The named.boot File

The named.boot file is generally very small and contains little else but pointers to master files containing zone information, and pointers to other name servers. Comments in the boot file start with a semicolon 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 figure-.

Figure: 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       72.191.in-addr.arpa      named.rev

The cache and primary commands shown in this example load information into named. This information is taken from the master files specified in the second argument. They contain textual representations of DNS resource records, which we will look at below.

In this example, we configured named as the primary name server for three domains, as indicated by the primary statements at the end of the file. The first of these lines, for instance, instructs named to act as a primary server for vbrew.com, taking the zone data from the file named.hosts. The directory keyword tells it that all zone files are located in /var/named.

The cache entry is very special and should be present on virtually all machines running a name server. Its function is two-fold: 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 below.

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

directory

This specifies a directory in which zone files reside. Names of files may be given relative to this directory. Several directories may be specified by repeatedly using directory. According to the Linux filesystem standard, this should be /var/named.

primary

This takes a domain name and a file name as an argument, declaring the local server authoritative for the named domain. As a primary server, named loads the zone information from the given master file. Generally, there will always be at least one primary entry in every boot file, namely for reverse mapping of net- work 127.0.0.0, which is the local loopback network.

secondary

This statement takes a domain name, an address list, and a file name as an argument. It declares the local server a sec- ondary master server for the domain specified. A secondary server holds authoritative data on the domain, too, but it doesn't gather it from files, but tries to download it from the primary server. The IP address of at least one primary server must thus be given to named in the address list. The local server will contact each of them in turn until it successfully transfers the zone database, which is then stored in the backup file given as the third argument. If none of the primary servers responds, the zone data is retrieved from the backup file instead. named will then attempt to refresh the zone data at regu- lar intervals. This is explained below along in connection with the SOA resource record type.

cache

This takes a domain and a file name as arguments. This file contains the root server hints, that is a list of records pointing to the root name servers. Only NS and A records will be recognized. The domain argument is generally the root domain name ``.''. This information is absolutely crucial to named: if the cache statement does not occur in the boot file, named will not develop a local cache at all. This will severely degrade performance and increase network load if the next server queried is not on the local net. Moreover, named will not be able to reach any root name servers, and thus it won't resolve any addresses except those it is authoritative for. An excep- tion from this rule is when using forwarding servers (cf. the forwarders option below).

forwarders

This statement takes an address list as an argument. The IP addresses in this list specify a list of name servers that named may query if it fails to resolve a query from its local cache. They are tried in order until one of them responds to the query.

slave

This statement makes the name server a slave server. That is, it will never perform recursive queries itself, but only for- wards them to servers specified with the forwarders statement.

There are two options which we will not describe here, being sortlist and domain. Additionally, there are two directives that may be used inside the zone database files. These are $INCLUDE and $ORIGIN. Since they are rarely needed, we will not describe them here, either.