<s>
In	O
computer	B-Architecture
networking	I-Architecture
,	O
a	O
host	B-Protocol
model	I-Protocol
is	O
an	O
option	O
of	O
designing	O
the	O
TCP/IP	B-Protocol
stack	I-Protocol
of	O
a	O
networking	O
operating	B-General_Concept
system	I-General_Concept
like	O
Microsoft	B-Application
Windows	I-Application
or	O
Linux	B-Application
.	O
</s>
<s>
When	O
a	O
unicast	B-Protocol
packet	B-Protocol
arrives	O
at	O
a	O
host	B-Device
,	O
IP	B-Protocol
must	O
determine	O
whether	O
the	O
packet	B-Protocol
is	O
locally	O
destined	O
(	O
its	O
destination	O
matches	O
an	O
address	O
that	O
is	O
assigned	O
to	O
an	O
interface	O
of	O
the	O
host	B-Device
)	O
.	O
</s>
<s>
If	O
the	O
IP	B-Protocol
stack	I-Protocol
is	O
implemented	O
with	O
a	O
weak	B-Protocol
host	I-Protocol
model	I-Protocol
,	O
it	O
accepts	O
any	O
locally	O
destined	O
packet	B-Protocol
regardless	O
of	O
the	O
network	O
interface	O
on	O
which	O
the	O
packet	B-Protocol
was	O
received	O
.	O
</s>
<s>
If	O
the	O
IP	B-Protocol
stack	I-Protocol
is	O
implemented	O
with	O
a	O
strong	B-Protocol
host	I-Protocol
model	I-Protocol
,	O
it	O
only	O
accepts	O
locally	O
destined	O
packets	B-Protocol
if	O
the	O
destination	O
IP	B-Protocol
address	O
in	O
the	O
packet	B-Protocol
matches	O
an	O
IP	B-Protocol
address	O
assigned	O
to	O
the	O
network	O
interface	O
on	O
which	O
the	O
packet	B-Protocol
was	O
received	O
.	O
</s>
<s>
The	O
weak	B-Protocol
host	I-Protocol
model	I-Protocol
provides	O
better	O
network	O
connectivity	O
(	O
for	O
example	O
,	O
it	O
can	O
be	O
easy	O
to	O
find	O
any	O
packet	B-Protocol
arriving	O
at	O
the	O
host	B-Device
using	O
ordinary	O
tools	O
)	O
,	O
but	O
it	O
also	O
makes	O
hosts	O
susceptible	O
to	O
multihome-based	O
network	O
attacks	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
some	O
configurations	O
when	O
a	O
system	O
running	O
a	O
weak	B-Protocol
host	I-Protocol
model	I-Protocol
is	O
connected	O
to	O
a	O
VPN	O
,	O
other	O
systems	O
on	O
the	O
same	O
subnet	O
can	O
compromise	O
the	O
security	O
of	O
the	O
VPN	O
connection	O
.	O
</s>
<s>
Systems	O
running	O
the	O
strong	B-Protocol
host	I-Protocol
model	I-Protocol
are	O
not	O
susceptible	O
to	O
this	O
type	O
of	O
attack	O
.	O
</s>
<s>
The	O
IPv4	B-Protocol
implementation	O
in	O
Microsoft	B-Application
Windows	I-Application
versions	O
prior	O
to	O
Windows	B-Application
Vista	I-Application
uses	O
the	O
weak	B-Protocol
host	I-Protocol
model	I-Protocol
.	O
</s>
<s>
The	O
Windows	B-Application
Vista	I-Application
and	O
Windows	B-Device
Server	I-Device
2008	I-Device
TCP/IP	B-Protocol
stack	I-Protocol
supports	O
the	O
strong	B-Protocol
host	I-Protocol
model	I-Protocol
for	O
both	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
and	O
is	O
configured	O
to	O
use	O
it	O
by	O
default	O
.	O
</s>
<s>
However	O
,	O
it	O
can	O
also	O
be	O
configured	O
to	O
use	O
a	O
weak	B-Protocol
host	I-Protocol
model	I-Protocol
.	O
</s>
<s>
The	O
IPv4	B-Protocol
implementation	O
in	O
Linux	B-Application
defaults	O
to	O
the	O
weak	B-Protocol
host	I-Protocol
model	I-Protocol
.	O
</s>
<s>
This	O
is	O
not	O
quite	O
the	O
same	O
as	O
the	O
strong	B-Protocol
host	I-Protocol
model	I-Protocol
,	O
but	O
defends	O
against	O
the	O
same	O
class	O
of	O
attacks	O
for	O
typical	O
multihomed	B-Protocol
hosts	O
.	O
</s>
<s>
Modern	O
BSDs	O
(	O
FreeBSD	B-Operating_System
,	O
NetBSD	B-Device
,	O
OpenBSD	B-Operating_System
,	O
and	O
DragonflyBSD	B-Application
)	O
all	O
default	O
to	O
the	O
weak	B-Protocol
host	I-Protocol
model	I-Protocol
.	O
</s>
<s>
OpenBSD	B-Operating_System
since	O
6.6-current	O
supports	O
strong	B-Protocol
host	I-Protocol
model	I-Protocol
by	O
default	O
"	O
if	O
and	O
only	O
IP	B-Protocol
forwarding	O
is	O
disabled	O
"	O
,	O
with	O
IP	B-Protocol
forwarding	O
enabled	O
(	O
and	O
for	O
older	O
versions	O
)	O
it	O
supports	O
reversed	O
path	O
source	O
validation	O
via	O
its	O
pf	B-Protocol
firewall	I-Protocol
,	O
using	O
the	O
urpf-failed	O
option	O
,	O
while	O
Free-	O
,	O
Net-	O
,	O
and	O
DragonflyBSD	B-Application
provide	O
a	O
global	O
sysctl	B-Application
options	O
.	O
</s>
