Who Else Is on Your Wi-Fi Network?

I got these two letters almost back to back, both asking pointed questions about figuring out whether someone else might be on their Wi-Fi network... or how to figure out who owned a nearby wireless network that was unsecured. Take a look.

Rob writes:  Is there a program (preferably a free download) that enables me to view the computers (users) connected to a wireless signal at any one particular time?

Meanwhile, Zac writes: One of my poor neighbors has their Linksys router set on default configuration. I can even view their configuration page. I am not wanting to do harm to their network, and I don't want to secure it for them without their knowledge. Is there a way to contact the PC listed under the client list, or a program that takes snapshots of their Internet usage (maybe I could see their email address), so I can get a hold of them?

First, you can find out who's connecting to your Wi-Fi network by looking in the DHCP Client List, which is part of your router's configuration and settings software. Just go to the configuration web page (where you set up encryption and passwords, etc.), and look for the DHCP Client List. You'll see the computer name and MAC address of everyone connected. If something fishy is there, you've got an interloper. Be advised that clever types can get around this rather easily: All you really need to do is use a static IP address instead of a dynamic one and you won't show up on the list. However, most computers use DHCP for simplicity, and any casual hangers-on will certainly be DHCP users.

But let's get more complicated: Say you've found an interloper, or like Zac you've found an open wireless network and want to help the owner close it. How do you connect a MAC address to a name?

It's not easily done. Ultra-hackers use advanced tools or directional antennas to figure out the source of a wireless signal, but those are beyond the reach of most users. (Check out Ethereal and AirSnare if you're determined to go this route. Both are software for experts only and are probably overkill for finding an accidental Wi-Fi leech.)

My best advice for beginners is to simply scour the network to see if you can find any shared computers or drives. You might be able to easily pick up a name or address from a file on the network. (This is probably not completely legal, but neither is using your neighbor's open signal, so tread cautiously here.)

With access to the router (as Zac mentioned), some innocuous tricks to get the owner's attention and let him know that his wireless setup is insecure. For example: Change the SSID (or network name) to heyneighborcall5551212, or turn on parental controls (if the router offers them), redirecting popular URLs like google.com, yahoo.com, msn.com, etc. to something like this page.

Unfortunately there's really no foolproof way to figure out who's on your wireless network, so remember to use security at all times and keep tabs on that DHCP list once in awhile. If your network starts to slow down or you fear a hacker intrusion, check out Ethereal and AirSnare at the links above.