When a secondary server cannot obtain updates from its master, it logs a master unreachable message. If the problem is not corrected, the secondary expires the zone and stops answering requests from clients. When that happens, users start seeing server failed messages.
Note that if the problem lies with a secondary server, some users could still be successfully obtaining DNS information from the master and thus operating without experiencing any difficulty.
The two most likely causes for these problems are network failure and a wrong IP address for the master in the secondary's boot file.
Diagnosis and solution:
Check that the secondary's boot file contains the correct IP address for the master. Check the line:
secondary domain IPaddress hostsfile
Make sure that the IP address of the master matches the master's actual IP address and the address for the master specified in the hosts file. If the IP address is wrong, correct it, and then reboot the secondary.
If the master's IP address is correct, make sure the master is up and running correctly by pinging the master's IP address: For example, to ping the master at IP address 220.127.116.11, you would enter:
% ping 18.104.22.168 -n 10
If the master does not respond to the ping, make sure it is up and running properly.
If the master is running okay, use ps to make sure it is running named. If it is not running named, reboot it.
If the master is correctly running named, you most likely have a network problem.