Troubleshooting with Ping
One important tool for troubleshooting networking is ping. To test if a device has proper networking, you can run ping on an IP address or a hostname. Once you are done, press CTRL+C (abbreviated ^C):
$ ping ircnow.org PING ircnow.org (209.141.39.173): 56 data bytes 64 bytes from 209.141.39.173: icmp_seq=0 ttl=244 time=251.636 ms 64 bytes from 209.141.39.173: icmp_seq=1 ttl=244 time=252.236 ms 64 bytes from 209.141.39.173: icmp_seq=2 ttl=244 time=249.526 ms 64 bytes from 209.141.39.173: icmp_seq=3 ttl=244 time=249.869 ms 64 bytes from 209.141.39.173: icmp_seq=4 ttl=244 time=248.836 ms 64 bytes from 209.141.39.173: icmp_seq=5 ttl=244 time=251.230 ms 64 bytes from 209.141.39.173: icmp_seq=6 ttl=244 time=249.227 ms 64 bytes from 209.141.39.173: icmp_seq=7 ttl=244 time=249.550 ms ^C --- ircnow.org ping statistics --- 8 packets transmitted, 8 packets received, 0.0% packet loss round-trip min/avg/max/std-dev = 248.836/250.264/252.236/1.174 ms
ping uses the Internet Control Message Protocol (ICMP) to test for problems. Here, we see it takes roughly 250 milliseconds for ircnow.org to reply to our messages. Of the 8 packets sent, all 8 were received, so we have 0 packet loss.
If you need to test out IPv6, then use the ping6 command. For both IPv4 and IPv6, you may find it helpful to bind to a specific IP address (send packets using a specific IP address). In that case, use the option -I. For example:
$ ping6 -I 1234:5678:abcd:ef::1 ircnow.org
This will force ping6 to send and receive packets using the (fake) IPv6 address 1234:5678:abcd:ef::1.
Sometimes, you may see ping results that require further investigation:
--- fake.ircnow.org ping statistics --- 2779223 packets transmitted, 2669400 packets received, 4.0% packet loss round-trip min/avg/max/std-dev = 510.548/600.687/535545.801/958.211 ms
Here, our ping suffers 4% packet loss. This is quite high and should be investigated. Perhaps there is an issue with the hardware or the networking equipment is faulty. The average latency of 600ms is also very high. Ideally, when the client and server are on the same continent, latency should be under 100ms. A latency of around 200ms is average, and anything above 300ms is quite high and feels laggy.
If you get packet loss that is less than 100%, then some packets are still arriving. Networking is working somewhat, but there may be a misconfiguration somewhere, or faulty hardware. However, if you are getting 100% packet loss, then either your hardware is altogether broken, or your configuration is wrong.
You also should check your firewall. Because ping and ping6 both use ICMP, any firewall rules that block ICMP should be disabled. Be careful: rate-limiting ICMP may cause issues.