System Administration Guide: Naming and Directory Services (DNS, NIS, and LDAP)

Chapter 6 DNS Troubleshooting (Reference)

This chapter described some common DNS problems and how to solve them.

Clients Can Find Machine by Name but Server Cannot


DNS clients can find machines by either IP address or by host name, but the server can only find machines by their IP addresses.

Probable cause and solution

This is most likely caused by omitting DNS from the hosts line of the server's nsswitch.conf file. For example, a bad hosts line might look like this: hosts: files

When using DNS you must include dns in the hosts record of every machine's nsswitch.conf file. For example:

hosts: dns nisplus files


hosts: nisplus dns files

Changes Do Not Take Effect or Are Erratic


You add or delete machines or servers but your changes are not recognized or do not take effect. Or in some instances the changes are recognized and at other times they are not yet in effect.

Probable cause

The most likely cause is that you forgot to increment the SOA serial number on the master server after you made your change. Since there is no new SOA number, your slave servers do not update their data to match that of the master so they are working with the old, unchanged data files.

Another possible cause is that the SOA serial number in one or more of the master data files was set to a value lower than the corresponding serial number on your slave servers. This could happen, for example, if you deleted a file on the master and then recreated it from scratch using an input file of some sort.

A third possible cause is that you forgot to send a HUP signal to the master server after making changes to the primary's data files.

Diagnosis and solution

First, check the SOA serial numbers in the data file that you changed and the corresponding file on the slave server.

DNS Client Cannot Lookup “Short” Names


Client can lookup fully qualified names but not short names.

Possible cause and solution

Check the client's /etc/resolv.conf file for spaces at the end of the domain name. No spaces or tabs are allowed at the end of the domain name.

Reverse Domain Data Not Correctly Transferred to slave

While zone domain-named data is properly transferred from the zone master server to a zone slave server, the reverse domain data is not being transferred. In other words, the host.rev file on the slave is not being properly updated from the primary.

Possible causes

Syntax error in the slave server's boot file.

Diagnosis and Solution

Check the slave server's boot file. Make sure that the master server's IP address is listed for the reverse zone entries just as it is for the hosts data.

Server Failed and Zone Expired Problems

When a slave server cannot obtain updates from its master, it logs a master unreachable message. If the problem is not corrected, the slave expires the zone and stops answering requests from clients. When that happens, users start seeing server failed messages.


Note that if the problem is with a slave server, some users could still be successfully obtaining DNS information from the master and thus operating without experiencing any difficulty.

Possible causes

The two most likely causes for these problems are network failure and a wrong IP address for the master in the slave's boot file.

Diagnosis and solution

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 slave.

rlogin, rsh, and ftp Problems


Possible causes

Diagnosis and solution

Check the appropriate hosts.rev file and make sure there is a PTR record for the user's machine. For example, if the user is working at the machine with an IP address of, the master server's doc.rev file should have an entry like:

46 	IN	 PTR

If the record is missing, add it to the hosts.rev file and then reboot the server or reload its data as explained in Forcing in.named to Reload DNS Data.

Check and correct the NS entries in the hosts.rev files and then reboot the server or reload its data as explained in Forcing in.named to Reload DNS Data.

Other DNS Syntax Errors


Error messages in console or syslog with operative phrases like the following are most often caused by syntax errors in DNS data and boot files.

Check the relevant files for spelling and syntax errors.

A common syntax error is misuse of the trailing dot in domain names (either using the dot when you should not, or not using it when you should). See Setting up DNS Servers.