Another typical symptom of failing reverse lookups is an abnormally long delay on connection attempts. This happens when the server's query for a PTR record is not answered and times out (often because of network problems or the nameserver being down). This can be baffling to diagnose, but you should suspect DNS problems whenever you hear questions such as 'Hey! Why is my web browser taking so long to connect?'
Maintaining Accurate Serial Numbers
Accurate serial numbers are very important to the correct operation of slave servers. An increase in the serial number of a zone causes slaves to reload the zone and update their local caches.
A common mistake that system administrators make is forgetting to increment the serial number after a change to the zone data. If you make this mistake, secondary nameservers don't reload the zone, and continue to serve old data. If you suspect that the data on the master and slave servers is out of sync, you can use dig to view the SOA record for the zone on each server (dig @master domain SOA and dig @slave domain SOA) and compare the serial numbers in the responses.
Another common problem is setting the serial number to an incorrect value—either too small or too large. A too-small serial number causes slaves to think that they possess a more up-to-date copy of the zone data, but this is easily corrected by increasing the serial number as necessary. A too-large serial number is more problematic and requires more elaborate measures to repair.
Serial number comparisons are defined in such a way that if a serial number — when subtracted from another with no overflow correction — results in a positive number, the second number is newer than the first, and a zone transfer is required. (See RFC 1982, 'Serial Number Arithmetic,' for details.) You can exploit this property by temporarily setting the serial number to 2?? (4,294,967,296), waiting for all the slaves to reload the zone, and then setting it to the correct number.
Troubleshooting Problems in Zone Files
The most common error in zone data is forgetting that names in a zone file are relative to the origin of the zone, not to the root. Writing www.example.com in the zone file for example.com and expecting it to be fully qualified causes names such as www.example.com.example.com to show up in the DNS. You should either write www, which is qualified to the correct www.example.com, or write www.example.com. (with the trailing period) to indicate that the name is fully qualified.
The SOA record should contain (as the first field) the domain name of the master server (not a CNAME) and a contact address (with the @ replaced by a .) to report problems to.
Mail sent to this address should be read frequently. The other fields should contain sensible values for your zone, and the serial number should be correctly incremented after each change.
As discussed earlier, A and PTR records should always match; that is, the A record pointed to by a PTR record should point back to the address of the PTR record. Remember to quote the two arguments of HINFO records if they contain any whitespace. Avoid the use of CNAME records for MX, NS, and SOA records.
In general, after making changes to zone data, it is a good idea to reload named and examine the logs for any errors that cause named to complain or reject the zone. Even better, you could use one of the verification tools, such as dnswalk, discussed briefly next.
Tools for Troubleshooting
BIND includes the always useful dig program, as well as named- checkconf (to check /etc/named.conf for syntax errors) and named- checkzone (to do the same for zone files). We also especially recommend dnswalk and nslint. dnswalk is a Perl script that scans the DNS setup of a given domain for problems. It should be used in conjunction with RFC 1912, which explains most of the problems it detects. nslint, like the analogous lint utility for C programs, searches for common BIND and zone file configuration errors.
By occasionally using these programs to troubleshoot DNS problems (especially after nontrivial zone changes), you go far toward keeping your DNS configuration healthy and trouble free.
Using Fedora's BIND Configuration Tool
Fedora provides a dozen or more different graphical configuration tools system administrators can use to configure network (and system) services. One of these tools is system-config-bind, a deceptively simple BIND configuration tool that requires an active X session and must be run with root privileges.
You can launch this client by using the command system-config-bind from a terminal window or by selecting the Domain Name Service menu item from the Server Settings menu. system- config-bind is automatically installed if you select the Fedora configuration tools.
Using system-config-bind and then saving any changes overwrites existing settings! If you prefer to manually edit your named configuration files, do not use system-config- bind. Always make a backup of the configuration files in any event — you'll be glad you did.
After you type the root password and press the Enter key, the client launches. You then see its main window, as shown in Figure 23.2.
FIGURE 23.2 Fedora's system-config-bind utility can be used to create, modify, and save basic domain nameserver settings.
system-config-bind can be used to add a forward master zone, reverse master zone, MX records, or slave zone. Click the New button to select an entry for configuration, as shown in Figure 23.3.
FIGURE 23.3 Use system-config-bind to add a new DNS record to your server or edit the existing settings.
You can edit or delete existing settings by first selecting and then clicking the Properties or Delete button in the system-config-bind dialog. When you finish entering or editing your custom settings, select the Save menu item from the File menu. Configuration files are saved in /etc/named.conf and under the /var/named directory.
