<s>
Microsoft	B-Protocol
DNS	I-Protocol
is	O
the	O
name	O
given	O
to	O
the	O
implementation	O
of	O
domain	B-Protocol
name	I-Protocol
system	I-Protocol
services	O
provided	O
in	O
Microsoft	B-Application
Windows	I-Application
operating	I-Application
systems	I-Application
.	O
</s>
<s>
The	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
support	O
in	O
Microsoft	B-Device
Windows	I-Device
NT	I-Device
,	O
and	O
thus	O
its	O
derivatives	O
Windows	B-Application
2000	I-Application
,	O
Windows	B-Application
XP	I-Application
,	O
and	O
Windows	B-Application
Server	I-Application
2003	I-Application
,	O
comprises	O
two	O
clients	O
and	O
a	O
server	O
.	O
</s>
<s>
Every	O
Microsoft	B-Application
Windows	I-Application
machine	O
has	O
a	O
DNS	B-Protocol
lookup	O
client	O
,	O
to	O
perform	O
ordinary	O
DNS	B-Protocol
lookups	O
.	O
</s>
<s>
Some	O
machines	O
have	O
a	O
Dynamic	B-Protocol
DNS	I-Protocol
client	O
,	O
to	O
perform	O
Dynamic	B-Protocol
DNS	I-Protocol
Update	I-Protocol
transactions	O
,	O
registering	O
the	O
machines	O
 '	O
names	O
and	O
IP	B-Protocol
addresses	I-Protocol
.	O
</s>
<s>
Some	O
machines	O
run	O
a	O
DNS	B-Protocol
server	O
,	O
to	O
publish	O
DNS	B-Protocol
data	O
,	O
to	O
service	O
DNS	B-Protocol
lookup	O
requests	O
from	O
DNS	B-Protocol
lookup	O
clients	O
,	O
and	O
to	O
service	O
DNS	B-Protocol
update	O
requests	O
from	O
DNS	B-Protocol
update	O
clients	O
.	O
</s>
<s>
Applications	O
perform	O
DNS	B-Protocol
lookups	O
with	O
the	O
aid	O
of	O
a	O
DLL	O
.	O
</s>
<s>
They	O
call	O
library	O
functions	O
in	O
the	O
DLL	O
,	O
which	O
in	O
turn	O
handle	O
all	O
communications	O
with	O
DNS	B-Protocol
servers	O
(	O
over	O
UDP	B-Protocol
or	O
TCP	B-Protocol
)	O
and	O
return	O
the	O
final	O
results	O
of	O
the	O
lookup	O
back	O
to	O
the	O
applications	O
.	O
</s>
<s>
Microsoft	O
's	O
DNS	B-Protocol
client	O
also	O
has	O
optional	O
support	O
for	O
local	O
caching	O
,	O
in	O
the	O
form	O
of	O
a	O
DNS	B-Protocol
Client	O
service	O
(	O
also	O
known	O
as	O
DNSCACHE	O
)	O
.	O
</s>
<s>
Before	O
they	O
attempt	O
to	O
directly	O
communicate	O
with	O
DNS	B-Protocol
servers	O
,	O
the	O
library	O
routines	O
first	O
attempt	O
to	O
make	O
a	O
local	B-Operating_System
IPC	I-Operating_System
connection	O
to	O
the	O
DNS	B-Protocol
Client	O
service	O
on	O
the	O
machine	O
.	O
</s>
<s>
If	O
there	O
is	O
one	O
,	O
and	O
if	O
such	O
a	O
connection	O
can	O
be	O
made	O
,	O
they	O
hand	O
the	O
actual	O
work	O
of	O
dealing	O
with	O
the	O
lookup	O
over	O
to	O
the	O
DNS	B-Protocol
Client	O
service	O
.	O
</s>
<s>
The	O
DNS	B-Protocol
Client	O
service	O
itself	O
communicates	O
with	O
DNS	B-Protocol
servers	O
,	O
and	O
caches	O
the	O
results	O
that	O
it	O
receives	O
.	O
</s>
<s>
Microsoft	O
's	O
DNS	B-Protocol
client	O
is	O
capable	O
of	O
talking	O
to	O
multiple	O
DNS	B-Protocol
servers	O
.	O
</s>
<s>
The	O
exact	O
algorithm	O
varies	O
according	O
to	O
the	O
version	O
,	O
and	O
service	O
pack	O
level	O
,	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
;	O
but	O
in	O
general	O
all	O
communication	O
is	O
with	O
a	O
preferred	O
DNS	B-Protocol
server	O
until	O
it	O
fails	O
to	O
answer	O
,	O
whereupon	O
communication	O
switches	O
to	O
one	O
of	O
several	O
alternative	O
DNS	B-Protocol
servers	O
.	O
</s>
<s>
There	O
are	O
several	O
minor	O
differences	O
in	O
system	O
behavior	O
depending	O
on	O
whether	O
the	O
DNS	B-Protocol
Client	O
service	O
is	O
started	O
:	O
</s>
<s>
Parsing	O
of	O
the	O
"	O
hosts	B-Protocol
"	O
file	O
:	O
The	O
lookup	O
functions	O
read	O
only	O
the	O
hosts	B-Protocol
file	I-Protocol
if	O
they	O
cannot	O
off-load	O
their	O
task	O
onto	O
the	O
DNS	B-Protocol
Client	O
service	O
and	O
have	O
to	O
fall	O
back	O
to	O
communicating	O
with	O
DNS	B-Protocol
servers	O
themselves	O
.	O
</s>
<s>
In	O
turn	O
,	O
the	O
DNS	B-Protocol
Client	O
service	O
reads	O
the	O
"	O
hosts	B-Protocol
"	O
file	O
once	O
,	O
at	O
startup	O
,	O
and	O
only	O
re-reads	O
it	O
if	O
it	O
notices	O
that	O
the	O
last	O
modification	O
timestamp	O
of	O
the	O
file	O
has	O
changed	O
since	O
it	O
last	O
read	O
it	O
.	O
</s>
<s>
With	O
the	O
DNS	B-Protocol
Client	O
service	O
running	O
:	O
The	O
"	O
hosts	B-Protocol
"	O
file	O
is	O
read	O
and	O
parsed	O
only	O
a	O
few	O
times	O
,	O
once	O
at	O
service	O
startup	O
,	O
and	O
thereafter	O
whenever	O
the	O
DNS	B-Protocol
Client	O
service	O
notices	O
that	O
it	O
has	O
been	O
modified	O
.	O
</s>
<s>
Additionally	O
,	O
running	O
the	O
DNS	B-Protocol
Client	O
service	O
in	O
conjunction	O
with	O
a	O
“	O
large	O
”	O
HOSTS	B-Protocol
file	I-Protocol
can	O
cause	O
it	O
to	O
put	O
a	O
100%	O
load	O
on	O
the	O
CPU/core	O
on	O
which	O
it	O
is	O
running	O
until	O
it	O
is	O
stopped	O
.	O
</s>
<s>
Without	O
the	O
DNS	B-Protocol
Client	O
service	O
running	O
:	O
The	O
"	O
hosts	B-Protocol
"	O
file	O
is	O
read	O
and	O
parsed	O
repeatedly	O
,	O
by	O
each	O
individual	O
application	O
program	O
as	O
it	O
makes	O
a	O
DNS	B-Protocol
lookup	O
.	O
</s>
<s>
The	O
effect	O
of	O
multiple	O
answers	O
in	O
the	O
"	O
hosts	B-Protocol
"	O
file	O
:	O
The	O
DNS	B-Protocol
Client	O
service	O
does	O
not	O
use	O
the	O
"	O
hosts	B-Protocol
"	O
file	O
directly	O
when	O
performing	O
lookups	O
.	O
</s>
<s>
When	O
the	O
lookup	O
functions	O
fall	O
back	O
to	O
doing	O
the	O
work	O
themselves	O
,	O
however	O
,	O
they	O
scan	O
the	O
"	O
hosts	B-Protocol
"	O
file	O
directly	O
and	O
sequentially	O
,	O
stopping	O
when	O
the	O
first	O
answer	O
is	O
found	O
.	O
</s>
<s>
With	O
the	O
DNS	B-Protocol
Client	O
service	O
running	O
:	O
If	O
the	O
"	O
hosts	B-Protocol
"	O
file	O
contains	O
multiple	O
lines	O
denoting	O
multiple	O
answers	O
for	O
a	O
given	O
lookup	O
,	O
all	O
of	O
the	O
answers	O
in	O
the	O
cache	O
will	O
be	O
returned	O
.	O
</s>
<s>
Without	O
the	O
DNS	B-Protocol
Client	O
service	O
running	O
:	O
If	O
the	O
"	O
hosts	B-Protocol
"	O
file	O
contains	O
multiple	O
lines	O
denoting	O
multiple	O
answers	O
for	O
a	O
given	O
lookup	O
,	O
only	O
the	O
first	O
answer	O
found	O
will	O
be	O
returned	O
.	O
</s>
<s>
Fallback	O
from	O
preferred	O
to	O
alternative	O
DNS	B-Protocol
servers	O
:	O
The	O
fallback	O
from	O
the	O
preferred	O
DNS	B-Protocol
server	O
to	O
the	O
alternative	O
DNS	B-Protocol
servers	O
is	O
done	O
by	O
whatever	O
entity	O
,	O
the	O
DNS	B-Protocol
Client	O
service	O
or	O
the	O
library	O
functions	O
themselves	O
,	O
is	O
actually	O
performing	O
the	O
communication	O
with	O
them	O
.	O
</s>
<s>
With	O
the	O
DNS	B-Protocol
Client	O
service	O
running	O
:	O
Fallback	O
to	O
the	O
alternative	O
DNS	B-Protocol
servers	O
happens	O
globally	O
.	O
</s>
<s>
If	O
the	O
preferred	O
DNS	B-Protocol
server	O
fails	O
to	O
answer	O
,	O
all	O
subsequent	O
communication	O
is	O
with	O
the	O
alternative	O
DNS	B-Protocol
servers	O
.	O
</s>
<s>
Without	O
the	O
DNS	B-Protocol
Client	O
service	O
running	O
:	O
Any	O
fallback	O
to	O
the	O
alternative	O
DNS	B-Protocol
servers	O
happens	O
locally	O
,	O
within	O
each	O
individual	O
process	O
that	O
is	O
making	O
DNS	B-Protocol
queries	I-Protocol
.	O
</s>
<s>
Different	O
processes	O
may	O
be	O
in	O
different	O
states	O
,	O
some	O
talking	O
to	O
the	O
preferred	O
DNS	B-Protocol
server	O
and	O
some	O
talking	O
to	O
alternative	O
DNS	B-Protocol
servers	O
.	O
</s>
<s>
Linux	B-Application
distributions	I-Application
and	O
various	O
versions	O
of	O
Unix	B-Application
have	O
a	O
generalized	O
name	O
resolver	O
layer	O
.	O
</s>
<s>
The	O
resolver	O
can	O
be	O
controlled	O
to	O
use	O
a	O
hosts	B-Protocol
file	I-Protocol
or	O
Network	B-Operating_System
Information	I-Operating_System
Service	I-Operating_System
(	O
NIS	O
)	O
,	O
by	O
configuring	O
the	O
Name	B-Protocol
Service	I-Protocol
Switch	I-Protocol
.	O
</s>
<s>
Whilst	O
DNS	B-Protocol
lookups	O
read	O
DNS	B-Protocol
data	O
,	O
DNS	B-Protocol
updates	O
write	O
them	O
.	O
</s>
<s>
Both	O
workstations	O
and	O
servers	O
running	O
Windows	O
attempt	O
to	O
send	O
Dynamic	B-Protocol
DNS	I-Protocol
update	I-Protocol
requests	O
to	O
DNS	B-Protocol
servers	O
.	O
</s>
<s>
Workstations	O
running	O
Windows	O
attempt	O
to	O
register	O
their	O
names	O
and	O
their	O
IP	B-Protocol
addresses	I-Protocol
with	O
DNS	B-Protocol
servers	O
,	O
so	O
that	O
other	O
machines	O
may	O
locate	O
them	O
by	O
name	O
.	O
</s>
<s>
Prior	O
to	O
Windows	O
Vista	O
(	O
and	O
Windows	O
Server	O
2008	O
)	O
,	O
this	O
registration	O
is	O
performed	O
by	O
the	O
DHCP	B-Protocol
Client	O
service	O
.	O
</s>
<s>
It	O
is	O
thus	O
necessary	O
to	O
run	O
the	O
DHCP	B-Protocol
Client	O
service	O
on	O
pre-Vista	O
machines	O
,	O
even	O
if	O
DHCP	B-Protocol
is	O
n't	O
being	O
used	O
to	O
configure	O
the	O
machine	O
in	O
order	O
to	O
dynamically	O
register	O
a	O
machine	O
's	O
name	O
and	O
address	O
for	O
DNS	B-Protocol
lookup	O
.	O
</s>
<s>
The	O
DHCP	B-Protocol
Client	O
service	O
registers	O
name	O
and	O
address	O
data	O
whenever	O
they	O
are	O
changed	O
(	O
either	O
manually	O
by	O
an	O
administrator	O
or	O
automatically	O
by	O
the	O
granting	O
or	O
revocation	O
of	O
a	O
DHCP	B-Protocol
lease	O
)	O
.	O
</s>
<s>
In	O
Windows	O
Vista	O
(	O
and	O
Windows	O
Server	O
2008	O
)	O
Microsoft	O
moved	O
the	O
registration	O
functionality	O
from	O
the	O
DHCP	B-Protocol
Client	O
service	O
to	O
the	O
DNS	B-Protocol
Client	O
service	O
.	O
</s>
<s>
Servers	O
running	O
Microsoft	B-Application
Windows	I-Application
also	O
attempt	O
to	O
register	O
other	O
information	O
,	O
in	O
addition	O
to	O
their	O
names	O
and	O
IP	B-Protocol
addresses	I-Protocol
,	O
such	O
as	O
the	O
locations	O
of	O
the	O
LDAP	B-Protocol
and	O
Kerberos	B-Protocol
services	O
that	O
they	O
provide	O
.	O
</s>
<s>
Microsoft	B-Application
Windows	I-Application
Server	O
operating	B-General_Concept
systems	I-General_Concept
can	O
run	O
the	O
DNS	B-Protocol
Server	O
service	O
.	O
</s>
<s>
This	O
is	O
a	O
monolithic	B-General_Concept
DNS	B-Protocol
server	O
that	O
provides	O
many	O
types	O
of	O
DNS	B-Protocol
service	O
,	O
including	O
caching	O
,	O
Dynamic	B-Protocol
DNS	I-Protocol
update	I-Protocol
,	O
zone	B-Protocol
transfer	I-Protocol
,	O
and	O
DNS	B-Protocol
notification	O
.	O
</s>
<s>
DNS	B-Protocol
notification	O
implements	O
a	O
push	O
mechanism	O
for	O
notifying	O
a	O
select	O
set	O
of	O
secondary	O
servers	O
for	O
a	O
zone	B-Protocol
when	O
it	O
is	O
updated	O
.	O
</s>
<s>
Microsoft	O
's	O
"	O
DNS	B-Protocol
Server	O
"	O
service	O
was	O
first	O
introduced	O
in	O
Windows	B-Device
NT	I-Device
3.51	O
as	O
an	O
add-on	O
with	O
Microsoft	O
's	O
collection	O
of	O
BackOffice	B-General_Concept
services	O
,	O
but	O
at	O
the	O
time	O
was	O
marked	O
to	O
be	O
used	O
for	O
testing	O
purposes	O
only	O
.	O
</s>
<s>
It	O
became	O
notorious	O
for	O
incompatibility	O
with	O
BIND	B-Protocol
configuration	O
files	O
,	O
in	O
particular	O
by	O
lacking	O
support	O
for	O
DNS	B-Protocol
wildcards	I-Protocol
and	O
differing	O
in	O
its	O
IPv6	B-Protocol
implementation	O
.	O
</s>
<s>
Since	O
the	O
introduction	O
,	O
Microsoft	O
has	O
taken	O
care	O
to	O
improve	O
interoperability	O
with	O
BIND	B-Protocol
and	O
other	O
implementations	O
in	O
terms	O
of	O
zone	B-Protocol
file	I-Protocol
format	O
,	O
zone	B-Protocol
transfer	I-Protocol
,	O
and	O
other	O
DNS	B-Protocol
protocol	I-Protocol
details	O
.	O
</s>
<s>
As	O
of	O
2004	O
,	O
it	O
was	O
the	O
fourth	O
most	O
popular	O
DNS	B-Protocol
server	O
(	O
counting	O
BIND	B-Protocol
version	O
9	O
separately	O
from	O
versions	O
8	O
and	O
4	O
)	O
for	O
the	O
publication	O
of	O
DNS	B-Protocol
data	O
.	O
</s>
<s>
Like	O
various	O
other	O
DNS	B-Protocol
servers	O
,	O
Microsoft	O
's	O
DNS	B-Protocol
server	O
supports	O
different	O
database	O
back	O
ends	O
.	O
</s>
<s>
Microsoft	O
's	O
DNS	B-Protocol
server	O
supports	O
two	O
such	O
back	O
ends	O
.	O
</s>
<s>
DNS	B-Protocol
data	O
can	O
be	O
stored	O
either	O
in	O
master	O
files	O
(	O
also	O
known	O
as	O
zone	B-Protocol
files	I-Protocol
)	O
or	O
in	O
the	O
Active	B-Protocol
Directory	I-Protocol
database	O
itself	O
.	O
</s>
<s>
In	O
the	O
latter	O
case	O
,	O
since	O
Active	B-Protocol
Directory	I-Protocol
(	O
rather	O
than	O
the	O
DNS	B-Protocol
server	O
)	O
handles	O
the	O
actual	O
replication	O
of	O
the	O
database	O
across	O
multiple	O
machines	O
,	O
the	O
database	O
can	O
be	O
modified	O
on	O
any	O
server	O
(	O
"	O
multiple-master	O
replication	O
"	O
)	O
,	O
and	O
the	O
addition	O
or	O
removal	O
of	O
a	O
zone	B-Protocol
will	O
be	O
immediately	O
propagated	O
to	O
all	O
other	O
DNS	B-Protocol
servers	O
within	O
the	O
appropriate	O
Active	B-Protocol
Directory	I-Protocol
"	O
replication	O
scope	O
"	O
.	O
</s>
<s>
(	O
Contrast	O
this	O
with	O
BIND	B-Protocol
,	O
where	O
when	O
such	O
changes	O
are	O
made	O
,	O
the	O
list	O
of	O
zones	O
,	O
in	O
the	O
file	O
,	O
has	O
to	O
be	O
explicitly	O
updated	O
on	O
each	O
individual	O
server	O
.	O
)	O
</s>
<s>
Microsoft	O
's	O
DNS	B-Protocol
server	O
can	O
be	O
administered	O
using	O
either	O
a	O
graphical	B-Application
user	I-Application
interface	I-Application
,	O
the	O
"	O
DNS	B-Protocol
Management	O
Console	O
"	O
,	O
or	O
a	O
command	B-Application
line	I-Application
interface	I-Application
,	O
the	O
dnscmd	O
utility	O
.	O
</s>
<s>
New	O
to	O
Windows	B-Device
Server	I-Device
2012	I-Device
is	O
a	O
fully	O
featured	O
PowerShell	B-Application
provider	O
for	O
DNS	B-Protocol
server	O
management	O
.	O
</s>
<s>
Prior	O
to	O
Windows	B-Application
Server	I-Application
2003	I-Application
and	O
Microsoft	B-Application
Windows	I-Application
2000	I-Application
Service	I-Application
Pack	I-Application
3	I-Application
,	O
the	O
most	O
common	O
problem	O
encountered	O
with	O
Microsoft	O
's	O
DNS	B-Protocol
server	O
was	O
cache	O
pollution	O
.	O
</s>
<s>
Although	O
Microsoft	O
's	O
DNS	B-Protocol
Server	O
had	O
a	O
mechanism	O
for	O
properly	O
dealing	O
with	O
cache	O
pollution	O
,	O
the	O
mechanism	O
was	O
turned	O
off	O
by	O
default	O
.	O
</s>
<s>
In	O
2004	O
,	O
a	O
common	O
problem	O
involved	O
the	O
feature	O
of	O
the	O
Windows	B-Application
Server	I-Application
2003	I-Application
version	O
of	O
Microsoft	O
's	O
DNS	B-Protocol
server	O
to	O
use	O
EDNS0	B-Protocol
,	O
which	O
a	O
large	O
number	O
of	O
firewalls	B-Application
could	O
not	O
cope	O
with	O
.	O
</s>
