<s>
VLAN	B-Protocol
hopping	I-Protocol
is	O
a	O
computer	O
security	O
exploit	O
,	O
a	O
method	O
of	O
attacking	O
networked	O
resources	O
on	O
a	O
virtual	B-General_Concept
LAN	I-General_Concept
(	O
VLAN	B-General_Concept
)	O
.	O
</s>
<s>
The	O
basic	O
concept	O
behind	O
all	O
VLAN	B-Protocol
hopping	I-Protocol
attacks	O
is	O
for	O
an	O
attacking	O
host	O
on	O
a	O
VLAN	B-General_Concept
to	O
gain	O
access	O
to	O
traffic	O
on	O
other	O
VLANs	B-General_Concept
that	O
would	O
normally	O
not	O
be	O
accessible	O
.	O
</s>
<s>
There	O
are	O
two	O
primary	O
methods	O
of	O
VLAN	B-Protocol
hopping	I-Protocol
:	O
switch	O
spoofing	O
and	O
double	O
tagging	O
.	O
</s>
<s>
Multiple	O
VLAN	B-General_Concept
Registration	O
Protocol	O
,	O
IEEE	O
802.1Q	O
,	O
Dynamic	B-Protocol
Trunking	I-Protocol
Protocol	I-Protocol
)	O
used	O
in	O
maintaining	O
a	O
VLAN	B-General_Concept
.	O
</s>
<s>
Traffic	O
for	O
multiple	O
VLANs	B-General_Concept
is	O
then	O
accessible	O
to	O
the	O
attacking	O
host	O
.	O
</s>
<s>
To	O
prevent	O
this	O
attack	O
on	O
Cisco	B-Protocol
IOS	I-Protocol
,	O
use	O
one	O
of	O
the	O
following	O
methods	O
:	O
</s>
<s>
In	O
a	O
double	O
tagging	O
attack	O
,	O
an	O
attacker	O
connected	O
to	O
an	O
802.1Q-enabled	O
port	O
prepends	O
two	O
VLAN	B-General_Concept
tags	O
to	O
a	O
frame	O
that	O
it	O
transmits	O
.	O
</s>
<s>
The	O
frame	O
(	O
externally	O
tagged	O
with	O
VLAN	B-General_Concept
ID	O
that	O
the	O
attacker	O
's	O
port	O
is	O
really	O
a	O
member	O
of	O
)	O
is	O
forwarded	O
without	O
the	O
first	O
tag	O
because	O
it	O
is	O
the	O
native	O
VLAN	B-General_Concept
of	O
a	O
trunk	O
interface	O
.	O
</s>
<s>
This	O
second	O
VLAN	B-General_Concept
tag	O
indicates	O
that	O
the	O
frame	O
is	O
destined	O
for	O
a	O
target	O
host	O
on	O
a	O
second	O
switch	O
.	O
</s>
<s>
The	O
frame	O
is	O
then	O
sent	O
to	O
the	O
target	O
host	O
as	O
though	O
it	O
originated	O
on	O
the	O
target	O
VLAN	B-General_Concept
,	O
effectively	O
bypassing	O
the	O
network	O
mechanisms	O
that	O
logically	O
isolate	O
VLANs	B-General_Concept
from	O
one	O
another	O
.	O
</s>
<s>
Double	O
Tagging	O
can	O
only	O
be	O
exploited	O
on	O
switch	O
ports	O
configured	O
to	O
use	O
native	O
VLANs	B-General_Concept
.	O
</s>
<s>
Trunk	O
ports	O
configured	O
with	O
a	O
native	O
VLAN	B-General_Concept
do	O
n't	O
apply	O
a	O
VLAN	B-General_Concept
tag	O
when	O
sending	O
these	O
frames	O
.	O
</s>
<s>
This	O
allows	O
an	O
attacker	O
's	O
fake	O
VLAN	B-General_Concept
tag	O
to	O
be	O
read	O
by	O
the	O
next	O
switch	O
.	O
</s>
<s>
IOS	B-Protocol
example	O
)	O
:	O
</s>
<s>
Simply	O
do	O
not	O
put	O
any	O
hosts	O
on	O
VLAN	B-General_Concept
1	O
(	O
The	O
default	O
VLAN	B-General_Concept
)	O
.	O
</s>
<s>
Change	O
the	O
native	O
VLAN	B-General_Concept
on	O
all	O
trunk	O
ports	O
to	O
an	O
unused	O
VLAN	B-General_Concept
ID	O
.	O
</s>
<s>
Explicit	O
tagging	O
of	O
the	O
native	O
VLAN	B-General_Concept
on	O
all	O
trunk	O
ports	O
.	O
</s>
<s>
As	O
an	O
example	O
of	O
a	O
double	O
tagging	O
attack	O
,	O
consider	O
a	O
secure	O
web	O
server	O
on	O
a	O
VLAN	B-General_Concept
called	O
VLAN2	O
.	O
</s>
<s>
An	O
attacking	O
host	O
on	O
a	O
separate	O
VLAN	B-General_Concept
,	O
called	O
VLAN1(Native )	O
,	O
creates	O
a	O
specially	O
formed	O
packet	O
to	O
attack	O
the	O
web	O
server	O
.	O
</s>
