How catch pc online in the LAN?

I need to know when a specific pc (which I know ip and MAC) is plugged on the LAN. I want to avoid all polling mecha开发者_如何学JAVAnisms, is it possible under linux to achieve this ? Maybe by sniffing icmp packet?

Thx

If you are in control of the gateway, you can just sniff traffic and see if there's any from/to that host (through MAC or IP address).

If not, things get complicated, and some sort of polling is needed in most cases - in a switched network, you generally won't see the traffic destined for another host; the surest way of checking "online-ness" would be ARP, possibly by using arping (or doing the ARP requests yourself and sniffing the traffic for ARP replies).

Note that neither of these approaches are "sure-fire" - with tools like powernap, (temporarily) offline hosts can appear to be online.

You can use NMAP to scan your network with a simple ping type scan to see what hosts are online and not. This is a polling mechanism, but unless you can program the routers/switches in your network to tell your when MAC or IP address XYZ has started sending traffic, I don't know of any other way to do this.