Linux Networking Cookbook

19.12. Troubleshooting DNS Servers

Problem

You're getting a lot of "unknown host" errors or timeouts, or mail bounces, or other signs of DNS problems. You can connect using the IP addresses, but not the hostnames. What do you do to track down the problem?

Solution

Use the dig and host commands to see what answers your DNS server is dishing out. Don't use nslookup; it has long been deprecated, and doesn't always work correctly anyway.

One of the biggest sources of trouble is having private and public authoritative name servers, or worse, a single server handling both jobs. So, you especially want to make sure that private name requests are not going out to the Internet. First, run the host command:

	$ host uberpc
	uberpc.alrac.net has address 192.168.1.76
	$ host 192.168.1.76
	76.1.168.192.in-addr.arpa domain name pointer uberpc.alrac.net.

This is good; the address is correct, and your reverse pointer is correctly configured. Now, run dig to see what server is being queried:

	$ dig uberpc
	; <<>> DiG 9.3.4 <<>> uberpc
	;; global options: printcmd
	;; Got answer:
	;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 46745
	;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

	;; QUESTION SECTION:
	;uberpc.                          IN      A

	;; ANSWER SECTION:
	uberpc.                   0       IN      A       192.168.1.76


	;; Query time: 42 msec
	;; SERVER: 192.168.1.50#53(192.168.1.50)
	;; WHEN: Sat Jul 14 23:17:02 2007
	;; MSG SIZE rcvd: 38

This shows the A record for Uberpc with a proper trailing dot, and that the server that is handling the request ...