<s>
An	O
ICMP	B-Application
tunnel	I-Application
establishes	O
a	O
covert	O
connection	O
between	O
two	O
remote	O
computers	O
(	O
a	O
client	O
and	O
proxy	O
)	O
,	O
using	O
ICMP	B-Protocol
echo	O
requests	O
and	O
reply	O
packets	O
.	O
</s>
<s>
An	O
example	O
of	O
this	O
technique	O
is	O
tunneling	B-Protocol
complete	O
TCP	B-Protocol
traffic	O
over	O
ping	O
requests	O
and	O
replies	O
.	O
</s>
<s>
ICMP	B-Protocol
tunneling	B-Protocol
works	O
by	O
injecting	O
arbitrary	O
data	O
into	O
an	O
echo	O
packet	O
sent	O
to	O
a	O
remote	O
computer	O
.	O
</s>
<s>
The	O
remote	O
computer	O
replies	O
in	O
the	O
same	O
manner	O
,	O
injecting	O
an	O
answer	O
into	O
another	O
ICMP	B-Protocol
packet	O
and	O
sending	O
it	O
back	O
.	O
</s>
<s>
The	O
client	O
performs	O
all	O
communication	O
using	O
ICMP	B-Protocol
echo	O
request	O
packets	O
,	O
while	O
the	O
proxy	O
uses	O
echo	O
reply	O
packets	O
.	O
</s>
<s>
In	O
theory	O
,	O
it	O
is	O
possible	O
to	O
have	O
the	O
proxy	O
use	O
echo	O
request	O
packets	O
(	O
which	O
makes	O
implementation	O
much	O
easier	O
)	O
,	O
but	O
these	O
packets	O
are	O
not	O
necessarily	O
forwarded	O
to	O
the	O
client	O
,	O
as	O
the	O
client	O
could	O
be	O
behind	O
a	O
translated	O
address	O
(	O
NAT	B-Protocol
)	O
.	O
</s>
<s>
ICMP	B-Protocol
tunneling	B-Protocol
is	O
possible	O
because	O
,	O
which	O
defines	O
the	O
structure	O
of	O
ICMP	B-Protocol
packets	O
,	O
allows	O
for	O
an	O
arbitrary	O
data	O
length	O
for	O
any	O
type	O
0	O
(	O
echo	O
reply	O
)	O
or	O
8	O
(	O
echo	O
message	O
)	O
ICMP	B-Protocol
packets	O
.	O
</s>
<s>
ICMP	B-Protocol
tunneling	B-Protocol
can	O
be	O
used	O
to	O
bypass	O
firewalls	O
rules	O
through	O
obfuscation	O
of	O
the	O
actual	O
traffic	O
.	O
</s>
<s>
Depending	O
on	O
the	O
implementation	O
of	O
the	O
ICMP	B-Protocol
tunneling	B-Protocol
software	I-Protocol
,	O
this	O
type	O
of	O
connection	O
can	O
also	O
be	O
categorized	O
as	O
an	O
encrypted	O
communication	O
channel	O
between	O
two	O
computers	O
.	O
</s>
<s>
Without	O
proper	O
deep	B-Application
packet	I-Application
inspection	I-Application
or	O
log	O
review	O
,	O
network	O
administrators	O
will	O
not	O
be	O
able	O
to	O
detect	O
this	O
type	O
of	O
traffic	O
through	O
their	O
network	O
.	O
</s>
<s>
One	O
way	O
to	O
prevent	O
this	O
type	O
of	O
tunneling	B-Protocol
is	O
to	O
block	O
ICMP	B-Protocol
traffic	O
,	O
at	O
the	O
cost	O
of	O
losing	O
some	O
network	O
functionality	O
that	O
people	O
usually	O
take	O
for	O
granted	O
(	O
e.g.	O
</s>
<s>
Another	O
method	O
for	O
mitigating	O
this	O
type	O
of	O
attack	O
is	O
to	O
only	O
allow	O
fixed	O
sized	O
ICMP	B-Protocol
packets	O
through	O
firewalls	O
,	O
which	O
can	O
impede	O
or	O
eliminate	O
this	O
type	O
of	O
behavior	O
.	O
</s>
<s>
ICMP-tunnels	O
are	O
sometimes	O
used	O
to	O
circumvent	O
firewalls	O
that	O
block	O
traffic	O
between	O
the	O
LAN	O
and	O
the	O
outside	O
world	O
.	O
</s>
<s>
If	O
the	O
network	O
operator	O
made	O
the	O
erroneous	O
assumption	O
that	O
it	O
is	O
enough	O
to	O
only	O
block	O
normal	O
transport	O
protocols	O
like	O
TCP	B-Protocol
and	O
UDP	O
,	O
but	O
not	O
core	O
protocols	O
such	O
as	O
ICMP	B-Protocol
,	O
then	O
it	O
is	O
sometimes	O
possible	O
to	O
use	O
an	O
ICMP-tunnel	O
to	O
access	O
the	O
internet	O
despite	O
not	O
having	O
been	O
authorized	O
for	O
network	O
access	O
.	O
</s>
<s>
Encryption	O
and	O
per-user	O
rules	O
that	O
disallow	O
users	O
exchanging	O
ICMP	B-Protocol
packets	O
(	O
and	O
all	O
other	O
types	O
of	O
packets	O
,	O
maybe	O
by	O
using	O
IEEE	O
802.1X	O
)	O
with	O
external	O
peers	O
before	O
authorization	O
solves	O
this	O
problem	O
.	O
</s>
