19.2. ping: check to see if a host is alive

The ping command is embarrassingly simple, but in many situations it is all you need. It sends an ICMP ECHO_REQUEST packet to a target host and waits to see if the host answers back. Despite its simplicity, ping is one of the workhorses of network debugging.

You can use ping to check the status of individual hosts and to test segments of the network. Routing tables, physical networks, and gateways are all involved in processing a ping, so the network must be more or less working for ping to succeed. If ping doesn’t work, you can be pretty sure that nothing more sophisticated will work either. However, this rule does not apply to networks that block ICMP echo requests with a firewall. Make sure that a firewall isn’t interfering with your debugging before you conclude that the target host is ignoring a ping. You might consider disabling a meddlesome firewall for a short period of time to facilitate debugging.