The sys admin's daily grind – Netdiscover

Swords to Plowshares

Article from Issue 170/2015

This month, sys admin Charly grabs the fairly ancient Netdiscover wardriving tool and takes care of his neighbors' WiFi networks.

"Charly, can you take a look at this? My (choose a computer term) isn't working!" This is a situation that eventually anyone who "does something in IT" gets into. By now, friends and neighbors have realized that I cannot help them with Windows problems. If their WiFi network is acting up, however, there's no escape.

So, in response, I connect my laptop to my neighbors' wireless LAN and check out the network. With what? Well, normally with Netdiscover [1]. This tool goes back to the days when wardriving was cool and widespread. For our younger readers: Wardrivers would walk or drive through the neighborhood armed with a laptop looking for open or poorly protected wireless networks and, for example, drop the King James version of the Bible into the confused owner's printer spooler just as a little warning.

Netdiscover supports active and passive modes. In passive mode, it does not send packets but evaluates the network traffic flying by. In a WLAN, in which all the devices transmit on the same radio channel, you can accumulate a fairly extensive list of devices in a short time. You launch Netdiscover in passive mode by typing

netdiscover -i wlan0 -p

(Figure 1) as root or using sudo. If you omit the -i wlan0 parameter, Netdiscover uses the first network interface that it finds.

Figure 1: In passive mode, Netdiscover collects data on all the devices that have advertised their existence in the observation period.

In active mode, Netdiscover sends ARP packets itself to find devices. You can opt to scan a specific network:

netdiscover -i wlan0 -c 3 -r

If I know that the WiFi network has poor coverage, I can set -c 3, as in the above example. This means that Netdiscover scans not once, but three times to compensate for any packet loss.

Throwing out the Nets

Using -r outlines the network that I want to take a closer look at. If you want to scan several networks, however, you need to write a file, and pass it into the tool using -l <filename>. Running Netdiscover against multiple WiFi IP ranges was popular back in the wardriving age. This process relied on the -f parameter, which enables fast mode and – in simple terms – means that the tool only verifies the existence of addresses that end with the octet .1, .100, or .254 for each network. Access point scavengers typically worked from moving cars and had to be fast. Then, if they did discover something interesting, they could park and investigate the network more closely.

Final tips: If you happen to be invited to a diagnostics session on your neighbor's network, see how many wireless access points or routers you can find. And, if you happen to find three, disable the DHCP server on two of them; this works wonders and boosts your reputation of doing "something in IT."

Charly Kühnast

Charly Kühnast is a Unix operating system administrator at the Data Center in Moers, Germany. His tasks include firewall and DMZ security and availability. He divides his leisure time into hot, wet, and eastern sectors, where he enjoys cooking, freshwater aquariums, and learning Japanese, respectively.

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