Chapter 14: DNS: Domain Name Service

Chapter 14

DNS: Domain Name Service

In this chapter:

	Configuring DNS
	Setting up a "caching only" name server
	Debugging your configuration

A machine on the Internet (or on your local area network) can usually be addressed using its name or its IP address. This is true from a user standpoint, but for applications, the IP address is the only way to reach a remote host. This implies the need of a translation of names to addresses. The Domain Name Service (DNS) provides exactly this: It maps a name to an address and vice versa. It also can give additional information about hosts, such as short descriptions and hints for9 mail routing.

SuSE Linux 6.1 ships with two versions of the DNS daemon. You can choose between version 4.9.7 and version 8.1.2 of the bind package that contains named, the daemon handling DNS requests. Use the newer version. It offers more functionality and improved security. Also note that the syntax for the bootup file has changed slightly. If remaining compatible is a major issue for you, or you set DNS up already and want only to move it over to the Linux machine, you might want to install the older version.

14.1 Configuring the Domain Name Service

This section reviews the latest version, bind 8.1.2. To get the name server started at all, the variable START_BIND, found in /etc/rc.config, has to be set to yes. The main configuration file read by named (the name server daemon), is /etc/named.conf. The named.conf that comes with SuSE is an extensive example of all possible features. Other than demonstrating what is possible, it's pretty useless. A much shorter and actually useful example follows.

#  
# Bind 8 configuration for a cache only server  
#  

 

options {  
  directory "/var/named";  

 

  forward first;  
  forwarders {  
    168.253.48.19;  
    198.6.1.1;  
    209.0.51.1;  
  };  
};  

 

# Root servers  
zone "." {  
  type hint;  
  file "root.hint";  
};  

 

# the loopback device  
zone "localhost" IN {  
  type master;  
  file "localhost.zone";  
  check-names fail;  
  allow-update { none; };  
};  

 

zone "0.0.127.in-addr.arpa" IN {  
  type master;  
  file "127.0.0.zone";  
  check-names fail;  
  allow-update { none; };  
};

This example setup shows a server without a local database. You use caching-only servers to minimize DNS traffic for your local network. If all local machines point to this server, a lookup will be made only when the requested entry has not been requested before. Any known address to name (or name to address) translations will be cached by this server, and it can answer requests directly without making a time-consuming lookup over the external link.

Even if you have a few machines (or even a single host), it may make sense to set up a caching-only DNS server. Not only will the traffic going to your ISP be reduced, but also the increased speed will be worth the effort. DNS queries are expensive in terms of time, and the server cache will reduce these to a minimum.

In the options section, you see that the directory /var/named is specified. All files given in the rest of the configuration are expected to be in this directory. A list of forwarders specifies name servers that are first queried before a real lookup to the root servers is made. This is done with the assumption that these forwarders are faster to reach than the root servers. You should replace these addresses with the addresses of the name servers of your ISP. The next section specifies the root servers, whose addresses can be found in the file /var/named/root.hint.

This file is made available by InterNIC registration services under anonymous FTP as file /domain/named.root on the server FTP.RS.INTERNIC.NET. It is also part of the bind8 package provided by SuSE. It's recommended that you use the newest version, so you may want to check whether a more recent version exists than the one provided on the CD.

The last section in the example is used to resolve the addresses used for the local loopback device. The zone files 127.0.0.zone and localhost.zone are also part of the SuSE bind8 package and are located in /var/named.

This example doesn't include any real host/address database (zone files), but it gives you an idea of the structure used in the named configuration file.

For a complete documentation of bind, look at the Web site of the Internet Software Consortium, http://www.isc.org/bind8/index.html. A good book on this subject is DNS and BIND, 3rd Edition, by Paul Albitz and Cricket Liu.

14.2 Debugging the Domain Name Service

If you have to set up a DNS server, you may come to the point at which you want to know what's going on inside the server. The named daemon has some nice debugging features, which are triggered by sending signals to the deamon:

	SIGHUP Causes server to reread `/etc/named.conf` and reload the database. It will also cause the server to check the serial number on all secondary zones; normally, the serial numbers are checked only at the SOA-specified intervals.
	SIGINT Dumps the current database and cache information to `/var/named/nameddump.db`. This gives you a list of all data known to the server at this moment.
	SIGILL Dumps statistics data into `/var/named/named.stats`. Statistics data is appended to the file.
	SIGTERM Dumps the primary and secondary database files. Used to save modified data on shutdown if the104 server is compiled with dynamic updating enabled.
	SIGUSR1 Turns on debugging; each SIGUSR1 sent to the server increments the debug level. This feature allows you to see what the server is doing and where it tries to look up the requested names or addresses. The more SIGUSR1 signals you send to the server, the more detailed is the output in `/var/named/named.run`.
	SIGUSR2 Turns off debugging completely.
	SIGWINCH Toggles logging of all incoming queries via syslog. Each DNS request will be logged in `/var/log/messages`.

Instead of sending these signals to the daemon, the tool ndc can be used. It supports the commands status, dumpdb, reload, stats, trace, notrace, querylog, start, stop and restart on the commanline. The function of these commands is equivalent to the signals listed abobve. For a detailed list of these commands refer to the man page ndc(8).

The man page gives different locations for the listed log files. The standard behavior would be to have all the output files in /var/tmp. However, SuSE decided to change this and have named dump its debugging output to /var/named.

Summary:

The domain name service translates machine names in actual IP addresses and vice versa. The bind package is the package containing the server and the tools to access the DNS information.

The DNS server uses ASCII file databases to hold the information about the domains it serves. You also can set up a server without local data as a caching server to minimize IP traffic and to speed up DNS queries. Signals can be sent to the server to make it write debugging output or dump its current database. This makes it easy to debug the local configuration and optimize the server setup.