7.1 Introduction

The name "ping" is taken from the sonar operation to locate objects. The Ping program was written by Mike Muuss and it tests whether another host is reachable . The program sends an ICMP echo request message to a host, expecting an ICMP echo reply to be returned. (Figure 6.3 lists all the ICMP message types.)

Normally if you can't Ping a host, you won't be able to Telnet or FTP to that host. Conversely, if you can't Telnet to a host, Ping is often the starting point to determine what the problem is. Ping also measures the round-trip time to the host, giving us some indication of how "far away" that host is.

In this chapter we'll use Ping as a diagnostic tool and to further explore ICMP. Ping also gives us an opportunity to examine the IP record route and timestamp options. Chapter 11 of [Stevens 1990] provides the source code for the Ping program.

Years ago we could make the unqualified statement that if we can't Ping a host, we can't Telnet or FTP to that host. With the increased awareness of security on the Internet, routers that provide access control lists, and firewall gateways, unqualified statements like this are no longer true. Reachability of a given host may depend not only on reachability at the IP layer, but also on what protocol is being used, and the port numbers involved. Ping may show a host as being unreachable, yet we might be able to Telnet to port 25 (the mail server).