Network scanners, such as Nmap (http://www.insecure.org/nmap/) or Nessus (http://
http://www.nessus.org), can scan for open ports on the local computer or on other computers. The
more sophisticated scanners, including Nessus, check for known vulnerabilities, so they can
tell you whether a server may be compromised should you decide to leave it running.
Nmap is capable of performing a basic check for open ports. Pass the -sT parameter and
the name of the target system to it, as shown here:
ami@amios:~$ nmap -sT google.com
Starting Nmap 6.00 ( http://nmap.org ) at 2014-03-01 12:40 UTC
Nmap scan report for google.com (22.214.171.124)
Host is up (0.0045s latency).
Other addresses for google.com (not scanned): 126.96.36.199 188.8.131.52 184.108.40.206 220.127.116.11 18.104.22.168
rDNS record for 22.214.171.124: ea-in-f102.1e100.net
Not shown: 998 filtered ports
PORT STATE SERVICE
80/tcp open http
443/tcp open https
Nmap done: 1 IP address (1 host up) scanned in 4.96 seconds
This output shows you 2 open port 80 (http) and 443 (https). You can use nmap to scan your server itself and then see if there are services running which does not support to be there.
When you use a network scanner, you should consider the fact that the ports you see
from your test system may not be the same as those that might be visible to an attacker.
This issue is particularly important if you’re testing a system that resides behind a fi rewall
from another system that’s behind the same firewall.
On the other hand, a
cracker on your local network would most likely have access similar to your own, so you
shouldn’t be complacent because you use a fi rewall. Nonetheless, fi rewalls can be important
tools for hiding servers without shutting them down.
You can use a stand-alone Linux boot CD-ROM to perform security
checks on a network. Tools intended for this purpose, such as BackTrack
(https://www.kali.org), provide easy access to Nmap and
other network security tools, enabling quick checks of network security
even if no computer on that network regularly runs Linux.
The latest current version is Kali Linux
You can also have a look on the following article if you are interested in VPN clients for Linux