The Penguin's Practical Network Troubleshooting Guide - page 2
Start with Cable Testing
Suppose you have a remote Web server that is not responding. I know this is horridly obvious, but sometimes people forget that the first step in network troubleshooting is always to confirm connectivity, and for this we have our little friend
ping. There is more to
ping than you may realize, so let's take a closer look. First, always make sure you are connected to the network. I've been bitten by this more than once. Next, ping localhost:
$ ping -c4 -a localhost
-c4 sends four ICMP ECHO_REQUESTs, and
-a makes it ping audibly. Then ping the IP of the box you're trying to connect to. Then ping the hostname. With three simple commands you have confirmed that your NIC is up, and tested both connectivity and DNS.
ping messages give some clues as to where the problem lies. This example shows that the hostname resolves, and there is a route to the host, but
ping is receiving no responses of any kind:
$ ping -c10 somename.com PING somename.com (22.214.171.124) 56(84) bytes of data. --- somename.com ping statistics --- 10 packets transmitted, 0 received, 100% packet loss, time 9999ms
You can try pinging the IP to see if it's a DNS problem:
$ ping -c10 126.96.36.199 PING 188.8.131.52 (184.108.40.206) 56(84) bytes of data. --- 220.127.116.11 ping statistics --- 10 packets transmitted, 0 received, 100% packet loss, time 8999ms
Nope, it's not DNS. Chances are the entire remote network is offline, because you should at least get a "Destination Host Unreachable" message from the network's border router. But it could be a problem anywhere between you and the remote machine. Trying to pinpoint an Internet trouble spot is difficult and frustrating. In the olden days
traceroute was a good tool for this, but in these here modern times a lot of network admins program their routers to not respond to
ping is often blocked as well.
A good alternative is to use
tcptraceroute sends TCP packets instead of UDP datagrams or ICMP ECHO requests like
traceroute, so it's unlikely they'll be blocked. And a nice bonus is
tcptraceroute traverses NAT firewalls. Use it like this:
$ tcptraceroute somename.com Selected device wan, address 18.104.22.168, port 32783 for outgoing packets Tracing the path to somename.com (22.214.171.124) on TCP port 80 (www), 30 hops max 1 126.96.36.199 18.383 ms 15.855 ms 14.915 ms 2 router.foo.net (188.8.131.52) 16.884 ms 15.412 ms 14.670 ms 3 184.108.40.206 15.942 ms 16.928 ms 14.914 ms 4 220.127.116.11 45.727 ms 44.255 ms 43.988 ms 5 18.104.22.168 57.315 ms 55.858 ms 63.676 ms 6 tbr1-p012501.st6wa.ip.att.net (22.214.171.124) 56.307 ms 60.762 ms 54.591 ms 7 126.96.36.199 60.220 ms 59.547 ms 52.862 ms 8 POS2-0.BR1.SEA1.ALTER.NET (188.8.131.52) 51.870 ms * 51.498 ms 9 0.so-4-2-0.XL1.SEA1.ALTER.NET (184.108.40.206) 55.560 ms 52.386 ms 55.570 ms 10 0.so-7-0-0.XL1.DCA6.ALTER.NET (220.127.116.11) 117.896 ms 115.958 ms 121.841 ms 11 0.so-6-0-0.WR1.IAD6.ALTER.NET (18.104.22.168) 119.130 ms 120.162 ms 136.860 ms 12 so-1-0-0.ur1.iad6.web.wcom.net (22.214.171.124) 117.899 ms 126.721 ms 118.613 ms 13 126.96.36.199 120.843 ms 117.494 ms 116.865 ms 14 * * * 15 uu-3-166.hostdomains.com (188.8.131.52) [open] 120.232 ms 145.716 ms 176.254 ms
This shows that
tcptraceroute can trace the remote server all the way to its origin, which is the fictional hostdomains.com, a Web hosting service. So now we know the pipeline is open end-to-end, but the somename.com server is not reachable, and we know which service provider to nag to fix it. If it were your own remote location, this would tell you that the problem is at your remote site.
If it's possible, set up a direct dial-in connection to any remote server that you are running. This will let you find out quickly if it's up or not, and also to perform troubleshooting tests from the other direction.
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