<s>
In	O
computing	O
,	O
a	O
firewall	B-Application
is	O
a	O
network	O
security	O
system	O
that	O
monitors	O
and	O
controls	O
incoming	O
and	O
outgoing	O
network	O
traffic	O
based	O
on	O
predetermined	O
security	O
rules	O
.	O
</s>
<s>
A	O
firewall	B-Application
typically	O
establishes	O
a	O
barrier	O
between	O
a	O
trusted	O
network	O
and	O
an	O
untrusted	O
network	O
,	O
such	O
as	O
the	O
Internet	O
.	O
</s>
<s>
The	O
term	O
firewall	B-Application
originally	O
referred	O
to	O
a	O
wall	O
intended	O
to	O
confine	O
a	O
fire	O
within	O
a	O
line	O
of	O
adjacent	O
buildings	O
.	O
</s>
<s>
The	O
predecessors	O
to	O
firewalls	B-Application
for	O
network	O
security	O
were	O
routers	B-Protocol
used	O
in	O
the	O
late	O
1980s	O
.	O
</s>
<s>
Because	O
they	O
already	O
segregated	O
networks	O
,	O
routers	B-Protocol
could	O
apply	O
filtering	O
to	O
packets	B-Protocol
crossing	O
them	O
.	O
</s>
<s>
Before	O
it	O
was	O
used	O
in	O
real-life	O
computing	O
,	O
the	O
term	O
appeared	O
in	O
the	O
1983	O
computer-hacking	O
movie	O
WarGames	B-Application
,	O
and	O
possibly	O
inspired	O
its	O
later	O
use	O
.	O
</s>
<s>
Firewalls	B-Application
are	O
categorized	O
as	O
a	O
network-based	O
or	O
a	O
host-based	O
system	O
.	O
</s>
<s>
Network-based	O
firewalls	B-Application
are	O
positioned	O
between	O
two	O
or	O
more	O
networks	O
,	O
typically	O
between	O
the	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
(	O
LAN	B-General_Concept
)	O
and	O
wide	O
area	O
network	O
(	O
WAN	O
)	O
.	O
</s>
<s>
They	O
are	O
either	O
a	O
software	B-Application
appliance	I-Application
running	O
on	O
general-purpose	O
hardware	O
,	O
a	O
hardware	O
appliance	O
running	O
on	O
special-purpose	O
hardware	O
,	O
or	O
a	O
virtual	B-Application
appliance	I-Application
running	O
on	O
a	O
virtual	O
host	B-Device
controlled	O
by	O
a	O
hypervisor	B-Operating_System
.	O
</s>
<s>
Firewall	B-Application
appliances	O
may	O
also	O
offer	O
non	O
firewall	B-Application
functionality	O
,	O
such	O
as	O
DHCP	B-Protocol
or	O
VPN	B-Application
services	O
.	O
</s>
<s>
Host-based	B-Application
firewalls	I-Application
are	O
deployed	O
directly	O
on	O
the	O
host	B-Device
itself	O
to	O
control	O
network	O
traffic	O
or	O
other	O
computing	O
resources	O
.	O
</s>
<s>
This	O
can	O
be	O
a	O
daemon	B-Operating_System
or	O
service	O
as	O
a	O
part	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
or	O
an	O
agent	O
application	O
for	O
protection	O
.	O
</s>
<s>
The	O
first	O
reported	O
type	O
of	O
network	B-Application
firewall	I-Application
is	O
called	O
a	O
packet	B-Protocol
filter	O
,	O
which	O
inspects	O
packets	B-Protocol
transferred	O
between	O
computers	O
.	O
</s>
<s>
The	O
firewall	B-Application
maintains	O
an	O
access-control	O
list	O
which	O
dictates	O
what	O
packets	B-Protocol
will	O
be	O
looked	O
at	O
and	O
what	O
action	O
should	O
be	O
applied	O
,	O
if	O
any	O
,	O
with	O
the	O
default	O
action	O
set	O
to	O
silent	O
discard	O
.	O
</s>
<s>
Three	O
basic	O
actions	O
regarding	O
the	O
packet	B-Protocol
consist	O
of	O
a	O
silent	O
discard	O
,	O
discard	O
with	O
Internet	B-Protocol
Control	I-Protocol
Message	I-Protocol
Protocol	I-Protocol
or	O
TCP	B-Protocol
reset	I-Protocol
response	O
to	O
the	O
sender	O
,	O
and	O
forward	O
to	O
the	O
next	O
hop	O
.	O
</s>
<s>
Packets	B-Protocol
may	O
be	O
filtered	O
by	O
source	O
and	O
destination	O
IP	B-Protocol
addresses	I-Protocol
,	O
protocol	O
,	O
source	O
and	O
destination	O
ports	B-Protocol
.	O
</s>
<s>
The	O
bulk	O
of	O
Internet	O
communication	O
in	O
20th	O
and	O
early	O
21st	O
century	O
used	O
either	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
or	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
in	O
conjunction	O
with	O
well-known	B-Protocol
ports	I-Protocol
,	O
enabling	O
firewalls	B-Application
of	O
that	O
era	O
to	O
distinguish	O
between	O
specific	O
types	O
of	O
traffic	O
such	O
as	O
web	O
browsing	O
,	O
remote	O
printing	O
,	O
email	O
transmission	O
,	O
and	O
file	O
transfers	O
.	O
</s>
<s>
The	O
first	O
paper	O
published	O
on	O
firewall	B-Application
technology	O
was	O
in	O
1987	O
when	O
engineers	O
from	O
Digital	O
Equipment	O
Corporation	O
(	O
DEC	O
)	O
developed	O
filter	O
systems	O
known	O
as	O
packet	B-Protocol
filter	O
firewalls	B-Application
.	O
</s>
<s>
At	O
AT&T	O
Bell	O
Labs	O
,	O
Bill	O
Cheswick	O
and	O
Steve	O
Bellovin	O
continued	O
their	O
research	O
in	O
packet	B-Application
filtering	I-Application
and	O
developed	O
a	O
working	O
model	O
for	O
their	O
own	O
company	O
based	O
on	O
their	O
original	O
first-generation	O
architecture	O
.	O
</s>
<s>
Van	O
Jacobson	O
released	O
a	O
paper	O
on	O
BSD	B-Protocol
Packet	I-Protocol
Filter	I-Protocol
(	O
BPF	O
)	O
while	O
at	O
Lawrence	O
Berkeley	O
Laboratory	O
.	O
</s>
<s>
From	O
1989	O
–	O
1990	O
,	O
three	O
colleagues	O
from	O
AT&T	O
Bell	O
Laboratories	O
,	O
Dave	O
Presotto	O
,	O
Janardan	O
Sharma	O
,	O
and	O
Kshitij	O
Nigam	O
,	O
developed	O
the	O
second	O
generation	O
of	O
firewalls	B-Application
,	O
calling	O
them	O
circuit-level	B-Protocol
gateways	I-Protocol
.	O
</s>
<s>
Second-generation	O
firewalls	B-Application
perform	O
the	O
work	O
of	O
their	O
first-generation	O
predecessors	O
but	O
also	O
maintain	O
knowledge	O
of	O
specific	O
conversations	O
between	O
endpoints	O
by	O
remembering	O
which	O
port	B-Protocol
number	I-Protocol
the	O
two	O
IP	B-Protocol
addresses	I-Protocol
are	O
using	O
at	O
layer	B-Architecture
4	I-Architecture
(	O
transport	B-Architecture
layer	I-Architecture
)	O
of	O
the	O
OSI	O
model	O
for	O
their	O
conversation	O
,	O
allowing	O
examination	O
of	O
the	O
overall	O
exchange	O
between	O
the	O
nodes	O
.	O
</s>
<s>
Marcus	O
Ranum	O
,	O
Wei	O
Xu	O
,	O
and	O
Peter	O
Churchyard	O
released	O
an	O
application	O
firewall	B-Application
known	O
as	O
Firewall	B-Application
Toolkit	O
(	O
FWTK	O
)	O
in	O
October	O
1993	O
.	O
</s>
<s>
This	O
became	O
the	O
basis	O
for	O
Gauntlet	O
firewall	B-Application
at	O
Trusted	O
Information	O
Systems	O
.	O
</s>
<s>
The	O
key	O
benefit	O
of	O
application	B-Application
layer	I-Application
filtering	O
is	O
that	O
it	O
can	O
understand	O
certain	O
applications	O
and	O
protocols	O
such	O
as	O
File	B-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
(	O
FTP	O
)	O
,	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
,	O
or	O
Hypertext	B-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
(	O
HTTP	B-Protocol
)	O
.	O
</s>
<s>
It	O
can	O
also	O
provide	O
unified	O
security	O
management	O
including	O
enforced	O
encrypted	B-Protocol
DNS	I-Protocol
and	O
virtual	B-Application
private	I-Application
networking	I-Application
.	O
</s>
<s>
As	O
of	O
2012	O
,	O
the	O
next-generation	B-Protocol
firewall	I-Protocol
provides	O
a	O
wider	O
range	O
of	O
inspection	O
at	O
the	O
application	B-Application
layer	I-Application
,	O
extending	O
deep	B-Application
packet	I-Application
inspection	I-Application
functionality	O
to	O
include	O
,	O
but	O
is	O
not	O
limited	O
to	O
:	O
</s>
<s>
Endpoint-based	O
application	O
firewalls	B-Application
function	O
by	O
determining	O
whether	O
a	O
process	O
should	O
accept	O
any	O
given	O
connection	O
.	O
</s>
<s>
Application	O
firewalls	B-Application
filter	O
connections	O
by	O
examining	O
the	O
process	O
ID	O
of	O
data	B-Protocol
packets	I-Protocol
against	O
a	O
rule	O
set	O
for	O
the	O
local	O
process	O
involved	O
in	O
the	O
data	O
transmission	O
.	O
</s>
<s>
Application	O
firewalls	B-Application
accomplish	O
their	O
function	O
by	O
hooking	O
into	O
socket	B-Protocol
calls	O
to	O
filter	O
the	O
connections	O
between	O
the	O
application	B-Application
layer	I-Application
and	O
the	O
lower	O
layers	O
.	O
</s>
<s>
Application	O
firewalls	B-Application
that	O
hook	O
into	O
socket	B-Protocol
calls	O
are	O
also	O
referred	O
to	O
as	O
socket	B-Protocol
filters	O
.	O
</s>
<s>
Setting	O
up	O
a	O
firewall	B-Application
is	O
a	O
complex	O
and	O
error-prone	O
task	O
.	O
</s>
<s>
Firewall	B-Application
policy	O
configuration	O
is	O
based	O
on	O
specific	O
network	O
type	O
(	O
e.g.	O
,	O
public	O
or	O
private	O
)	O
,	O
and	O
can	O
be	O
set	O
up	O
using	O
firewall	B-Application
rules	O
that	O
either	O
block	O
or	O
allow	O
access	O
to	O
prevent	O
potential	O
attacks	O
from	O
hackers	O
or	O
malware	O
.	O
</s>
