<s>
A	O
reverse	B-Architecture
connection	I-Architecture
is	O
usually	O
used	O
to	O
bypass	O
firewall	B-Application
restrictions	O
on	O
open	B-Protocol
ports	I-Protocol
.	O
</s>
<s>
A	O
firewall	B-Application
usually	O
blocks	O
incoming	O
connections	O
on	O
open	B-Protocol
ports	I-Protocol
,	O
</s>
<s>
In	O
a	O
normal	O
forward	O
connection	O
,	O
a	O
client	B-Protocol
connects	O
to	O
a	O
server	B-Application
through	O
the	O
server	B-Application
's	O
open	B-Protocol
port	I-Protocol
,	O
</s>
<s>
but	O
in	O
the	O
case	O
of	O
a	O
reverse	B-Architecture
connection	I-Architecture
,	O
the	O
client	B-Protocol
opens	O
the	O
port	O
that	O
the	O
server	B-Application
connects	O
to	O
.	O
</s>
<s>
The	O
most	O
common	O
way	O
a	O
reverse	B-Architecture
connection	I-Architecture
is	O
used	O
is	O
to	O
bypass	O
firewall	B-Application
and	O
router	B-Protocol
security	O
restrictions	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
backdoor	O
running	O
on	O
a	O
computer	O
behind	O
a	O
firewall	B-Application
that	O
blocks	O
incoming	O
connections	O
can	O
easily	O
open	O
an	O
outbound	O
connection	O
to	O
a	O
remote	O
host	O
on	O
the	O
Internet	O
.	O
</s>
<s>
Remote	B-Protocol
administration	I-Protocol
tools	I-Protocol
(	O
RAT	O
)	O
that	O
use	O
a	O
reverse	B-Architecture
connection	I-Architecture
usually	O
send	O
SYN	O
packets	O
to	O
the	O
client	B-Protocol
's	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
The	O
client	B-Protocol
listens	O
for	O
these	O
SYN	O
packets	O
and	O
accepts	O
the	O
desired	O
connections	O
.	O
</s>
<s>
If	O
a	O
computer	O
is	O
sending	O
SYN	O
packets	O
or	O
is	O
connected	O
to	O
the	O
client	B-Protocol
's	O
computer	O
,	O
the	O
connections	O
can	O
be	O
discovered	O
by	O
using	O
the	O
netstat	O
command	O
or	O
a	O
common	O
port	O
listener	O
like	O
“	O
Active	O
Ports	O
”	O
.	O
</s>
<s>
There	O
are	O
honest	O
uses	O
for	O
using	O
reverse	B-Architecture
connections	I-Architecture
,	O
for	O
example	O
to	O
allow	O
hosts	O
behind	O
a	O
NAT	O
firewall	B-Application
to	O
be	O
administered	O
remotely	O
.	O
</s>
<s>
These	O
hosts	O
do	O
not	O
normally	O
have	O
public	O
IP	B-Protocol
addresses	I-Protocol
,	O
and	O
so	O
must	O
either	O
have	O
ports	O
forwarded	O
at	O
the	O
firewall	B-Application
,	O
or	O
open	O
reverse	B-Architecture
connections	I-Architecture
to	O
a	O
central	O
administration	O
server	B-Application
.	O
</s>
