Linux Network Administrator Guide, Second Edition

Example 6.7: An Excerpt from the named.rev File for Network 149.76

; the 76.149.in-addr.arpa domain.
@ IN SOA vax12.gcc.groucho.edu. joe.vax12.gcc.groucho.edu. {
 1999070100 360000 3600 3600000 3600
}
…
; subnet 4: Mathematics Dept.
1.4 IN PTR sophus.maths.groucho.edu.
17.4 IN PTR erdos.maths.groucho.edu.
23.4 IN PTR gauss.maths.groucho.edu
…
; subnet 12: Physics Dept, separate zone
12 IN NS niels.physics.groucho.edu.
 IN NS gauss.maths.groucho.edu.
niels.physics.groucho.edu. IN A 149.76.12.1
gauss.maths.groucho.edu. IN A 149.76.4.23
…

in-addr.arpa system zones can only be created as supersets of IP networks. An even more severe restriction is that these networks' netmasks have to be on byte boundaries. All subnets at Groucho Marx University have a netmask of 255.255.255.0, hence an in- addr.arpa zone could be created for each subnet. However, if the netmask were 255.255.255.128 instead, creating zones for the subnet 149.76.12.128 would be impossible, because there's no way to tell DNS that the 12.76.149.in-addr.arpa domain has been split into two zones of authority, with hostnames ranging from 1 through 127, and 128 through 255, respectively.

Running named

named (pronounced name-dee) provides DNS on most Unix machines. It is a server program originally developed for BSD to provide name service to clients, and possibly to other name servers. BIND Version 4 was around for some time and appeared in most Linux distributions. The new release, Version 8, has been introduced in most Linux distributions, and is a big change from previous versions.[42] It has many new features, such as support for DNS dynamic updates, DNS change notifications, much improved performance, and a new configuration file syntax. Please check the documentation contained in the source distribution for details.

This section requires some understanding of the way DNS works. If the following discussion is all Greek to you, you may want to reread the section 'How DNS Works'.

named is usually started at system boot time and runs until the machine goes down again. Implementations of BIND prior to Version 8 take their information from a configuration file called /etc/named.boot and various files that map domain names to addresses. The latter are called zone files. Versions of BIND from Version 8 onwards use /etc/named.conf in place of /etc/named.boot.

To run named at the prompt, enter:

# /usr/sbin/named

named will come up and read the named.boot file and any zone files specified therein. It writes its process ID to /var/run/named.pid in ASCII, downloads any zone files from primary servers, if necessary, and starts listening on port 53 for DNS queries.

The named.boot File

The BIND configuration file prior to Version 8 was very simple in structure. BIND Version 8 has a very different configuration file syntax to deal with many of the new features introduced. The name of the configuration file changed from /etc/named.boot, in older versions of BIND, to /etc/named.conf in BIND Version 8. We'll focus on configuring the older version because it is probably what most distributions are still using, but we'll present an equivalent named.conf to illustrate the differences, and we'll talk about how to convert the old format into the new one.

The named.boot file is generally small and contains little but pointers to master files containing zone information and pointers to other name servers. Comments in the boot file start with the (#) or (;) characters and extend to the next newline. Before we discuss the format of named.boot in more detail, we will take a look at the sample file for vlager given in Example 6.8.

Example 6.8: The named.boot File for vlager

;
; /etc/named.boot file for vlager.vbrew.com
;
directory /var/named
;
; domain file
;____________________
cache  .  named.ca
primary vbrew.com named.hosts
primary 0.0.127.in-addr.arpa named.local
primary 16.172.in-addr.arpa named.rev

Let's look at each statement individually. The directory keyword tells named that all filenames referred to later in this file, zone files for example, are located in the /var/named directory. This saves a little typing.

The primary keyword shown in this example loads information into named. This information is taken from the master files specified as the last of the parameters. These files represent DNS resource records, which we will look at next.

In this example, we configured named as the primary name server for three domains, as indicated by the three primary statements. The first of these statements instructs named to act as a primary server for vbrew.com, taking the zone data from the file named.hosts.

The cache keyword is very special and should be present on virtually all machines running a name server. It instructs named to enable its cache and to load the root name server hints from the cache file specified (named.ca in our example). We will come back to the name server hints in the following list.

Here's a list of the most important options you can use in named.boot:

directory

This option specifies a directory in which zone files reside. Names of files in other options may be given relative to this directory. Several directories may be specified by repeatedly using