Java Reference
In-Depth Information
Example 8-5. Find out which of the first 1024 ports seem to be hosting TCP servers on a
specified host
import
java.net.*
;
import
java.io.*
;
public
class
LowPortScanner
{
public
static
void
main
(
String
[]
args
)
{
String
host
=
args
.
length
>
0
?
args
[
0
]
:
"localhost"
;
for
(
int
i
=
1
;
i
<
1024
;
i
++)
{
try
{
Socket
s
=
new
Socket
(
host
,
i
);
System
.
out
.
println
(
"There is a server on port "
+
i
+
" of "
+
host
);
s
.
close
();
}
catch
(
UnknownHostException
ex
)
{
System
.
err
.
println
(
ex
);
break
;
}
catch
(
IOException
ex
)
{
// must not be a server on this port
}
}
}
}
Here's the output this program produces on my local host (your results will vary, de‐
pending on which ports are occupied):
$
java LowPortScanner
There is a server on port 21 of localhost
There is a server on port 22 of localhost
There is a server on port 23 of localhost
There is a server on port 25 of localhost
There is a server on port 37 of localhost
There is a server on port 111 of localhost
There is a server on port 139 of localhost
There is a server on port 210 of localhost
There is a server on port 515 of localhost
There is a server on port 873 of localhost
If you're curious about what servers are running on these ports, try experimenting with
Telnet. On a Unix system, you may be able to find out which services reside on which
ports by looking in the file
/etc/services
. If
LowPortScanner
finds any ports that are
running servers but are not listed in
/etc/services
, then that's interesting.
Although this program looks simple, it's not without its uses. The first step to securing
a system is understanding it. This program helps you understand what your system is
doing so you can find (and close) possible entrance points for attackers. You may also
