<s>
In	O
computer	O
security	O
,	O
a	O
DMZ	B-General_Concept
or	O
demilitarized	B-General_Concept
zone	I-General_Concept
(	O
sometimes	O
referred	O
to	O
as	O
a	O
perimeter	B-General_Concept
network	I-General_Concept
or	O
screened	O
subnet	B-Protocol
)	O
is	O
a	O
physical	O
or	O
logical	O
subnetwork	B-Protocol
that	O
contains	O
and	O
exposes	O
an	O
organization	O
's	O
external-facing	O
services	O
to	O
an	O
untrusted	O
,	O
usually	O
larger	O
,	O
network	O
such	O
as	O
the	O
Internet	O
.	O
</s>
<s>
The	O
purpose	O
of	O
a	O
DMZ	B-General_Concept
is	O
to	O
add	O
an	O
additional	O
layer	O
of	O
security	O
to	O
an	O
organization	O
's	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
(	O
LAN	B-General_Concept
)	O
:	O
an	O
external	O
network	B-Protocol
node	I-Protocol
can	O
access	O
only	O
what	O
is	O
exposed	O
in	O
the	O
DMZ	B-General_Concept
,	O
while	O
the	O
rest	O
of	O
the	O
organization	O
's	O
network	O
is	O
protected	O
behind	O
a	O
firewall	B-Application
.	O
</s>
<s>
The	O
DMZ	B-General_Concept
functions	O
as	O
a	O
small	O
,	O
isolated	O
network	O
positioned	O
between	O
the	O
Internet	O
and	O
the	O
private	O
network	O
.	O
</s>
<s>
This	O
is	O
not	O
to	O
be	O
confused	O
with	O
a	O
DMZ	B-General_Concept
host	B-Device
,	O
a	O
feature	O
present	O
in	O
some	O
home	O
routers	B-Protocol
which	O
frequently	O
differs	O
greatly	O
from	O
an	O
ordinary	O
DMZ	B-General_Concept
.	O
</s>
<s>
The	O
name	O
is	O
from	O
the	O
term	O
demilitarized	B-General_Concept
zone	I-General_Concept
,	O
an	O
area	O
between	O
states	O
in	O
which	O
military	O
operations	O
are	O
not	O
permitted	O
.	O
</s>
<s>
The	O
DMZ	B-General_Concept
is	O
seen	O
as	O
not	O
belonging	O
to	O
either	O
network	O
bordering	O
it	O
.	O
</s>
<s>
This	O
metaphor	O
applies	O
to	O
the	O
computing	O
use	O
as	O
the	O
DMZ	B-General_Concept
acts	O
as	O
a	O
gateway	O
to	O
the	O
public	O
Internet	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
the	O
hosts	O
most	O
vulnerable	O
to	O
attack	O
are	O
those	O
that	O
provide	O
services	O
to	O
users	O
outside	O
of	O
the	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
,	O
such	O
as	O
e-mail	O
,	O
Web	O
and	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
servers	O
.	O
</s>
<s>
Because	O
of	O
the	O
increased	O
potential	O
of	O
these	O
hosts	O
suffering	O
an	O
attack	O
,	O
they	O
are	O
placed	O
into	O
this	O
specific	O
subnetwork	B-Protocol
in	O
order	O
to	O
protect	O
the	O
rest	O
of	O
the	O
network	O
in	O
case	O
any	O
of	O
them	O
become	O
compromised	O
.	O
</s>
<s>
Hosts	O
in	O
the	O
DMZ	B-General_Concept
are	O
permitted	O
to	O
have	O
only	O
limited	O
connectivity	O
to	O
specific	O
hosts	O
in	O
the	O
internal	O
network	O
,	O
as	O
the	O
content	O
of	O
DMZ	B-General_Concept
is	O
not	O
as	O
secure	O
as	O
the	O
internal	O
network	O
.	O
</s>
<s>
Similarly	O
,	O
communication	O
between	O
hosts	O
in	O
the	O
DMZ	B-General_Concept
and	O
to	O
the	O
external	O
network	O
is	O
also	O
restricted	O
to	O
make	O
the	O
DMZ	B-General_Concept
more	O
secure	O
than	O
the	O
Internet	O
and	O
suitable	O
for	O
housing	O
these	O
special	O
purpose	O
services	O
.	O
</s>
<s>
This	O
allows	O
hosts	O
in	O
the	O
DMZ	B-General_Concept
to	O
communicate	O
with	O
both	O
the	O
internal	O
and	O
external	O
network	O
,	O
while	O
an	O
intervening	O
firewall	B-Application
controls	O
the	O
traffic	O
between	O
the	O
DMZ	B-General_Concept
servers	O
and	O
the	O
internal	O
network	O
clients	O
,	O
and	O
another	O
firewall	B-Application
would	O
perform	O
some	O
level	O
of	O
control	O
to	O
protect	O
the	O
DMZ	B-General_Concept
from	O
the	O
external	O
network	O
.	O
</s>
<s>
A	O
DMZ	B-General_Concept
configuration	O
provides	O
additional	O
security	O
from	O
external	O
attacks	O
,	O
but	O
it	O
typically	O
has	O
no	O
bearing	O
on	O
internal	O
attacks	O
such	O
as	O
sniffing	O
communication	O
via	O
a	O
packet	B-Protocol
analyzer	I-Protocol
or	O
spoofing	O
such	O
as	O
e-mail	O
spoofing	O
.	O
</s>
<s>
It	O
is	O
also	O
sometimes	O
good	O
practice	O
to	O
configure	O
a	O
separate	O
classified	O
militarized	O
zone	O
(	O
CMZ	O
)	O
,	O
a	O
highly	O
monitored	O
militarized	O
zone	O
comprising	O
mostly	O
Web	B-Application
servers	I-Application
(	O
and	O
similar	O
servers	O
that	O
interface	O
to	O
the	O
external	O
world	O
i.e.	O
</s>
<s>
the	O
Internet	O
)	O
that	O
are	O
not	O
in	O
the	O
DMZ	B-General_Concept
but	O
contain	O
sensitive	O
information	O
about	O
accessing	O
servers	O
within	O
the	O
LAN	B-General_Concept
(	O
like	O
database	B-Application
servers	I-Application
)	O
.	O
</s>
<s>
In	O
such	O
architecture	O
,	O
the	O
DMZ	B-General_Concept
usually	O
has	O
the	O
application	B-Protocol
firewall	I-Protocol
and	O
the	O
FTP	B-Protocol
while	O
the	O
CMZ	O
hosts	O
the	O
Web	B-Application
servers	I-Application
.	O
</s>
<s>
(	O
The	O
database	B-Application
servers	I-Application
could	O
be	O
in	O
the	O
CMZ	O
,	O
in	O
the	O
LAN	B-General_Concept
,	O
or	O
in	O
a	O
separate	O
VLAN	O
altogether	O
.	O
)	O
</s>
<s>
Any	O
service	O
that	O
is	O
being	O
provided	O
to	O
users	O
on	O
the	O
external	O
network	O
can	O
be	O
placed	O
in	O
the	O
DMZ	B-General_Concept
.	O
</s>
<s>
Web	B-Application
servers	I-Application
that	O
communicate	O
with	O
an	O
internal	O
database	O
require	O
access	O
to	O
a	O
database	B-Application
server	I-Application
,	O
which	O
may	O
not	O
be	O
publicly	O
accessible	O
and	O
may	O
contain	O
sensitive	O
information	O
.	O
</s>
<s>
The	O
web	B-Application
servers	I-Application
can	O
communicate	O
with	O
database	B-Application
servers	I-Application
either	O
directly	O
or	O
through	O
an	O
application	B-Protocol
firewall	I-Protocol
for	O
security	O
reasons	O
.	O
</s>
<s>
E-mail	O
messages	O
and	O
particularly	O
the	O
user	O
database	O
are	O
confidential	O
,	O
so	O
they	O
are	O
typically	O
stored	O
on	O
servers	O
that	O
cannot	O
be	O
accessed	O
from	O
the	O
Internet	O
(	O
at	O
least	O
not	O
in	O
an	O
insecure	O
manner	O
)	O
,	O
but	O
can	O
be	O
accessed	O
from	O
email	B-Protocol
servers	I-Protocol
that	O
are	O
exposed	O
to	O
the	O
Internet	O
.	O
</s>
<s>
The	O
mail	B-Protocol
server	I-Protocol
inside	O
the	O
DMZ	B-General_Concept
passes	O
incoming	O
mail	O
to	O
the	O
secured/internal	O
mail	B-Protocol
servers	I-Protocol
.	O
</s>
<s>
For	O
security	O
,	O
compliance	O
with	O
legal	O
standards	O
such	O
as	O
HIPAA	O
,	O
and	O
monitoring	O
reasons	O
,	O
in	O
a	O
business	O
environment	O
,	O
some	O
enterprises	O
install	O
a	O
proxy	B-Application
server	I-Application
within	O
the	O
DMZ	B-General_Concept
.	O
</s>
<s>
Obliges	O
internal	O
users	O
(	O
usually	O
employees	O
)	O
to	O
use	O
the	O
proxy	B-Application
server	I-Application
for	O
Internet	O
access	O
.	O
</s>
<s>
Reduced	O
Internet	O
access	O
bandwidth	O
requirements	O
since	O
some	O
web	O
content	O
may	O
be	O
cached	O
by	O
the	O
proxy	B-Application
server	I-Application
.	O
</s>
<s>
A	O
reverse	B-General_Concept
proxy	I-General_Concept
server	I-Application
,	O
like	O
a	O
proxy	B-Application
server	I-Application
,	O
is	O
an	O
intermediary	O
,	O
but	O
is	O
used	O
the	O
other	O
way	O
around	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
back	O
office	O
application	O
access	O
,	O
such	O
as	O
an	O
email	O
system	O
,	O
could	O
be	O
provided	O
to	O
external	O
users	O
(	O
to	O
read	O
emails	O
while	O
outside	O
the	O
company	O
)	O
but	O
the	O
remote	O
user	O
would	O
not	O
have	O
direct	O
access	O
to	O
their	O
email	B-Protocol
server	I-Protocol
(	O
only	O
the	O
reverse	B-General_Concept
proxy	I-General_Concept
server	O
can	O
physically	O
access	O
the	O
internal	O
email	B-Protocol
server	I-Protocol
)	O
.	O
</s>
<s>
Usually	O
such	O
a	O
reverse	B-General_Concept
proxy	I-General_Concept
mechanism	O
is	O
provided	O
by	O
using	O
an	O
application	B-Protocol
layer	I-Protocol
firewall	I-Protocol
that	O
focuses	O
on	O
the	O
specific	O
shape	O
and	O
contents	O
of	O
the	O
traffic	O
rather	O
than	O
just	O
controlling	O
access	O
to	O
specific	O
TCP	B-Protocol
and	I-Protocol
UDP	I-Protocol
ports	I-Protocol
(	O
as	O
a	O
packet	O
filter	O
firewall	B-Application
would	O
do	O
)	O
,	O
but	O
a	O
reverse	B-General_Concept
proxy	I-General_Concept
is	O
usually	O
not	O
a	O
good	O
substitute	O
for	O
a	O
well	O
thought	O
out	O
DMZ	B-General_Concept
design	O
as	O
it	O
has	O
to	O
rely	O
on	O
continuous	O
signature	O
updates	O
for	O
updated	O
attack	O
vectors	O
.	O
</s>
<s>
There	O
are	O
many	O
different	O
ways	O
to	O
design	O
a	O
network	O
with	O
a	O
DMZ	B-General_Concept
.	O
</s>
<s>
Two	O
of	O
the	O
most	O
basic	O
methods	O
are	O
with	O
a	O
single	O
firewall	B-Application
,	O
also	O
known	O
as	O
the	O
three	O
legged	O
model	O
,	O
and	O
with	O
dual	O
firewalls	B-Application
,	O
also	O
known	O
as	O
back	O
to	O
back	O
.	O
</s>
<s>
A	O
single	O
firewall	B-Application
with	O
at	O
least	O
3	O
network	O
interfaces	O
can	O
be	O
used	O
to	O
create	O
a	O
network	O
architecture	O
containing	O
a	O
DMZ	B-General_Concept
.	O
</s>
<s>
The	O
external	O
network	O
is	O
formed	O
from	O
the	O
ISP	O
to	O
the	O
firewall	B-Application
on	O
the	O
first	O
network	O
interface	O
,	O
the	O
internal	O
network	O
is	O
formed	O
from	O
the	O
second	O
network	O
interface	O
,	O
and	O
the	O
DMZ	B-General_Concept
is	O
formed	O
from	O
the	O
third	O
network	O
interface	O
.	O
</s>
<s>
The	O
firewall	B-Application
becomes	O
a	O
single	O
point	O
of	O
failure	O
for	O
the	O
network	O
and	O
must	O
be	O
able	O
to	O
handle	O
all	O
of	O
the	O
traffic	O
going	O
to	O
the	O
DMZ	B-General_Concept
as	O
well	O
as	O
the	O
internal	O
network	O
.	O
</s>
<s>
The	O
zones	O
are	O
usually	O
marked	O
with	O
colors	O
-for	O
example	O
,	O
purple	O
for	O
LAN	B-General_Concept
,	O
green	O
for	O
DMZ	B-General_Concept
,	O
red	O
for	O
Internet	O
(	O
with	O
often	O
another	O
color	O
used	O
for	O
wireless	O
zones	O
)	O
.	O
</s>
<s>
The	O
most	O
secure	O
approach	O
,	O
according	O
to	O
Colton	O
Fralick	O
,	O
is	O
to	O
use	O
two	O
firewalls	B-Application
to	O
create	O
a	O
DMZ	B-General_Concept
.	O
</s>
<s>
The	O
first	O
firewall	B-Application
(	O
also	O
called	O
the	O
"	O
front-end	O
"	O
or	O
"	O
perimeter	O
"	O
firewall	B-Application
)	O
must	O
be	O
configured	O
to	O
allow	O
traffic	O
destined	O
to	O
the	O
DMZ	B-General_Concept
only	O
.	O
</s>
<s>
The	O
second	O
firewall	B-Application
(	O
also	O
called	O
"	O
back-end	O
"	O
or	O
"	O
internal	O
"	O
firewall	B-Application
)	O
only	O
allows	O
traffic	O
to	O
the	O
DMZ	B-General_Concept
from	O
the	O
internal	O
network	O
.	O
</s>
<s>
There	O
is	O
even	O
more	O
protection	O
if	O
the	O
two	O
firewalls	B-Application
are	O
provided	O
by	O
two	O
different	O
vendors	O
,	O
because	O
it	O
makes	O
it	O
less	O
likely	O
that	O
both	O
devices	O
suffer	O
from	O
the	O
same	O
security	O
vulnerabilities	O
.	O
</s>
<s>
The	O
practice	O
of	O
using	O
different	O
firewalls	B-Application
from	O
different	O
vendors	O
is	O
sometimes	O
described	O
as	O
a	O
component	O
of	O
a	O
"	O
defense	O
in	O
depth	O
"	O
security	O
strategy	O
.	O
</s>
<s>
Some	O
home	O
routers	B-Protocol
refer	O
to	O
a	O
DMZ	B-General_Concept
host	B-Device
,	O
which	O
—	O
in	O
many	O
cases	O
—	O
is	O
actually	O
a	O
misnomer	O
.	O
</s>
<s>
A	O
home	O
router	B-Protocol
DMZ	B-General_Concept
host	B-Device
is	O
a	O
single	O
address	O
(	O
e.g.	O
,	O
IP	O
address	O
)	O
on	O
the	O
internal	O
network	O
that	O
has	O
all	O
traffic	O
sent	O
to	O
it	O
which	O
is	O
not	O
otherwise	O
forwarded	O
to	O
other	O
LAN	B-General_Concept
hosts	O
.	O
</s>
<s>
By	O
definition	O
,	O
this	O
is	O
not	O
a	O
true	O
DMZ	B-General_Concept
(	O
demilitarized	B-General_Concept
zone	I-General_Concept
)	O
,	O
since	O
the	O
router	B-Protocol
alone	O
does	O
not	O
separate	O
the	O
host	B-Device
from	O
the	O
internal	O
network	O
.	O
</s>
<s>
That	O
is	O
,	O
the	O
DMZ	B-General_Concept
host	B-Device
is	O
able	O
to	O
connect	O
to	O
other	O
hosts	O
on	O
the	O
internal	O
network	O
,	O
whereas	O
hosts	O
within	O
a	O
real	O
DMZ	B-General_Concept
are	O
prevented	O
from	O
connecting	O
with	O
the	O
internal	O
network	O
by	O
a	O
firewall	B-Application
that	O
separates	O
them	O
unless	O
the	O
firewall	B-Application
permits	O
the	O
connection	O
.	O
</s>
<s>
A	O
firewall	B-Application
may	O
allow	O
this	O
if	O
a	O
host	B-Device
on	O
the	O
internal	O
network	O
first	O
requests	O
a	O
connection	O
to	O
the	O
host	B-Device
within	O
the	O
DMZ	B-General_Concept
.	O
</s>
<s>
The	O
DMZ	B-General_Concept
host	B-Device
provides	O
none	O
of	O
the	O
security	O
advantages	O
that	O
a	O
subnet	B-Protocol
provides	O
and	O
is	O
often	O
used	O
as	O
an	O
easy	O
method	O
of	O
forwarding	O
all	O
ports	O
to	O
another	O
firewall	B-Application
/	O
NAT	B-Protocol
device	I-Protocol
.	O
</s>
<s>
This	O
tactic	O
(	O
establishing	O
a	O
DMZ	B-General_Concept
host	B-Device
)	O
is	O
also	O
used	O
with	O
systems	O
which	O
do	O
not	O
interact	O
properly	O
with	O
normal	O
firewalling	O
rules	O
or	O
NAT	B-Protocol
.	O
</s>
<s>
This	O
can	O
be	O
because	O
no	O
forwarding	O
rule	O
can	O
be	O
formulated	O
ahead	O
of	O
time	O
(	O
varying	O
TCP	O
or	O
UDP	O
port	B-Protocol
numbers	I-Protocol
for	O
example	O
,	O
as	O
opposed	O
to	O
a	O
fixed	O
number	O
or	O
fixed	O
range	O
)	O
.	O
</s>
<s>
This	O
is	O
also	O
used	O
for	O
network	O
protocols	O
for	O
which	O
the	O
router	B-Protocol
has	O
no	O
programming	O
to	O
handle	O
(	O
6in4	B-Protocol
or	O
GRE	B-Protocol
tunnels	I-Protocol
are	O
prototypical	O
examples	O
)	O
.	O
</s>
