<s>
A	O
virtual	B-Protocol
firewall	I-Protocol
(	O
VF	O
)	O
is	O
a	O
network	B-Application
firewall	I-Application
service	O
or	O
appliance	O
running	O
entirely	O
within	O
a	O
virtualized	B-General_Concept
environment	I-General_Concept
and	O
which	O
provides	O
the	O
usual	O
packet	B-Application
filtering	I-Application
and	O
monitoring	O
provided	O
via	O
a	O
physical	O
network	B-Application
firewall	I-Application
.	O
</s>
<s>
The	O
VF	O
can	O
be	O
realized	O
as	O
a	O
traditional	O
software	O
firewall	B-Application
on	O
a	O
guest	O
virtual	B-Architecture
machine	I-Architecture
already	O
running	O
,	O
a	O
purpose-built	O
virtual	O
security	O
appliance	O
designed	O
with	O
virtual	B-Protocol
network	I-Protocol
security	O
in	O
mind	O
,	O
a	O
virtual	B-Architecture
switch	I-Architecture
with	O
additional	O
security	O
capabilities	O
,	O
or	O
a	O
managed	O
kernel	O
process	O
running	O
within	O
the	O
host	O
hypervisor	B-Operating_System
.	O
</s>
<s>
As	O
such	O
it	O
can	O
be	O
protected	O
by	O
physical	O
firewalls	B-Application
and	O
fire	O
walls	O
alike	O
;	O
the	O
first	O
and	O
most	O
important	O
protection	O
for	O
a	O
physical	O
computer	O
network	O
always	O
was	O
and	O
remains	O
a	O
physical	O
,	O
locked	O
,	O
flame-resistant	O
door	O
.	O
</s>
<s>
Since	O
the	O
inception	O
of	O
the	O
Internet	O
this	O
was	O
the	O
case	O
,	O
and	O
structural	O
fire	O
walls	O
and	O
network	B-Application
firewalls	I-Application
were	O
for	O
a	O
long	O
time	O
both	O
necessary	O
and	O
sufficient	O
.	O
</s>
<s>
Since	O
about	O
1998	O
there	O
has	O
been	O
an	O
explosive	O
increase	O
in	O
the	O
use	O
of	O
virtual	B-Architecture
machines	I-Architecture
(	O
VM	O
)	O
in	O
addition	O
to	O
—	O
sometimes	O
instead	O
of	O
—	O
physical	O
machines	O
to	O
offer	O
many	O
kinds	O
of	O
computer	O
and	O
communications	O
services	O
on	O
local	B-General_Concept
area	I-General_Concept
networks	I-General_Concept
and	O
over	O
the	O
broader	O
Internet	O
.	O
</s>
<s>
The	O
advantages	O
of	O
virtual	B-Architecture
machines	I-Architecture
are	O
well	O
explored	O
elsewhere	O
.	O
</s>
<s>
Virtual	B-Architecture
machines	I-Architecture
can	O
operate	O
in	O
isolation	O
(	O
for	O
example	O
as	O
a	O
guest	B-General_Concept
operating	I-General_Concept
system	I-General_Concept
on	O
a	O
personal	O
computer	O
)	O
or	O
under	O
a	O
unified	O
virtualized	B-General_Concept
environment	I-General_Concept
overseen	O
by	O
a	O
supervisory	O
virtual	B-Operating_System
machine	I-Operating_System
monitor	I-Operating_System
or	O
"	O
hypervisor	B-Operating_System
"	O
process	O
.	O
</s>
<s>
In	O
the	O
case	O
where	O
many	O
virtual	B-Architecture
machines	I-Architecture
operate	O
under	O
the	O
same	O
virtualized	B-General_Concept
environment	I-General_Concept
they	O
might	O
be	O
connected	O
together	O
via	O
a	O
virtual	B-Protocol
network	I-Protocol
consisting	O
of	O
virtualized	B-Architecture
network	I-Architecture
switches	I-Architecture
between	O
machines	O
and	O
virtualized	B-General_Concept
network	I-General_Concept
interfaces	I-General_Concept
within	O
machines	O
.	O
</s>
<s>
The	O
resulting	O
virtual	B-Protocol
network	I-Protocol
could	O
then	O
implement	O
traditional	O
network	O
protocols	O
(	O
for	O
example	O
TCP	B-Protocol
)	O
or	O
virtual	B-Protocol
network	I-Protocol
provisioning	O
such	O
as	O
VLAN	B-General_Concept
or	O
VPN	B-Application
,	O
though	O
the	O
latter	O
while	O
useful	O
for	O
their	O
own	O
reasons	O
are	O
in	O
no	O
way	O
required	O
.	O
</s>
<s>
There	O
is	O
a	O
continued	O
perception	O
that	O
virtual	B-Architecture
machines	I-Architecture
are	O
inherently	O
secure	O
because	O
they	O
are	O
seen	O
as	O
"	O
sandboxed	O
"	O
within	O
the	O
host	O
operating	O
system	O
.	O
</s>
<s>
It	O
is	O
often	O
believed	O
that	O
the	O
host	O
,	O
in	O
like	O
manner	O
,	O
is	O
secured	O
against	O
exploitation	O
from	O
the	O
virtual	B-Architecture
machine	I-Architecture
itself	O
and	O
that	O
the	O
host	O
is	O
no	O
threat	O
to	O
the	O
virtual	B-Architecture
machine	I-Architecture
because	O
it	O
is	O
a	O
physical	O
asset	O
protected	O
by	O
traditional	O
physical	O
and	O
network	O
security	O
.	O
</s>
<s>
Even	O
when	O
this	O
is	O
not	O
explicitly	O
assumed	O
,	O
early	O
testing	O
of	O
virtual	O
infrastructures	O
often	O
proceeds	O
in	O
isolated	O
lab	O
environments	O
where	O
security	O
is	O
not	O
as	O
a	O
rule	O
an	O
immediate	O
concern	O
,	O
or	O
security	O
may	O
only	O
come	O
to	O
the	O
fore	O
when	O
the	O
same	O
solution	O
is	O
moving	O
into	O
production	O
or	O
onto	O
a	O
computer	B-Architecture
cloud	I-Architecture
,	O
where	O
suddenly	O
virtual	B-Architecture
machines	I-Architecture
of	O
different	O
trust	O
levels	O
may	O
wind	O
up	O
on	O
the	O
same	O
virtual	B-Protocol
network	I-Protocol
running	O
across	O
any	O
number	O
of	O
physical	O
hosts	O
.	O
</s>
<s>
Because	O
they	O
are	O
true	O
networks	O
,	O
virtual	B-Protocol
networks	I-Protocol
may	O
end	O
up	O
suffering	O
the	O
same	O
kinds	O
of	O
vulnerabilities	O
long	O
associated	O
with	O
a	O
physical	O
network	O
,	O
some	O
of	O
which	O
being	O
:	O
</s>
<s>
Users	O
on	O
machines	O
within	O
the	O
virtual	B-Protocol
network	I-Protocol
have	O
access	O
to	O
all	O
other	O
machines	O
on	O
the	O
same	O
virtual	B-Protocol
network	I-Protocol
.	O
</s>
<s>
Compromising	O
or	O
misappropriating	O
one	O
virtual	B-Architecture
machine	I-Architecture
on	O
a	O
virtual	B-Protocol
network	I-Protocol
is	O
sufficient	O
to	O
provide	O
a	O
platform	O
for	O
additional	O
attacks	O
against	O
other	O
machines	O
on	O
the	O
same	O
network	O
segment	O
.	O
</s>
<s>
If	O
a	O
virtual	B-Protocol
network	I-Protocol
is	O
internetworked	O
to	O
the	O
physical	O
network	O
or	O
broader	O
Internet	O
then	O
machines	O
on	O
the	O
virtual	B-Protocol
network	I-Protocol
might	O
have	O
access	O
to	O
external	O
resources	O
(	O
and	O
external	O
exploits	O
)	O
that	O
could	O
leave	O
them	O
open	O
to	O
exploitation	O
.	O
</s>
<s>
The	O
problems	O
created	O
by	O
the	O
near	O
invisibility	O
of	O
between-virtual	O
machine	O
(	O
VM-to-VM	O
)	O
traffic	O
on	O
a	O
virtual	B-Protocol
network	I-Protocol
are	O
exactly	O
like	O
those	O
found	O
in	O
physical	O
networks	O
,	O
complicated	O
by	O
the	O
fact	O
that	O
the	O
packets	O
may	O
be	O
moving	O
entirely	O
inside	O
the	O
hardware	O
of	O
a	O
single	O
physical	O
host	O
:	O
</s>
<s>
Because	O
the	O
virtual	B-Protocol
network	I-Protocol
traffic	O
may	O
never	O
leave	O
the	O
physical	O
host	O
hardware	O
,	O
security	O
administrators	O
cannot	O
observe	O
VM-to-VM	O
traffic	O
,	O
cannot	O
intercept	O
it	O
,	O
and	O
so	O
cannot	O
know	O
what	O
that	O
traffic	O
is	O
for	O
.	O
</s>
<s>
Logging	O
of	O
VM-to-VM	O
network	O
activity	O
within	O
a	O
single	O
host	O
and	O
verification	O
of	O
virtual	B-Architecture
machine	I-Architecture
access	O
for	O
regulatory	O
compliance	O
purposes	O
becomes	O
difficult	O
.	O
</s>
<s>
Inappropriate	O
uses	O
of	O
virtual	B-Protocol
network	I-Protocol
resources	O
and	O
bandwidth	O
consumption	O
VM-to-VM	O
are	O
difficult	O
to	O
discover	O
or	O
rectify	O
.	O
</s>
<s>
Unusual	O
or	O
inappropriate	O
services	O
running	O
on	O
or	O
within	O
the	O
virtual	B-Protocol
network	I-Protocol
could	O
go	O
undetected	O
.	O
</s>
<s>
There	O
are	O
security	O
issues	O
known	O
only	O
in	O
virtualized	O
environments	O
that	O
wreak	O
havoc	O
with	O
physical	O
security	O
measures	O
and	O
practices	O
,	O
and	O
some	O
of	O
these	O
are	O
touted	O
as	O
actual	O
advantages	O
of	O
virtual	B-Architecture
machine	I-Architecture
technology	O
over	O
physical	O
machines	O
:	O
</s>
<s>
VMs	O
and/or	O
virtual	O
storage	O
volumes	O
can	O
be	O
easily	O
cloned	O
and	O
the	O
clone	O
made	O
to	O
run	O
on	O
any	O
part	O
of	O
the	O
virtualized	B-General_Concept
environment	I-General_Concept
,	O
including	O
a	O
DMZ	B-General_Concept
.	O
</s>
<s>
Since	O
virtual	B-Architecture
machines	I-Architecture
can	O
be	O
created	O
in	O
a	O
few	O
minutes	O
by	O
any	O
authorized	O
user	O
and	O
set	O
running	O
without	O
a	O
paper	O
trail	O
,	O
they	O
can	O
in	O
these	O
cases	O
bypass	O
established	O
"	O
first	O
boot	O
"	O
IT	O
security	O
practices	O
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
network	O
traffic	O
visibility	O
issues	O
and	O
uncoordinated	O
VM	O
sprawl	O
,	O
a	O
rogue	O
VM	O
using	O
just	O
the	O
virtual	B-Protocol
network	I-Protocol
,	O
switches	O
and	O
interfaces	O
(	O
all	O
of	O
which	O
run	O
in	O
a	O
process	O
on	O
the	O
host	O
physical	O
hardware	O
)	O
can	O
potentially	O
break	O
the	O
network	O
as	O
could	O
any	O
physical	O
machine	O
on	O
a	O
physical	O
network	O
—	O
and	O
in	O
the	O
usual	O
ways	O
—	O
though	O
now	O
by	O
consuming	O
host	O
CPU	O
cycles	O
it	O
can	O
additionally	O
bring	O
down	O
the	O
entire	O
virtualized	B-General_Concept
environment	I-General_Concept
and	O
all	O
the	O
other	O
VMs	O
with	O
it	O
simply	O
by	O
overpowering	O
the	O
host	O
physical	O
resources	O
the	O
rest	O
of	O
the	O
virtualized	B-General_Concept
environment	I-General_Concept
depend	O
upon	O
.	O
</s>
<s>
One	O
method	O
to	O
secure	O
,	O
log	O
and	O
monitor	O
VM-to-VM	O
traffic	O
involved	O
routing	O
the	O
virtualized	O
network	O
traffic	O
out	O
of	O
the	O
virtual	B-Protocol
network	I-Protocol
and	O
onto	O
the	O
physical	O
network	O
via	O
VLANs	B-General_Concept
,	O
and	O
hence	O
into	O
a	O
physical	O
firewall	B-Application
already	O
present	O
to	O
provide	O
security	O
and	O
compliance	O
services	O
for	O
the	O
physical	O
network	O
.	O
</s>
<s>
The	O
VLAN	B-General_Concept
traffic	O
could	O
be	O
monitored	O
and	O
filtered	O
by	O
the	O
physical	O
firewall	B-Application
and	O
then	O
passed	O
back	O
into	O
the	O
virtual	B-Protocol
network	I-Protocol
(	O
if	O
deemed	O
legitimate	O
for	O
that	O
purpose	O
)	O
and	O
on	O
to	O
the	O
target	O
virtual	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
Not	O
surprisingly	O
,	O
LAN	B-General_Concept
managers	O
,	O
security	O
experts	O
and	O
network	O
security	O
vendors	O
began	O
to	O
wonder	O
if	O
it	O
might	O
be	O
more	O
efficient	O
to	O
keep	O
the	O
traffic	O
entirely	O
within	O
the	O
virtualized	B-General_Concept
environment	I-General_Concept
and	O
secure	O
it	O
from	O
there	O
.	O
</s>
<s>
A	O
virtual	B-Protocol
firewall	I-Protocol
then	O
is	O
a	O
firewall	B-Application
service	O
or	O
appliance	O
running	O
entirely	O
within	O
a	O
virtualised	O
environment	O
—	O
even	O
as	O
another	O
virtual	B-Architecture
machine	I-Architecture
,	O
but	O
just	O
as	O
readily	O
within	O
the	O
hypervisor	B-Operating_System
itself	O
—	O
providing	O
the	O
usual	O
packet	B-Application
filtering	I-Application
and	O
monitoring	O
that	O
a	O
physical	O
firewall	B-Application
provides	O
.	O
</s>
<s>
The	O
VF	O
can	O
be	O
installed	O
as	O
a	O
traditional	O
software	O
firewall	B-Application
on	O
a	O
guest	O
VM	O
already	O
running	O
within	O
the	O
virtualized	B-General_Concept
environment	I-General_Concept
;	O
or	O
it	O
can	O
be	O
a	O
purpose-built	O
virtual	O
security	O
appliance	O
designed	O
with	O
virtual	B-Protocol
network	I-Protocol
security	O
in	O
mind	O
;	O
or	O
it	O
can	O
be	O
a	O
virtual	B-Architecture
switch	I-Architecture
with	O
additional	O
security	O
capabilities	O
;	O
or	O
it	O
can	O
be	O
a	O
managed	O
kernel	O
process	O
running	O
within	O
the	O
host	O
hypervisor	B-Operating_System
that	O
sits	O
atop	O
all	O
VM	O
activity	O
.	O
</s>
<s>
The	O
current	O
direction	O
in	O
virtual	B-Protocol
firewall	I-Protocol
technology	O
is	O
a	O
combination	O
of	O
security-capable	O
virtual	B-Architecture
switches	I-Architecture
,	O
and	O
virtual	O
security	O
appliances	O
.	O
</s>
<s>
Some	O
virtual	B-Protocol
firewalls	I-Protocol
integrate	O
additional	O
networking	O
functions	O
such	O
as	O
site-to-site	O
and	O
remote	O
access	O
VPN	B-Application
,	O
QoS	O
,	O
URL	O
filtering	O
and	O
more	O
.	O
</s>
<s>
Virtual	B-Protocol
firewalls	I-Protocol
can	O
operate	O
in	O
different	O
modes	O
to	O
provide	O
security	O
services	O
,	O
depending	O
on	O
the	O
point	O
of	O
deployment	O
.	O
</s>
<s>
Typically	O
these	O
are	O
either	O
bridge-mode	O
or	O
hypervisor-mode	O
(	O
hypervisor-based	O
,	O
hypervisor-resident	O
)	O
.	O
</s>
<s>
Both	O
may	O
come	O
shrink	O
wrapped	O
as	O
a	O
virtual	O
security	O
appliance	O
and	O
may	O
install	O
a	O
virtual	B-Architecture
machine	I-Architecture
for	O
management	O
purposes	O
.	O
</s>
<s>
A	O
virtual	B-Protocol
firewall	I-Protocol
operating	O
in	O
bridge-mode	O
acts	O
like	O
its	O
physical-world	O
firewall	B-Application
analog	O
;	O
it	O
sits	O
in	O
a	O
strategic	O
part	O
of	O
the	O
network	O
infrastructure	O
—	O
usually	O
at	O
an	O
inter-network	O
virtual	B-Architecture
switch	I-Architecture
or	O
bridge	O
—	O
and	O
intercepts	O
network	O
traffic	O
destined	O
for	O
other	O
network	O
segments	O
and	O
needing	O
to	O
travel	O
over	O
the	O
bridge	O
.	O
</s>
<s>
By	O
examining	O
the	O
source	O
origin	O
,	O
the	O
destination	O
,	O
the	O
type	O
of	O
packet	O
it	O
is	O
and	O
even	B-Application
the	I-Application
payload	I-Application
the	O
VF	O
can	O
decide	O
if	O
the	O
packet	O
is	O
to	O
be	O
allowed	O
passage	O
,	O
dropped	O
,	O
rejected	O
,	O
or	O
forwarded	O
or	O
mirrored	O
to	O
some	O
other	O
device	O
.	O
</s>
<s>
Initial	O
entrants	O
into	O
the	O
virtual	B-Protocol
firewall	I-Protocol
field	O
were	O
largely	O
bridge-mode	O
,	O
and	O
many	O
offers	O
retain	O
this	O
feature	O
.	O
</s>
<s>
By	O
contrast	O
,	O
a	O
virtual	B-Protocol
firewall	I-Protocol
operating	O
in	O
hypervisor-mode	O
is	O
not	O
actually	O
part	O
of	O
the	O
virtual	B-Protocol
network	I-Protocol
at	O
all	O
,	O
and	O
as	O
such	O
has	O
no	O
physical-world	O
device	O
analog	O
.	O
</s>
<s>
A	O
hypervisor-mode	O
virtual	B-Protocol
firewall	I-Protocol
resides	O
in	O
the	O
virtual	B-Operating_System
machine	I-Operating_System
monitor	I-Operating_System
or	O
hypervisor	B-Operating_System
where	O
it	O
is	O
well	O
positioned	O
to	O
capture	O
VM	O
activity	O
including	O
packet	O
injections	O
.	O
</s>
<s>
The	O
entire	O
monitored	O
VM	O
and	O
all	O
its	O
virtual	B-General_Concept
hardware	I-General_Concept
,	O
software	O
,	O
services	O
,	O
memory	O
and	O
storage	O
can	O
be	O
examined	O
,	O
as	O
can	O
changes	O
in	O
these	O
.	O
</s>
<s>
Further	O
,	O
since	O
a	O
hypervisor-based	O
virtual	B-Protocol
firewall	I-Protocol
is	O
not	O
part	O
of	O
the	O
network	O
proper	O
and	O
is	O
not	O
a	O
virtual	B-Architecture
machine	I-Architecture
its	O
functionality	O
cannot	O
be	O
monitored	O
in	O
turn	O
or	O
altered	O
by	O
users	O
and	O
software	O
limited	O
to	O
running	O
under	O
a	O
VM	O
or	O
having	O
access	O
only	O
to	O
the	O
virtualized	O
network	O
.	O
</s>
<s>
Bridge-mode	O
virtual	B-Protocol
firewalls	I-Protocol
can	O
be	O
installed	O
just	O
as	O
any	O
other	O
virtual	B-Architecture
machine	I-Architecture
in	O
the	O
virtualized	O
infrastructure	O
.	O
</s>
<s>
Since	O
it	O
is	O
then	O
a	O
virtual	B-Architecture
machine	I-Architecture
itself	O
,	O
the	O
relationship	O
of	O
the	O
VF	O
to	O
all	O
the	O
other	O
VM	O
may	O
become	O
complicated	O
over	O
time	O
due	O
to	O
VMs	O
disappearing	O
and	O
appearing	O
in	O
random	O
ways	O
,	O
migrating	O
between	O
different	O
physical	O
hosts	O
,	O
or	O
other	O
uncoordinated	O
changes	O
allowed	O
by	O
the	O
virtualized	O
infrastructure	O
.	O
</s>
<s>
Hypervisor-mode	O
virtual	B-Protocol
firewalls	I-Protocol
require	O
a	O
modification	O
to	O
the	O
physical	O
host	O
hypervisor	B-Operating_System
kernel	O
in	O
order	O
to	O
install	O
process	O
hooks	O
or	O
modules	O
allowing	O
the	O
virtual	B-Protocol
firewall	I-Protocol
system	O
access	O
to	O
VM	O
information	O
and	O
direct	O
access	O
to	O
the	O
virtual	B-Protocol
network	I-Protocol
switches	O
and	O
virtualized	B-General_Concept
network	I-General_Concept
interfaces	I-General_Concept
moving	O
packet	O
traffic	O
between	O
VMs	O
or	O
between	O
VMs	O
and	O
the	O
network	O
gateway	O
.	O
</s>
<s>
The	O
hypervisor-resident	O
virtual	B-Protocol
firewall	I-Protocol
can	O
use	O
the	O
same	O
hooks	O
to	O
then	O
perform	O
all	O
the	O
customary	O
firewall	B-Application
functions	O
like	O
packet	O
inspection	O
,	O
dropping	O
,	O
and	O
forwarding	O
but	O
without	O
actually	O
touching	O
the	O
virtual	B-Protocol
network	I-Protocol
at	O
any	O
point	O
.	O
</s>
<s>
Hypervisor-mode	O
virtual	B-Protocol
firewalls	I-Protocol
can	O
be	O
very	O
much	O
faster	O
than	O
the	O
same	O
technology	O
running	O
in	O
bridge-mode	O
because	O
they	O
are	O
not	O
doing	O
packet	O
inspection	O
in	O
a	O
virtual	B-Architecture
machine	I-Architecture
,	O
but	O
rather	O
from	O
within	O
the	O
kernel	O
at	O
native	O
hardware	O
speeds	O
.	O
</s>
