<s>
On	O
Microsoft	B-General_Concept
Servers	I-General_Concept
,	O
a	O
domain	B-General_Concept
controller	I-General_Concept
(	O
DC	O
)	O
is	O
a	O
server	B-Application
computer	I-Application
that	O
responds	O
to	O
security	O
authentication	O
requests	O
(	O
logging	O
in	O
,	O
etc	O
.	O
)	O
</s>
<s>
within	O
a	O
Windows	B-General_Concept
domain	I-General_Concept
.	O
</s>
<s>
A	O
domain	O
is	O
a	O
concept	O
introduced	O
in	O
Windows	B-Device
NT	I-Device
whereby	O
a	O
user	O
may	O
be	O
granted	O
access	O
to	O
a	O
number	O
of	O
computer	O
resources	O
with	O
the	O
use	O
of	O
a	O
single	O
username	O
and	O
password	O
combination	O
.	O
</s>
<s>
With	O
Windows	B-Device
NT	I-Device
4	I-Device
Server	I-Device
,	O
one	O
domain	B-General_Concept
controller	I-General_Concept
per	O
domain	O
was	O
configured	O
as	O
the	O
primary	B-General_Concept
domain	I-General_Concept
controller	I-General_Concept
(	O
PDC	O
)	O
;	O
all	O
other	O
domain	B-General_Concept
controllers	I-General_Concept
were	O
backup	B-General_Concept
domain	I-General_Concept
controllers	I-General_Concept
(	O
BDC	O
)	O
.	O
</s>
<s>
Windows	B-Application
2000	I-Application
and	O
later	O
versions	O
introduced	O
Active	B-Protocol
Directory	I-Protocol
(	O
"	O
AD	O
"	O
)	O
,	O
which	O
largely	O
eliminated	O
the	O
concept	O
of	O
PDC	O
and	O
BDC	O
in	O
favor	O
of	O
multi-master	B-Operating_System
replication	I-Operating_System
.	O
</s>
<s>
However	O
,	O
there	O
are	O
still	O
several	O
roles	O
that	O
only	O
one	O
domain	B-General_Concept
controller	I-General_Concept
can	O
perform	O
,	O
called	O
the	O
Flexible	B-General_Concept
single	I-General_Concept
master	I-General_Concept
operation	I-General_Concept
roles	O
.	O
</s>
<s>
In	O
Windows	B-Device
NT	I-Device
4	I-Device
,	O
one	O
DC	O
serves	O
as	O
the	O
primary	B-General_Concept
domain	I-General_Concept
controller	I-General_Concept
(	O
PDC	O
)	O
.	O
</s>
<s>
Others	O
,	O
if	O
they	O
exist	O
,	O
are	O
usually	O
a	O
backup	B-General_Concept
domain	I-General_Concept
controller	I-General_Concept
(	O
BDC	O
)	O
.	O
</s>
<s>
The	O
PDC	O
will	O
usually	O
be	O
the	O
first	O
domain	B-General_Concept
controller	I-General_Concept
that	O
was	O
created	O
unless	O
it	O
was	O
replaced	O
by	O
a	O
promoted	O
BDC	O
.	O
</s>
<s>
In	O
modern	O
releases	O
of	O
Windows	O
,	O
domains	O
have	O
been	O
supplemented	O
by	O
the	O
use	O
of	O
Active	B-Protocol
Directory	I-Protocol
services	I-Protocol
.	O
</s>
<s>
In	O
Active	B-Protocol
Directory	I-Protocol
domains	O
,	O
the	O
concept	O
of	O
primary	O
and	O
secondary	O
domain	B-General_Concept
controller	I-General_Concept
relationships	O
no	O
longer	O
applies	O
.	O
</s>
<s>
The	O
PDC	O
emulator	O
master	O
acts	O
in	O
place	O
of	O
the	O
PDC	O
if	O
there	O
are	O
Windows	B-Device
NT	I-Device
4.0	I-Device
domain	B-General_Concept
controllers	I-General_Concept
(	O
BDCs	O
)	O
remaining	O
within	O
the	O
domain	O
,	O
acting	O
as	O
a	O
source	O
for	O
them	O
to	O
replicate	O
from	O
.	O
</s>
<s>
As	O
password	O
changes	O
take	O
time	O
to	O
replicate	O
across	O
all	O
the	O
domain	B-General_Concept
controllers	I-General_Concept
in	O
an	O
Active	B-Protocol
Directory	I-Protocol
domain	O
,	O
the	O
PDC	O
emulator	O
master	O
receives	O
notification	O
of	O
password	O
changes	O
immediately	O
,	O
and	O
if	O
a	O
logon	O
attempt	O
fails	O
at	O
another	O
domain	B-General_Concept
controller	I-General_Concept
,	O
that	O
domain	B-General_Concept
controller	I-General_Concept
will	O
forward	O
the	O
logon	O
request	O
to	O
the	O
PDC	O
emulator	O
master	O
before	O
rejecting	O
it	O
.	O
</s>
<s>
The	O
PDC	O
emulator	O
master	O
also	O
serves	O
as	O
the	O
machine	O
to	O
which	O
all	O
domain	B-General_Concept
controllers	I-General_Concept
in	O
the	O
domain	O
will	O
synchronise	O
their	O
clocks	O
.	O
</s>
<s>
It	O
,	O
in	O
turn	O
,	O
should	O
be	O
configured	O
to	O
synchronise	O
to	O
an	O
external	O
NTP	B-Protocol
time	O
source	O
.	O
</s>
<s>
Primary	B-General_Concept
Domain	I-General_Concept
Controllers	I-General_Concept
(	O
PDC	O
)	O
have	O
been	O
faithfully	O
recreated	O
on	O
the	O
Samba	B-Language
emulation	O
of	O
Microsoft	O
's	O
SMB	B-Operating_System
client/server	O
system	O
.	O
</s>
<s>
Samba	B-Language
has	O
the	O
capability	O
to	O
emulate	O
an	O
NT	B-Device
4.0	I-Device
domain	O
,	O
as	O
well	O
as	O
modern	O
Active	B-Protocol
Directory	I-Protocol
Domain	I-Protocol
Services	I-Protocol
on	O
a	O
Linux	B-Application
machine	O
.	O
</s>
<s>
In	O
Windows	B-Device
NT	I-Device
4	I-Device
domains	O
,	O
the	O
backup	B-General_Concept
domain	I-General_Concept
controller	I-General_Concept
(	O
BDC	O
)	O
is	O
a	O
computer	O
that	O
has	O
a	O
copy	O
of	O
the	O
user	O
accounts	O
database	O
.	O
</s>
<s>
These	O
additional	O
domain	B-General_Concept
controllers	I-General_Concept
exist	O
to	O
provide	O
fault	O
tolerance	O
.	O
</s>
<s>
When	O
Windows	B-Application
2000	I-Application
was	O
released	O
,	O
the	O
NT	B-General_Concept
domain	I-General_Concept
as	O
found	O
in	O
NT	B-Device
4	I-Device
and	O
prior	O
versions	O
was	O
replaced	O
by	O
Active	B-Protocol
Directory	I-Protocol
.	O
</s>
<s>
In	O
Active	B-Protocol
Directory	I-Protocol
domains	O
running	O
in	O
native	O
mode	O
,	O
the	O
concept	O
of	O
the	O
PDC	O
and	O
BDC	O
do	O
not	O
exist	O
.	O
</s>
<s>
In	O
these	O
domains	O
,	O
all	O
domain	B-General_Concept
controllers	I-General_Concept
are	O
considered	O
equals	O
.	O
</s>
<s>
A	O
side	O
effect	O
of	O
this	O
change	O
is	O
the	O
loss	O
of	O
ability	O
to	O
create	O
a	O
"	O
read-only	O
"	O
domain	B-General_Concept
controller	I-General_Concept
.	O
</s>
<s>
Windows	B-Device
Server	I-Device
2008	I-Device
reintroduced	O
this	O
capability	O
.	O
</s>
<s>
Windows	O
Server	O
can	O
be	O
one	O
of	O
three	O
kinds	O
:	O
Active	B-Protocol
Directory	I-Protocol
"	O
domain	B-General_Concept
controllers	I-General_Concept
"	O
(	O
ones	O
that	O
provide	O
identity	O
and	O
authentication	O
)	O
,	O
Active	B-Protocol
Directory	I-Protocol
"	O
member	O
servers	O
"	O
(	O
ones	O
that	O
provide	O
complementary	O
services	O
such	O
as	O
file	O
repositories	O
and	O
schema	O
)	O
and	O
Windows	O
Workgroup	O
"	O
stand-alone	O
servers	O
"	O
.	O
</s>
<s>
The	O
term	O
"	O
Active	B-Protocol
Directory	I-Protocol
Server	O
"	O
is	O
sometimes	O
used	O
by	O
Microsoft	O
as	O
synonymous	O
to	O
"	O
Domain	B-General_Concept
Controller	I-General_Concept
"	O
but	O
the	O
term	O
is	O
discouraged	O
.	O
</s>
