Many users have misconfigured firewalls such that they are all but useless in defending against attacks. You can determine the state of your firewall by using the same tool hackers use to find its weaknesses, Nmap. To install Nmap, use the Package Management Tool to install the System Tools package group, and the extra package nmap.
Unfortunately, Nmap doesn't have a place on the GNOME or KDE menus. Instead, you use Nmap by issuing shell commands from a terminal window.
Nmap has enough features to be the topic of a book. You can learn more about Nmap from its author's web site, http://www.insecure.org/. A vanilla use of Nmap involves configuring it to send TCP or UDP packets to every important port of a specified system. The target system's responses reveal whether it has services listening on scanned ports.
To scan your system, open a terminal window, become the root user and issue a command having the form:
# nmap -sT -sU -p 1-1023 xxx.xxx.xxx.xxx where xxx.xxx.xxx.xxx is the IP address of your system. After a few seconds, Nmap reports the ports on which services are listening:
Starting nmap V. 3.00 ( www.insecure.org/nmap/ )
Interesting ports on (192.168.83.131):
(The 2042 ports scanned but not shown below are in state: closed)
Port State Service
22/tcp open ssh
68/udp open dhcpclient
80/tcp open http
12 3/udp open ntp
Nmap run completed -- 1 IP address (1 host up) scanned in 5 seconds
Some ports listed in Nmap's report may not be remotely accessible. Therefore, to accurately determine the state of a system, you must scan it remotely. If a remote scan of your system discloses unexpected open ports, you should check your firewall and service configurations to determine whether anything is amiss.
Open ports above port 1023 that come and go with successive scans are not generally cause for concern. Usually, these are associated with established connections and are not really open. However, they can also be an early indication of the presence of a Trojan horse installed by an attacker. It's good practice to scan your hosts after you configure them so that you can later distinguish normal from abnormal behavior.
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