27.6 Domain Name System (DNS)

27.6.6.1 Using make-localhost

To configure a master zone for the localhost visit the /etc/namedb directory and run the following command:

# sh make-localhost

If all went well, a new file should exist in the master subdirectory. The filenames should be localhost.rev for the local domain name and localhost-v6.rev for IPv6 configurations. As the default configuration file, required information will be present in the named.conf file.

27.6.6.2 /etc/namedb/named.conf

// $FreeBSD$
//
// Refer to the named.conf(5) and named(8) man pages, and the documentation
// in /usr/share/doc/bind9 for more details.
//
// If you are going to set up an authoritative server, make sure you
// understand the hairy details of how DNS works.  Even with
// simple mistakes, you can break connectivity for affected parties,
// or cause huge amounts of useless Internet traffic.

options {
    directory   "/etc/namedb";
    pid-file    "/var/run/named/pid";
    dump-file   "/var/dump/named_dump.db";
    statistics-file "/var/stats/named.stats";

// If named is being used only as a local resolver, this is a safe default.
// For named to be accessible to the network, comment this option, specify
// the proper IP address, or delete this option.
    listen-on   { 127.0.0.1; };

// If you have IPv6 enabled on this system, uncomment this option for
// use as a local resolver.  To give access to the network, specify
// an IPv6 address, or the keyword "any".
//  listen-on-v6    { ::1; };

// In addition to the "forwarders" clause, you can force your name
// server to never initiate queries of its own, but always ask its
// forwarders only, by enabling the following line:
//
//  forward only;

// If you've got a DNS server around at your upstream provider, enter
// its IP address here, and enable the line below.  This will make you
// benefit from its cache, thus reduce overall DNS traffic in the Internet.
/*
    forwarders {
        127.0.0.1;
    };
*/

Just as the comment says, to benefit from an uplink's cache, forwarders can be enabled here. Under normal circumstances, a name server will recursively query the Internet looking at certain name servers until it finds the answer it is looking for. Having this enabled will have it query the uplink's name server (or name server provided) first, taking advantage of its cache. If the uplink name server in question is a heavily trafficked, fast name server, enabling this may be worthwhile.

   /*
     * If there is a firewall between you and nameservers you want
     * to talk to, you might need to uncomment the query-source
     * directive below.  Previous versions of BIND always asked
     * questions using port 53, but BIND versions 8 and later
     * use a pseudo-random unprivileged UDP port by default.
     */
     // query-source address * port 53;
};

// If you enable a local name server, don't forget to enter 127.0.0.1
// first in your /etc/resolv.conf so this server will be queried.
// Also, make sure to enable it in /etc/rc.conf.

zone "." {
    type hint;
    file "named.root";
};

zone "0.0.127.IN-ADDR.ARPA" {
    type master;
    file "master/localhost.rev";
};

// RFC 3152
zone "1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA" {
    type master;
    file "master/localhost-v6.rev";
};

// NB: Do not use the IP addresses below, they are faked, and only
// serve demonstration/documentation purposes!
//
// Example slave zone config entries.  It can be convenient to become
// a slave at least for the zone your own domain is in.  Ask
// your network administrator for the IP address of the responsible
// primary.
//
// Never forget to include the reverse lookup (IN-ADDR.ARPA) zone!
// (This is named after the first bytes of the IP address, in reverse
// order, with ".IN-ADDR.ARPA" appended.)
//
// Before starting to set up a primary zone, make sure you fully
// understand how DNS and BIND works.  There are sometimes
// non-obvious pitfalls.  Setting up a slave zone is simpler.
//
// NB: Don't blindly enable the examples below. :-)  Use actual names
// and addresses instead.

/* An example master zone
zone "example.net" {
    type master;
    file "master/example.net";
};
*/

/* An example dynamic zone
key "exampleorgkey" {
    algorithm hmac-md5;
    secret "sf87HJqjkqh8ac87a02lla==";
};
zone "example.org" {
    type master;
    allow-update {
        key "exampleorgkey";
    };
    file "dynamic/example.org";
};
*/

/* Examples of forward and reverse slave zones
zone "example.com" {
    type slave;
    file "slave/example.com";
    masters {
        192.168.1.1;
    };
};
zone "1.168.192.in-addr.arpa" {
    type slave;
    file "slave/1.168.192.in-addr.arpa";
    masters {
        192.168.1.1;
    };
};
*/

In named.conf, these are examples of slave entries for a forward and reverse zone.

For each new zone served, a new zone entry must be added to named.conf.

For example, the simplest zone entry for example.org can look like:

zone "example.org" {
    type master;
    file "master/example.org";
};

The zone is a master, as indicated by the type statement, holding its zone information in /etc/namedb/master/example.org indicated by the file statement.

zone "example.org" {
    type slave;
    file "slave/example.org";
};

In the slave case, the zone information is transferred from the master name server for the particular zone, and saved in the file specified. If and when the master server dies or is unreachable, the slave name server will have the transferred zone information and will be able to serve it.

27.6.6.3 Zone Files

An example master zone file for example.org (existing within /etc/namedb/master/example.org) is as follows:

$TTL 3600        ; 1 hour
example.org.    IN      SOA      ns1.example.org. admin.example.org. (
                                2006051501      ; Serial
                                10800           ; Refresh
                                3600            ; Retry
                                604800          ; Expire
                                86400           ; Minimum TTL
                        )

; DNS Servers
                IN      NS      ns1.example.org.
                IN      NS      ns2.example.org.

; MX Records
                IN      MX 10   mx.example.org.
                IN      MX 20   mail.example.org.

                IN      A       192.168.1.1

; Machine Names
localhost       IN      A       127.0.0.1
ns1             IN      A       192.168.1.2
ns2             IN      A       192.168.1.3
mx              IN      A       192.168.1.4
mail            IN      A       192.168.1.5

; Aliases
www             IN      CNAME   @

Note that every hostname ending in a “.” is an exact hostname, whereas everything without a trailing “.” is referenced to the origin. For example, www is translated into www.origin. In our fictitious zone file, our origin is example.org., so www would translate to www.example.org.

The format of a zone file follows:

recordname      IN recordtype   value

The most commonly used DNS records:

example.org. IN SOA ns1.example.org. admin.example.org. (
                        2006051501      ; Serial
                        10800           ; Refresh after 3 hours
                        3600            ; Retry after 1 hour
                        604800          ; Expire after 1 week
                        86400 )         ; Minimum TTL of 1 day

       IN NS           ns1.example.org.

This is an NS entry. Every name server that is going to reply authoritatively for the zone must have one of these entries.

localhost       IN      A       127.0.0.1
ns1             IN      A       192.168.1.2
ns2             IN      A       192.168.1.3
mx              IN      A       192.168.1.4
mail            IN      A       192.168.1.5

The A record indicates machine names. As seen above, ns1.example.org would resolve to 192.168.1.2.

                IN      A       192.168.1.1

This line assigns IP address 192.168.1.1 to the current origin, in this case example.org.

www             IN CNAME        @

The canonical name record is usually used for giving aliases to a machine. In the example, www is aliased to the “master” machine which name equals to domain name example.org (192.168.1.1). CNAMEs can be used to provide alias hostnames, or round robin one hostname among multiple machines.

               IN MX   10      mail.example.org.

The MX record indicates which mail servers are responsible for handling incoming mail for the zone. mail.example.org is the hostname of the mail server, and 10 being the priority of that mail server.

One can have several mail servers, with priorities of 10, 20 and so on. A mail server attempting to deliver to example.org would first try the highest priority MX (the record with the lowest priority number), then the second highest, etc, until the mail can be properly delivered.

For in-addr.arpa zone files (reverse DNS), the same format is used, except with PTR entries instead of A or CNAME.

$TTL 3600

1.168.192.in-addr.arpa. IN SOA ns1.example.org. admin.example.org. (
                        2006051501      ; Serial
                        10800           ; Refresh
                        3600            ; Retry
                        604800          ; Expire
                        3600 )          ; Minimum

        IN      NS      ns1.example.org.
        IN      NS      ns2.example.org.

1       IN      PTR     example.org.
2       IN      PTR     ns1.example.org.
3       IN      PTR     ns2.example.org.
4       IN      PTR     mx.example.org.
5       IN      PTR     mail.example.org.

This file gives the proper IP address to hostname mappings of our above fictitious domain.