Hackers use "port scanning
" to search for vulnerable computers with open ports using IP addresses or a group of random IP address ranges so they can break in and install malicious programs (viruses, Trojans). Botnets
and Zombie computers
scour the net, randomly scanning a block of IP addresses, searching for vulnerable ports
- commonly probed ports
and make repeated attempts to access them. If your computer is sending out large amounts of data, this usually indicates that your system may have a virus or a Trojan horse.
If your firewall provides an alert
which indicates it has blocked access to a port that does not necessarily mean your system has been compromised. The alerts allow the firewall to notify you in various ways about possible penetration and intrusion attempts on your computer. It is not unusual for a firewall to provide numerous alerts regarding such attempted access
. However, not all unrequested traffic is malevolent. Even your ISP will send out regular checks to see if your computer is still there, so you may need to investigate an attempted intrusion.
You can use netstat
, a command-line tool that displays incoming and outgoing network connections, from a command prompt
to obtain Local/Foreign Addresses, PID and listening state.
- netstat /? lists all available parameters that can be used.
- netstat -a lists all active TCP connections and the TCP and UDP ports on which the computer is listening.
- netstat -b lists all active TCP connections, Foreign Address, State and process ID (PID) for each connection.
- netstat -n lists active TCP connections. Addresses and port numbers are expressed numerically and no attempt is made to determine names.
- netstat -o lists active TCP connections and includes the process ID (PID) for each connection. You can find the application based on the PID on the Processes tab in Windows Task Manager. This parameter can be combined with -a, -n, and -p (example: netstat -ano).
-- If the port in question is listed as "Listening" there is a possibility that it is in use by a Trojan server but your firewall, if properly configured, should have blocked any attempt to access it.
You can use Process Monitor
, an advanced monitoring tool for Windows that shows real-time file system, Registry and process/thread activity or various network traffic monitoring tools
for troubleshooting and malware investigation.
There are third party utilities that will allow you to manage, block, and view detailed listings of all TCP and UDP endpoints on your system, including local/remote addresses, state of TCP connections and the process that opened the port:Caution: If you're going to start blocking ports, be careful which ones you block or you may lose Internet connectivity. For a list of TCP/UDP ports and notes about them, please refer to
You can investigate IP addresses and gather additional information at: