Wireless Hacks

Quickly Poll Wireless Clients with ping

A quick and dirty method for determining who is on your local subnet.

This is a simple, quick hack, but useful in many circumstances. Suppose you are associated with a wireless network, and are curious about who else is also using the network. You could fire up a network sniffer (like Ethereal [Hack #38] or tcpdump [Hack #37]), or manually scan for associated clients (using nmap [Hack #40]), although that might be construed as antisocial. You’re not so much interested in what people are doing, just how many people are online.

It is simple to find clients on your local network using the ubiquitous ping utility. Simply ping the broadcast address of your network, and see who responds.

You can find the broadcast address by running ifconfig like so:

rob@florian:~$ ifconfig eth0
eth0   Link encap:Ethernet HWaddr 00:40:63:C0:AA:4B 
     inet addr:10.15.6.1 Bcast:10.15.6.255 Mask:255.255.255.0
     UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
     RX packets:13425489 errors:0 dropped:33 overruns:0 frame:0
     TX packets:19603221 errors:1118 dropped:0 overruns:0 carrier:0
     collisions:0 txqueuelen:100 
     RX bytes:3073225705 (2930.8 Mb) TX bytes:1301320438 (1241.0 Mb)
     Interrupt:10 Base address:0xe800

There it is, the Bcast address. This is the broadcast address for your local subnet, which every machine is listening to. In Mac OS X and BSD, it is simply listed as the broadcast address:

rob@caligula:~$ ifconfig en1 en1: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> ...