One of the most important concepts in computer security is “knowing thy system”. This essentially means that in order to be able to protect something you need to have some idea of what it’s doing and/or how it works.
Your computer’s connections to the outside world is among the most important information you can have about your system. In addition to what connections are currently established, you also want to know what ports your computer is “listening” on, or in other words, what ways other systems are able to interact with your computer.
Below I’ll cover how to see who your Windows or Linux computer is currently talking to, and the ways your computer is willing to talk through open, listening ports.
Ports
There is often some confusion about what network ports are, and what it means for them to be “open”. Think of network ports as spring-loaded windows on a house. So if someone doesn’t actively hold the window open, it’ll shut automatically and remain closed until it’s opened again.
If a port is open, it means there’s someone (an application) in the window waiting to speak with someone outside the house. Imagine that each open window has a midget in it, and each midget is waiting to have a certain type of conversation with an outsider.
If it’s port 25 that’s open on your machine, then you’ve likely1 got an email midget in the window waiting to process mail for you. If it’s port 445 that’s open, you’ve probably got a Windows Networking midget in there waiting to send and receive files, etc.
The important thing to remember is that when you see a port open on your system, it’s because something opened it. Remember, if there wasn’t a midget in the window it would just close by itself. The issue then becomes finding out what program opened the port, and whether or not it’s legitimate.
Windows
Windows has a built-in tool called netstat that can show a decent amount of information. If you just have a quick question about a certain port you can use it right from the command line and avoid using a third party application:
netstat -an | find "LISTENING"
TCP 0.0.0.0:135 0.0.0.0:0 LISTENING
TCP 0.0.0.0:445 0.0.0.0:0 LISTENING
TCP 0.0.0.0:1049 0.0.0.0:0 LISTENING
TCP 0.0.0.0:9000 0.0.0.0:0 LISTENING
TCP 0.0.0.0:33333 0.0.0.0:0 LISTENING
You want to take note of the red portions: those are the ports that your system is listening for connections on. You can do the same thing and search for established connections as well:
netstat -an | find "ESTABLISHED"
TCP 1.2.3.4:4095 66.102.7.99:80 ESTABLISHED
TCP 1.2.3.4:8324 209.73.177.115:25 ESTABLISHED
Here were seeing the systems we’re currently connected to, and which the ports the connections are using. Notice that the colon “:” is used to show an ip / port pair. So this is showing that we (1.2.3.4) are connected to Google (66.102.7.99) on port 80 — which means we are browsing the Google website.
Tcpview
For those that want more information about their network connections and/or are graphically inclined, there’s a free tool called Tcpview that’s a must for any serious Windows user.
Tcpview allows you to view, in real time, the connections that are open on your system. Not only does it update constantly as connections spawn or die off, but it also shows you what program is responsible for opening a given port on your system. [For those bent on command line kung-fu, you can get similar functionality from netstat -anb]
Linux
Being a Linux/OS X guy myself I would deserve a good pumelling if I didn’t show how to get similar information from a *nix system. The best way to do this is with the lsof command:
lsof -i
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
dhcpcd 6061 root 4u IPv4 4510 UDP *:bootpc
sshd 7703 root 3u IPv6 6499 TCP *:ssh (LISTEN)
sshd 7892 root 3u IPv6 6757 TCP 10.10.1.5:ssh->
192.168.1.5:49901 (ESTABLISHED)
Using
lsof you can ask to see only TCP or UDP connections, only connections to a certain host, only connections using a certain port, as well as a ton of other options. Here are a few examples:
lsof -iTCP // only TCP
lsof -iUDP // only UDP
lsof -i :22 // involving port 22
lsof -i :@attacker.com // connections with attacker.com
lsof -i :1.2.3.4 // connections to 1.2.3.4
lsof -i :mail.com:25 // connections to mail.com on the SMTP port
lsof -i | grep LISTEN // see what’s listening
lsof -i | grep ESTABLISHED // see what’s established
Conclusion
Knowing who your system is talking to (and who it’s willing to talk to) is crucial to your overall computer security. Using the short guide above you can now gather this information in both Windows and *nix environments.:
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1 Not true in all cases.
You can then switch between the windows with 
