Application
name	I-Application
servers	I-Application
maintained	O
by	O
delegation	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
.	O
</s>
<s>
DNS	B-Protocol
servers	I-Protocol
,	O
which	O
are	O
located	O
all	O
over	O
the	O
world	O
,	O
translate	O
domain	O
names	O
into	O
IP	O
addresses	O
,	O
giving	O
them	O
control	O
over	O
which	O
server	O
a	O
user	O
may	O
access	O
via	O
a	O
given	O
domain	O
.	O
</s>
<s>
A	O
DNS	B-Protocol
name	B-Protocol
server	I-Protocol
is	O
a	O
server	O
that	O
stores	O
the	O
DNS	B-Protocol
records	I-Protocol
,	O
such	O
as	O
address	O
(	O
A	O
,	O
AAAA	O
)	O
records	O
,	O
name	B-Protocol
server	I-Protocol
(	O
NS	O
)	O
records	O
,	O
and	O
mail	O
exchanger	O
(	O
MX	O
)	O
records	O
for	O
a	O
domain	O
name	O
(	O
see	O
also	O
List	B-Protocol
of	I-Protocol
DNS	I-Protocol
record	I-Protocol
types	I-Protocol
)	O
and	O
responds	O
with	O
answers	O
to	O
queries	O
against	O
its	O
database	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
gives	O
answers	O
in	O
response	O
to	O
questions	O
asked	O
about	O
names	O
in	O
a	O
zone	B-Protocol
.	O
</s>
<s>
An	O
authoritative-only	O
name	B-Protocol
server	I-Protocol
returns	O
answers	O
only	O
to	O
queries	O
about	O
domain	O
names	O
that	O
have	O
been	O
specifically	O
configured	O
by	O
the	O
administrator	O
.	O
</s>
<s>
Name	B-Protocol
servers	I-Protocol
can	O
also	O
be	O
configured	O
to	O
give	O
authoritative	O
answers	O
to	O
queries	O
in	O
some	O
zones	O
,	O
while	O
acting	O
as	O
a	O
caching	O
name	B-Protocol
server	I-Protocol
for	O
all	O
other	O
zones	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>
A	O
primary	O
server	O
for	O
a	O
zone	B-Protocol
is	O
the	O
server	O
that	O
stores	O
the	O
definitive	O
versions	O
of	O
all	O
records	O
in	O
that	O
zone	B-Protocol
.	O
</s>
<s>
A	O
secondary	O
server	O
for	O
a	O
zone	B-Protocol
uses	O
an	O
automatic	O
updating	O
mechanism	O
to	O
maintain	O
an	O
identical	O
copy	O
of	O
the	O
primary	O
server	O
's	O
database	O
for	O
a	O
zone	B-Protocol
.	O
</s>
<s>
Examples	O
of	O
such	O
mechanisms	O
include	O
DNS	B-Protocol
zone	I-Protocol
transfers	I-Protocol
and	O
file	O
transfer	O
protocols	O
.	O
</s>
<s>
DNS	B-Protocol
provides	O
a	O
mechanism	O
whereby	O
the	O
primary	O
for	O
a	O
zone	B-Protocol
can	O
notify	O
all	O
the	O
known	O
secondaries	O
for	O
that	O
zone	B-Protocol
when	O
the	O
contents	O
of	O
the	O
zone	B-Protocol
have	O
changed	O
.	O
</s>
<s>
The	O
contents	O
of	O
a	O
zone	B-Protocol
are	O
either	O
manually	O
configured	O
by	O
an	O
administrator	O
,	O
or	O
managed	O
using	O
Dynamic	B-Protocol
DNS	I-Protocol
.	O
</s>
<s>
Every	O
domain	O
name	O
appears	O
in	O
a	O
zone	B-Protocol
served	O
by	O
one	O
or	O
more	O
authoritative	O
name	B-Protocol
servers	I-Protocol
.	O
</s>
<s>
The	O
fully	B-Protocol
qualified	I-Protocol
domain	I-Protocol
names	I-Protocol
of	O
the	O
authoritative	O
name	B-Protocol
servers	I-Protocol
of	O
a	O
zone	B-Protocol
are	O
listed	O
in	O
the	O
NS	O
records	O
of	O
that	O
zone	B-Protocol
.	O
</s>
<s>
If	O
the	O
server	O
for	O
a	O
zone	B-Protocol
is	O
not	O
also	O
authoritative	O
for	O
its	O
parent	O
zone	B-Protocol
,	O
the	O
server	O
for	O
the	O
parent	O
zone	B-Protocol
must	O
be	O
configured	O
with	O
a	O
delegation	O
for	O
the	O
zone	B-Protocol
.	O
</s>
<s>
When	O
a	O
domain	O
is	O
registered	O
with	O
a	O
domain	B-Application
name	I-Application
registrar	I-Application
,	O
the	O
zone	B-Protocol
administrator	O
provides	O
the	O
list	O
of	O
name	B-Protocol
servers	I-Protocol
(	O
typically	O
at	O
least	O
two	O
,	O
for	O
redundancy	O
)	O
that	O
are	O
authoritative	O
for	O
the	O
zone	B-Protocol
that	O
contains	O
the	O
domain	O
.	O
</s>
<s>
The	O
registrar	O
provides	O
the	O
names	O
of	O
these	O
servers	O
to	O
the	O
domain	B-Protocol
registry	I-Protocol
for	O
the	O
top-level	O
domain	O
containing	O
the	O
zone	B-Protocol
.	O
</s>
<s>
The	O
domain	B-Protocol
registry	I-Protocol
in	O
turn	O
configures	O
the	O
authoritative	O
name	B-Protocol
servers	I-Protocol
for	O
that	O
top-level	O
domain	O
with	O
delegations	O
for	O
each	O
server	O
for	O
the	O
zone	B-Protocol
.	O
</s>
<s>
If	O
the	O
fully	B-Protocol
qualified	I-Protocol
domain	I-Protocol
name	I-Protocol
of	O
any	O
name	B-Protocol
server	I-Protocol
for	O
a	O
zone	B-Protocol
appears	O
within	O
that	O
zone	B-Protocol
,	O
the	O
zone	B-Protocol
administrator	O
provides	O
IP	O
addresses	O
for	O
that	O
name	B-Protocol
server	I-Protocol
,	O
which	O
are	O
installed	O
in	O
the	O
parent	O
zone	B-Protocol
as	O
glue	O
records	O
;	O
otherwise	O
,	O
the	O
delegation	O
consists	O
of	O
the	O
list	O
of	O
NS	O
records	O
for	O
that	O
zone	B-Protocol
.	O
</s>
<s>
A	O
name	B-Protocol
server	I-Protocol
indicates	O
that	O
its	O
response	O
is	O
authoritative	O
by	O
setting	O
the	O
Authoritative	O
Answer	O
(	O
AA	O
)	O
bit	O
in	O
the	O
response	O
to	O
a	O
query	O
on	O
a	O
name	O
for	O
which	O
it	O
is	O
authoritative	O
.	O
</s>
<s>
Name	B-Protocol
servers	I-Protocol
providing	O
answers	O
for	O
which	O
they	O
are	O
not	O
authoritative	O
(	O
for	O
example	O
,	O
name	B-Protocol
servers	I-Protocol
for	O
parent	O
zones	O
)	O
do	O
not	O
set	O
the	O
AA	O
bit	O
.	O
</s>
<s>
If	O
a	O
name	B-Protocol
server	I-Protocol
cannot	O
answer	O
a	O
query	O
because	O
it	O
does	O
not	O
contain	O
an	O
entry	O
for	O
the	O
host	O
in	O
its	O
DNS	B-Protocol
cache	O
,	O
it	O
may	O
recursively	O
query	O
name	B-Protocol
servers	I-Protocol
higher	O
up	O
in	O
the	O
hierarchy	O
.	O
</s>
<s>
A	O
server	O
providing	O
recursive	O
queries	O
is	O
known	O
as	O
a	O
recursive	O
name	B-Protocol
server	I-Protocol
or	O
recursive	O
DNS	B-Protocol
,	O
sometimes	O
abbreviated	O
as	O
recdns	O
.	O
</s>
<s>
In	O
principle	O
,	O
authoritative	O
name	B-Protocol
servers	I-Protocol
suffice	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-servers	O
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
must	O
implement	O
resolver	O
software	O
capable	O
of	O
recursive	O
operation	O
.	O
</s>
<s>
Caching	O
name	B-Protocol
servers	I-Protocol
(	O
DNS	B-Protocol
caches	O
)	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	O
)	O
of	O
each	O
domain-name	O
record	O
.	O
</s>
<s>
DNS	B-Protocol
caches	O
improve	O
the	O
efficiency	O
of	O
the	O
DNS	B-Protocol
by	O
reducing	O
DNS	B-Protocol
traffic	O
across	O
the	O
Internet	O
,	O
and	O
by	O
reducing	O
load	O
on	O
authoritative	O
name-servers	O
,	O
particularly	O
root	O
name-servers	O
.	O
</s>
<s>
Because	O
they	O
can	O
answer	O
questions	O
more	O
quickly	O
,	O
they	O
also	O
increase	O
the	O
performance	O
of	O
end-user	O
applications	O
that	O
use	O
the	O
DNS	B-Protocol
.	O
</s>
<s>
Recursive	O
name	B-Protocol
servers	I-Protocol
resolve	O
any	O
query	O
they	O
receive	O
,	O
even	O
if	O
they	O
are	O
not	O
authoritative	O
for	O
the	O
question	O
being	O
asked	O
,	O
by	O
consulting	O
the	O
server	O
or	O
servers	O
that	O
are	O
authoritative	O
for	O
the	O
question	O
.	O
</s>
<s>
Caching	O
name	B-Protocol
servers	I-Protocol
are	O
often	O
also	O
recursive	O
name	B-Protocol
servers	I-Protocol
—	O
they	O
perform	O
every	O
step	O
necessary	O
to	O
answer	O
any	O
DNS	B-Protocol
query	I-Protocol
they	O
receive	O
.	O
</s>
<s>
To	O
do	O
this	O
the	O
name	B-Protocol
server	I-Protocol
queries	O
each	O
authoritative	O
name-server	O
in	O
turn	O
,	O
starting	O
from	O
the	O
DNS	B-Protocol
root	I-Protocol
zone	I-Protocol
.	O
</s>
<s>
It	O
continues	O
until	O
it	O
reaches	O
the	O
authoritative	O
server	O
for	O
the	O
zone	B-Protocol
that	O
contains	O
the	O
queried	O
domain	O
name	O
.	O
</s>
<s>
The	O
authority	O
,	O
resolving	O
and	O
caching	O
functions	O
can	O
all	O
be	O
present	O
in	O
a	O
DNS	B-Protocol
server	I-Protocol
implementation	O
,	O
but	O
this	O
is	O
not	O
required	O
:	O
a	O
DNS	B-Protocol
server	I-Protocol
can	O
implement	O
any	O
one	O
of	O
these	O
functions	O
alone	O
,	O
without	O
implementing	O
the	O
others	O
.	O
</s>
