<s>
In	O
computer	B-Architecture
networking	I-Architecture
,	O
split-horizon	B-Protocol
DNS	I-Protocol
(	O
also	O
known	O
as	O
split-view	B-Protocol
DNS	I-Protocol
,	O
split-brain	O
DNS	B-Protocol
,	O
or	O
split	O
DNS	B-Protocol
)	O
is	O
the	O
facility	O
of	O
a	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
implementation	O
to	O
provide	O
different	O
sets	O
of	O
DNS	B-Protocol
information	O
,	O
usually	O
selected	O
by	O
the	O
source	O
address	O
of	O
the	O
DNS	B-Protocol
request	O
.	O
</s>
<s>
This	O
facility	O
can	O
provide	O
a	O
mechanism	O
for	O
security	O
and	O
privacy	O
management	O
by	O
logical	O
or	O
physical	O
separation	O
of	O
DNS	B-Protocol
information	O
for	O
network-internal	O
access	O
(	O
within	O
an	O
administrative	B-Protocol
domain	I-Protocol
,	O
e.g.	O
,	O
company	O
)	O
and	O
access	O
from	O
an	O
unsecure	O
,	O
public	O
network	O
(	O
e.g.	O
</s>
<s>
Implementation	O
of	O
split-horizon	B-Protocol
DNS	I-Protocol
can	O
be	O
accomplished	O
with	O
hardware-based	O
separation	O
or	O
by	O
software	O
solutions	O
.	O
</s>
<s>
Hardware-based	O
implementations	O
run	O
distinct	O
DNS	B-Protocol
server	O
devices	O
for	O
the	O
desired	O
access	O
granularity	O
within	O
the	O
networks	O
involved	O
.	O
</s>
<s>
Software	O
solutions	O
use	O
either	O
multiple	O
DNS	B-Protocol
server	O
processes	O
on	O
the	O
same	O
hardware	O
or	O
special	O
server	O
software	O
with	O
the	O
built-in	O
capability	O
of	O
discriminating	O
access	O
to	O
DNS	B-Protocol
zone	I-Protocol
records	O
.	O
</s>
<s>
The	O
latter	O
is	O
a	O
common	O
feature	O
of	O
many	O
server	O
software	O
implementations	O
of	O
the	O
DNS	B-Protocol
protocol	I-Protocol
(	O
cf	O
.	O
</s>
<s>
Comparison	B-Protocol
of	I-Protocol
DNS	I-Protocol
server	I-Protocol
software	I-Protocol
)	O
and	O
is	O
sometimes	O
the	O
implied	O
meaning	O
of	O
the	O
term	O
split-horizon	B-Protocol
DNS	I-Protocol
,	O
since	O
all	O
other	O
forms	O
of	O
implementation	O
can	O
be	O
achieved	O
with	O
any	O
DNS	B-Protocol
server	O
software	O
.	O
</s>
<s>
Split-horizon	B-Protocol
DNS	I-Protocol
can	O
provide	O
a	O
mechanism	O
for	O
security	O
and	O
privacy	O
management	O
by	O
logical	O
or	O
physical	O
separation	O
of	O
DNS	B-Protocol
information	O
for	O
network-internal	O
access	O
(	O
within	O
an	O
administrative	B-Protocol
domain	I-Protocol
,	O
e.g.	O
,	O
company	O
)	O
and	O
access	O
from	O
an	O
unsecure	O
,	O
public	O
network	O
(	O
e.g.	O
</s>
<s>
One	O
common	O
use	O
case	O
for	O
split-horizon	B-Protocol
DNS	I-Protocol
is	O
when	O
a	O
server	O
has	O
both	O
a	O
private	O
IP	O
address	O
on	O
a	O
local	O
area	O
network	O
(	O
not	O
reachable	O
from	O
most	O
of	O
the	O
Internet	O
)	O
and	O
a	O
public	O
address	O
,	O
i.e.	O
</s>
<s>
By	O
using	O
split-horizon	B-Protocol
DNS	I-Protocol
the	O
same	O
name	O
can	O
lead	O
to	O
either	O
the	O
private	O
IP	O
address	O
or	O
the	O
public	O
one	O
,	O
depending	O
on	O
which	O
client	O
sends	O
the	O
query	O
.	O
</s>
<s>
As	O
an	O
example	O
,	O
the	O
DNS	B-Protocol
server	O
could	O
be	O
configured	O
to	O
return	O
two	O
different	O
sets	O
of	O
records	O
for	O
the	O
host	O
host1.example.net	O
for	O
requestees	O
internal	O
and	O
external	O
to	O
a	O
corporate	O
network	O
.	O
</s>
<s>
Split-horizon	B-Protocol
DNS	I-Protocol
is	O
designed	O
to	O
provide	O
different	O
authoritative	O
answers	O
to	O
an	O
identical	O
query	O
and	O
DNSSEC	B-Protocol
is	O
used	O
to	O
ensure	O
veracity	O
of	O
data	O
returned	O
by	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
.	O
</s>
<s>
Research	O
has	O
produced	O
recommendations	O
to	O
properly	O
combine	O
these	O
two	O
DNS	B-Protocol
features	O
.	O
</s>
<s>
Implementation	O
of	O
split-horizon	B-Protocol
DNS	I-Protocol
can	O
be	O
accomplished	O
with	O
hardware-based	O
separation	O
or	O
by	O
software	O
solutions	O
.	O
</s>
<s>
Hardware-based	O
implementations	O
run	O
distinct	O
DNS	B-Protocol
server	O
devices	O
for	O
the	O
desired	O
access	O
granularity	O
within	O
the	O
networks	O
involved	O
.	O
</s>
<s>
Software	O
solutions	O
use	O
either	O
multiple	O
DNS	B-Protocol
server	O
processes	O
on	O
the	O
same	O
hardware	O
or	O
special	O
server	O
software	O
with	O
the	O
built-in	O
capability	O
of	O
discriminating	O
access	O
to	O
DNS	B-Protocol
zone	I-Protocol
records	O
.	O
</s>
<s>
The	O
latter	O
is	O
a	O
common	O
feature	O
of	O
many	O
server	O
software	O
implementations	O
of	O
the	O
DNS	B-Protocol
protocol	I-Protocol
(	O
cf	O
.	O
</s>
<s>
Comparison	B-Protocol
of	I-Protocol
DNS	I-Protocol
server	I-Protocol
software	I-Protocol
)	O
and	O
is	O
sometimes	O
the	O
implied	O
meaning	O
of	O
the	O
term	O
split-horizon	B-Protocol
DNS	I-Protocol
,	O
since	O
all	O
other	O
forms	O
of	O
implementation	O
can	O
be	O
achieved	O
with	O
any	O
DNS	B-Protocol
server	O
software	O
.	O
</s>
