If your broadband Internet connection is being shared using a router – which is a small device where you plug in your modem and the computers you want to share the Internet connection, usually coming with four network ports –, you are lucky. These home routers act as Firewalls and block any incoming request for folders and files. Thus, you can share files on your home or office network without worrying of being invaded by the file sharing method.
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Figure 2: Broadband router, front view.
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Figure 3: Broadband router, back view.
But if you don't use a router, you have to be very careful. In this case, probably the computer that has the Internet connection also has a second network card connecting this computer to another computer (usually through a cross-over cable) or even to a hub or switch, to share its Internet connection with more than one computer. In this case, the PC that has the Internet connection is vulnerable, because all computers on the Internet can access its files if file sharing is enabled!
There are some solutions. The best is to spend some bucks and buy a broadband router. As we said, this device also acts as a Firewall, protecting your whole network. Installing this device is very easy, just plug in your Internet connection (the cable that comes from cable or xDSL modem) in a jack called WAN, Broadband or similar and then plug in the computers in your network to the other available jacks. If you need more ports – since these devices usually have only four ports – just buy a network switch with the number of ports you want and connect it to one of the available ports on the router. The switch will act as a "port expander". Some routers come with wireless antennas for sharing your internet connection with laptop and desktop computers with wireless capability. By the way, we've written a specific tutorial on wireless security, read it if you have wireless connection.
But if you don't have the money, you can simple disable file sharing from this computer that has the Internet connection. But you may need to share its files! There are two solutions. First, put all files to be shared on another computer and enable file sharing on this other computer. Or simply move the broadband modem to a computer that doesn't need its file sharing to be activated.
You may be thinking why is safe to enable file sharing on the other computers but the computer that has the modem. What happens is that normally the computer with the modem gets a public IP address. A computer with a public IP address can be viewed by anyone on the net. The other computers, on the other hand, get a "magical" IP address (usually 192.168.x.x or 10.x.x.x, for example), that are IP addresses that work only in a local network. Computers with one magical IP address cannot be reached from the net, so they are protected. At least for file sharing hacking. Don't forget that there are a lot of hacking methods, like phishing.
Personal firewalls are also a good idea. If you use Windows XP, install Service Pack 2 (SP2), because it comes with Windows Firewall, a software that can stop malicious software from trying to access your computer and if you install a Trojan Horse by mistake the firewall can block its attempt to send data outside your computer.
Another menace are the infamous spywares and malwares. There are lots of software that can detect and remove this kind of junk. The new AntiSpyware from Microsoft is pretty good.