ping and reachability

ping is the first tool anyone reaches for when *the network is down*. It sends small ICMP echo request packets to a host and waits for echo replies. Every reply proves that your packet got there, that the host answered, and that the answer found its way back. It also measures the round-trip time.

~% ping -c 3 127.0.0.1
PING 127.0.0.1 (127.0.0.1): 56 data bytes
64 bytes from 127.0.0.1: seq=0 ttl=64 time=1.284 ms
64 bytes from 127.0.0.1: seq=1 ttl=64 time=0.541 ms
64 bytes from 127.0.0.1: seq=2 ttl=64 time=0.613 ms

--- 127.0.0.1 ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max = 0.541/0.812/1.284 ms

Each line is one reply: seq numbers the packets so you notice gaps, ttl is the packet's remaining hop budget (lesson 80), time the round trip. The summary at the end, printed when ping finishes or when you press Ctrl-C, gives the loss percentage and min/avg/max times. Without -c Linux's ping never stops on its own, so in scripts and exercises always give a count.

The exit status

ping exits with 0 if at least one reply arrived and non-zero otherwise. That makes it a building block for scripts: ping -c 1 -W 1 HOST >/dev/null 2>&1 && echo up || echo down. Throw away the output, keep the verdict. Add -W 1 so a dead host costs you one second, not ten.

Reading failures

Failures come in different flavours, and each points somewhere else. ping: sendto: Network is unreachable appears instantly: your own machine has no route for that address. Try it after ifconfig lo down and even 127.0.0.1 is unreachable; bring lo up and it answers. ping: bad address 'name' means the name could not be resolved, a DNS problem (next lesson). Lines that never come, ending in 100% packet loss, mean the packets left but nothing answered: the host is down, or a firewall drops ICMP, which many servers do. No reply does not prove a host is down.

ping inside TempMV

The live part of this lesson needs the machine's network switched on (Built-in mode) and an address from udhcpc -i eth0 (lesson 75). Then try ping -c 2 192.168.86.1, the virtual router. Now try ping -c 2 8.8.8.8: it answers in under a millisecond. That is impossible for a real trip across the internet, and it is the giveaway: a web page cannot send ICMP, so the emulator's network stack answers every echo request itself. Here a reply proves that your NIC, address and route work, nothing more. To check that a real site is reachable you will use HTTP, with wget, in lesson 78.