arping For Mac

arping is a util to find out it a specific IP address on the LAN is 'taken' and what MAC address owns it. Sure, you *could* just use 'ping' to find out if it's taken and even if the computer blocks ping (and everything else) you still get an entry in your ARP cache.

But what if you aren't on a routable net? Or the host blocks ping (all ICMP even)? Then you're stuck. Or you use arping.

arping sends arp and/or ip pings to a given host. The arping utility sends ARP and/or ICMP requests to the specified host and displays the replies. The host may be specified by its hostname, its MAC address, or its IP address.

One request is sent each second. When pinging an IP an ARP who-has query is sent. When pinging a MAC address a directed broadcast ICMP Echo request is sent.

ARP packets are usually replied to (on a LAN) so fast that the OS task scheduler can't keep up to get exact enough timing. On an idle system the roundtrip times will be pretty much accurate, but with more load the timing gets less exact. To get more exact timing on a non-idle system, re-nice arping to -15 or so.