What’s done wrong here?

$ curl foo.example.com
curl: (6) Could not resolve host: foo.example.com
$ host foo.example.com
Host foo.example.com not found: 3(NXDOMAIN)
$ ping foo.example.com
ping: foo.example.com: Name or service not known
$ traceroute foo.example.com
foo.example.com: Name or service not known

Another admin trying to be thorough when reporting an issus. They could have stopped after the first command. The curl error messages tells you everything you need to know. If curl can’t resolve the name, the other tools will very likely also not be able to resovle the name.

Another troubleshooting tale.

When you look at the following output, what is your first guess?

root@mail:~# postqueue -p
-Queue ID-  --Size-- ----Arrival Time---- -Sender/Recipient-------
D94947FBE83    1331 Thu Apr  3 15:18:58  jens@example.net
(Host or domain name not found. Name service error for name=example.com type=MX: Host not found, try again)
                                         foobar@example.com

DNS right? Nope! Well kind off.

In reality it was another domain and not example.com, and it was of many mails I wrote that day and the only one that couldn’t be delivered. What looked like a DNS problem at first glance was accutaly an IPv4 routing problem inside the provider network.