arping command

arping command is a very useful Linux command. It's purpose is to send ARP request on the local network and to print received responses. It's a sort like ping command, but it works on layer 2 (data link layer) instead on a network layer like ping. Unlike ping command you have to specify egress interface to this command since otherwise it doesn't know where to send request, and the command itself is programmed to send it to some predetermined interface, e.g. eth0.

Most frequently, you'll use it like this:

arping -I wlan0 192.168.1.1

which, in this particular case, tells arping command to send requests asking for a link layer address corresponding to IP address 192.168.1.1. Requests should go via wlan0 interface.

Now you may wonder what's so special about this command. Well, the special part is that there is no way you can block it, unlike ping command. For example, Windows 7 by default blocks protocol packets used by ping command and thus you wont be able to check if host is alive using ping. With arping you can definitely determine if it is alive or not. But there is a restriction, you can use it only on a local, Ethernet-like, network! So there is no way it can be used over the network, at least not without some heavy tricks. Additional restriction is that you need administrative (root) privileges on a host, unlike ping command that can be run by any user.