<s>
Firewalking	O
is	O
a	O
technique	O
developed	O
by	O
Mike	O
Schiffman	O
and	O
David	O
Goldsmith	O
that	O
utilizes	O
traceroute	B-Protocol
techniques	O
and	O
TTL	B-Protocol
values	O
to	O
analyze	O
IP	B-Protocol
packet	I-Protocol
responses	O
in	O
order	O
to	O
determine	O
gateway	O
ACL	O
(	O
Access	O
Control	O
List	O
)	O
filters	O
and	O
map	O
networks	O
.	O
</s>
<s>
It	O
is	O
an	O
active	O
reconnaissance	O
network	O
security	O
analysis	O
technique	O
that	O
attempts	O
to	O
determine	O
which	O
layer	O
4	O
protocols	O
a	O
specific	O
firewall	B-Application
will	O
allow	O
.	O
</s>
<s>
Firewalk	B-General_Concept
is	O
a	O
software	O
tool	O
that	O
performs	O
Firewalking	O
.	O
</s>
<s>
To	O
protect	O
a	O
firewall	B-Application
or	O
gateway	O
against	O
firewalking	O
one	O
can	O
block	O
ICMP	O
Time	O
Exceeded	O
messages	O
.	O
</s>
