<s>
An	O
application	B-Protocol
firewall	I-Protocol
is	O
a	O
form	O
of	O
firewall	B-Application
that	O
controls	O
input/output	B-General_Concept
or	O
system	B-Operating_System
calls	I-Operating_System
of	O
an	O
application	O
or	O
service	O
.	O
</s>
<s>
The	O
application	B-Protocol
firewall	I-Protocol
can	O
control	O
communications	O
up	O
to	O
the	O
application	B-Application
layer	I-Application
of	O
the	O
OSI	O
model	O
,	O
which	O
is	O
the	O
highest	O
operating	O
layer	O
,	O
and	O
where	O
it	O
gets	O
its	O
name	O
.	O
</s>
<s>
The	O
two	O
primary	O
categories	O
of	O
application	B-Protocol
firewalls	I-Protocol
are	O
network-based	O
and	O
host-based	O
.	O
</s>
<s>
Gene	O
Spafford	O
of	O
Purdue	O
University	O
,	O
Bill	O
Cheswick	O
at	O
AT&T	O
Laboratories	O
,	O
and	O
Marcus	O
Ranum	O
described	O
a	O
third-generation	O
firewall	B-Application
known	O
as	O
an	O
application	B-Protocol
layer	I-Protocol
firewall	I-Protocol
.	O
</s>
<s>
Marcus	O
Ranum	O
's	O
work	O
,	O
based	O
on	O
the	O
firewall	B-Application
created	O
by	O
Paul	O
Vixie	O
,	O
Brian	O
Reid	O
,	O
and	O
Jeff	O
Mogul	O
,	O
spearheaded	O
the	O
creation	O
of	O
the	O
first	O
commercial	O
product	O
.	O
</s>
<s>
Under	O
a	O
broader	O
DARPA	O
contract	O
at	O
TIS	O
,	O
Marcus	O
Ranum	O
,	O
Wei	O
Xu	O
,	O
and	O
Peter	O
Churchyard	O
developed	O
the	O
Firewall	B-Application
Toolkit	O
(	O
FWTK	O
)	O
and	O
made	O
it	O
freely	O
available	O
under	O
license	O
in	O
October	O
1993	O
.	O
</s>
<s>
The	O
purposes	O
for	O
releasing	O
the	O
freely	O
available	O
,	O
not	O
for	O
commercial	O
use	O
,	O
FWTK	O
were	O
:	O
to	O
demonstrate	O
,	O
via	O
the	O
software	O
,	O
documentation	O
,	O
and	O
methods	O
used	O
,	O
how	O
a	O
company	O
with	O
(	O
at	O
the	O
time	O
)	O
11	O
years	O
experience	O
in	O
formal	O
security	O
methods	O
,	O
and	O
individuals	O
with	O
firewall	B-Application
experience	O
,	O
developed	O
firewall	B-Application
software	I-Application
;	O
to	O
create	O
a	O
common	O
base	O
of	O
very	O
good	O
firewall	B-Application
software	I-Application
for	O
others	O
to	O
build	O
on	O
(	O
so	O
people	O
did	O
not	O
have	O
to	O
continue	O
to	O
"	O
roll	O
their	O
own	O
"	O
from	O
scratch	O
)	O
;	O
to	O
"	O
raise	O
the	O
bar	O
"	O
of	O
firewall	B-Application
software	I-Application
being	O
used	O
.	O
</s>
<s>
This	O
was	O
the	O
first	O
transparent	B-Protocol
firewall	I-Protocol
,	O
known	O
as	O
the	O
inception	O
of	O
the	O
third	O
generation	O
firewall	B-Application
,	O
beyond	O
a	O
traditional	O
application	O
proxy	O
(	O
the	O
second	O
generation	O
firewall	B-Application
)	O
,	O
released	O
as	O
the	O
commercial	O
product	O
known	O
as	O
Gauntlet	O
firewall	B-Application
.	O
</s>
<s>
Gauntlet	O
firewall	B-Application
was	O
rated	O
one	O
of	O
the	O
top	O
application	B-Protocol
firewalls	I-Protocol
from	O
1995	O
until	O
1998	O
,	O
the	O
year	O
it	O
was	O
acquired	O
by	O
Network	O
Associates	O
Inc	O
,	O
(	O
NAI	O
)	O
.	O
</s>
<s>
Network	O
Associates	O
continued	O
to	O
claim	O
that	O
Gauntlet	O
was	O
the	O
"	O
worlds	O
most	O
secure	O
firewall	B-Application
"	O
but	O
in	O
May	O
2000	O
,	O
security	O
researcher	O
Jim	O
Stickley	O
discovered	O
a	O
large	O
vulnerability	O
in	O
the	O
firewall	B-Application
,	O
allowing	O
remote	O
access	O
to	O
the	O
operating	O
system	O
and	O
bypassing	O
the	O
security	O
controls	O
.	O
</s>
<s>
Stickley	O
discovered	O
a	O
second	O
vulnerability	O
a	O
year	O
later	O
,	O
effectively	O
ending	O
Gauntlet	O
firewalls	B-Application
 '	O
security	O
dominance	O
.	O
</s>
<s>
Application	B-Protocol
layer	I-Protocol
filtering	I-Protocol
operates	O
at	O
a	O
higher	O
level	O
than	O
traditional	O
security	O
appliances	O
.	O
</s>
<s>
Network-based	O
application	B-Protocol
firewalls	I-Protocol
operate	O
at	O
the	O
application	B-Application
layer	I-Application
of	O
a	O
TCP/IP	B-Protocol
stack	I-Protocol
and	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>
Modern	O
versions	O
of	O
network-based	O
application	B-Protocol
firewalls	I-Protocol
can	O
include	O
the	O
following	O
technologies	O
:	O
</s>
<s>
Web	O
application	B-Protocol
firewalls	I-Protocol
(	O
WAF	O
)	O
are	O
a	O
specialized	O
version	O
of	O
a	O
network-based	O
appliance	O
that	O
acts	O
as	O
a	O
reverse	B-General_Concept
proxy	I-General_Concept
,	O
inspecting	O
traffic	O
before	O
being	O
forwarded	O
to	O
an	O
associated	O
server	O
.	O
</s>
<s>
A	O
host-based	O
application	B-Protocol
firewall	I-Protocol
monitors	O
application	O
system	B-Operating_System
calls	I-Operating_System
or	O
other	O
general	O
system	O
communication	O
.	O
</s>
<s>
Many	O
host-based	O
application	B-Protocol
firewalls	I-Protocol
are	O
combined	O
or	O
used	O
in	O
conjunction	O
with	O
a	O
packet	O
filter	O
.	O
</s>
<s>
Due	O
to	O
technological	O
limitations	O
,	O
modern	O
solutions	O
such	O
as	O
sandboxing	O
are	O
being	O
used	O
as	O
a	O
replacement	O
of	O
host-based	O
application	B-Protocol
firewalls	I-Protocol
to	O
protect	O
system	O
processes	O
.	O
</s>
<s>
There	O
are	O
various	O
application	B-Protocol
firewalls	I-Protocol
available	O
,	O
including	O
both	O
free	O
and	O
open	O
source	O
software	O
and	O
commercial	O
products	O
.	O
</s>
<s>
The	O
TrustedBSD	O
MAC	O
framework	O
is	O
used	O
to	O
sandbox	O
services	O
and	O
provides	O
a	O
firewall	B-Application
layer	O
given	O
the	O
configuration	O
of	O
the	O
sharing	O
services	O
in	O
Mac	O
OS	O
X	O
Leopard	O
and	O
Snow	O
Leopard	O
.	O
</s>
<s>
ModSecurity	B-Language
-	O
also	O
works	O
under	O
Windows	O
,	O
Mac	O
OS	O
X	O
,	O
Solaris	B-Application
and	O
other	O
versions	O
of	O
Unix	B-Application
.	O
</s>
<s>
ModSecurity	B-Language
is	O
designed	O
to	O
work	O
with	O
the	O
web-servers	O
IIS	O
,	O
Apache2	O
and	O
NGINX	O
.	O
</s>
<s>
Next-Generation	O
Firewalls	B-Application
:	O
</s>
