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This file is required for the simple reason that some software—especially the popular DNS program, nslookup—will not work unless it can do a reverse lookup of the nameservers address. The pz part of the name is a directory name, so the file resides in the subdirectory named pz (named as such because "primary zone" used to be the name of what is now known as a "master zone"). Some DNS admins like to organize their zone files into separate directory master zones, and slave, or secondary, zones. There are many different conventions for naming zone files. If you are exploring an existing BIND installation while reading this, you may find a completely different naming convention in use. If so, you may want to continue using the naming convention currently in use at your site.

As shown in the named.conf file, this is a master zone; according to the naming convention I like, this master zone goes in the pz directory and gets a name of the network in which it resolves hosts: "127.0.0":

$ORIGIN 0.0.127.in-addr.arpa.
@               IN      SOA     ns.penguin.bv. hostmaster.penguin.bv. (
                                1       ; Serial
                                8H      ; Refresh
                                2H      ; Retry
                                1W      ; Expire
                                1D)     ; Minimum TTL
                        NS      ns.penguin.bv.
1                       PTR     localhost.

The first line explicitly states that the domain name, origin, of this zone is 0.0.127.in-addr.arpa. This is the name of the reverse zone for the 127.0.0 network. This origin is also implied by the argument to the zone directive in the named.conf file, and so it is superfluous. What the file defines should also be obvious from the filename. The $ORIGIN line is considered good form by many, though, because it explicitly documents what the file you're looking at is about.

The $TTL 1D line states that records looked up and cached in a caching server from this file have a TTL (Time To Live) of one day. The cached entry expires after 24 hours and is removed from the cache when that much time has passed.

The line starting with "@" defines the SOA RR, which spans several lines and ends on the line with the closing parenthesis.

The last line starting with "1" defines a PTR record for the domain name It should point to "localhost.", the traditional name for the loopback interface address on Unix systems.

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