29.6 Domain Name System (DNS)

29.6.6.1 /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 {
    // Relative to the chroot directory, if any
    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; };

// These zones are already covered by the empty zones listed below.
// If you remove the related empty zones below, comment these lines out.
    disable-empty-zone "255.255.255.255.IN-ADDR.ARPA";
    disable-empty-zone "0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA";
    disable-empty-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";

// 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;
    };
*/

// If the 'forwarders' clause is not empty the default is to 'forward first'
// which will fall back to sending a query from your local server if the name
// servers in 'forwarders' do not have the answer.  Alternatively you can
// force your name server to never initiate queries of its own by enabling the
// following line:
//  forward only;

// If you wish to have forwarding configured automatically based on
// the entries in /etc/resolv.conf, uncomment the following line and
// set named_auto_forward=yes in /etc/rc.conf.  You can also enable
// named_auto_forward_only (the effect of which is described above).
//  include "/etc/namedb/auto_forward.conf";

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.

   /*
       Modern versions of BIND use a random UDP port for each outgoing
       query by default in order to dramatically reduce the possibility
       of cache poisoning.  All users are strongly encouraged to utilize
       this feature, and to configure their firewalls to accommodate it.

       AS A LAST RESORT in order to get around a restrictive firewall
       policy you can try enabling the option below.  Use of this option
       will significantly reduce your ability to withstand cache poisoning
       attacks, and should be avoided if at all possible.

       Replace NNNNN in the example with a number between 49160 and 65530.
    */
    // query-source address * port NNNNN;
};

// 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.

// The traditional root hints mechanism. Use this, OR the slave zones below.
zone "." { type hint; file "named.root"; };

/*  Slaving the following zones from the root name servers has some
    significant advantages:
    1. Faster local resolution for your users
    2. No spurious traffic will be sent from your network to the roots
    3. Greater resilience to any potential root server failure/DDoS

    On the other hand, this method requires more monitoring than the
    hints file to be sure that an unexpected failure mode has not
    incapacitated your server.  Name servers that are serving a lot
    of clients will benefit more from this approach than individual
    hosts.  Use with caution.

    To use this mechanism, uncomment the entries below, and comment
    the hint zone above.
*/
/*
zone "." {
    type slave;
    file "slave/root.slave";
    masters {
        192.5.5.241;    // F.ROOT-SERVERS.NET.
    };
    notify no;
};
zone "arpa" {
    type slave;
    file "slave/arpa.slave";
    masters {
        192.5.5.241;    // F.ROOT-SERVERS.NET.
    };
    notify no;
};
zone "in-addr.arpa" {
    type slave;
    file "slave/in-addr.arpa.slave";
    masters {
        192.5.5.241;    // F.ROOT-SERVERS.NET.
    };
    notify no;
};
*/

/*  Serving the following zones locally will prevent any queries
    for these zones leaving your network and going to the root
    name servers.  This has two significant advantages:
    1. Faster local resolution for your users
    2. No spurious traffic will be sent from your network to the roots
*/
// RFC 1912
zone "localhost"    { type master; file "master/localhost-forward.db"; };
zone "127.in-addr.arpa" { type master; file "master/localhost-reverse.db"; };
zone "255.in-addr.arpa" { type master; file "master/empty.db"; };

// RFC 1912-style zone for IPv6 localhost address
zone "0.ip6.arpa"   { type master; file "master/localhost-reverse.db"; };

// "This" Network (RFCs 1912 and 3330)
zone "0.in-addr.arpa"       { type master; file "master/empty.db"; };

// Private Use Networks (RFC 1918)
zone "10.in-addr.arpa"      { type master; file "master/empty.db"; };
zone "16.172.in-addr.arpa"  { type master; file "master/empty.db"; };
zone "17.172.in-addr.arpa"  { type master; file "master/empty.db"; };
zone "18.172.in-addr.arpa"  { type master; file "master/empty.db"; };
zone "19.172.in-addr.arpa"  { type master; file "master/empty.db"; };
zone "20.172.in-addr.arpa"  { type master; file "master/empty.db"; };
zone "21.172.in-addr.arpa"  { type master; file "master/empty.db"; };
zone "22.172.in-addr.arpa"  { type master; file "master/empty.db"; };
zone "23.172.in-addr.arpa"  { type master; file "master/empty.db"; };
zone "24.172.in-addr.arpa"  { type master; file "master/empty.db"; };
zone "25.172.in-addr.arpa"  { type master; file "master/empty.db"; };
zone "26.172.in-addr.arpa"  { type master; file "master/empty.db"; };
zone "27.172.in-addr.arpa"  { type master; file "master/empty.db"; };
zone "28.172.in-addr.arpa"  { type master; file "master/empty.db"; };
zone "29.172.in-addr.arpa"  { type master; file "master/empty.db"; };
zone "30.172.in-addr.arpa"  { type master; file "master/empty.db"; };
zone "31.172.in-addr.arpa"  { type master; file "master/empty.db"; };
zone "168.192.in-addr.arpa" { type master; file "master/empty.db"; };

// Link-local/APIPA (RFCs 3330 and 3927)
zone "254.169.in-addr.arpa" { type master; file "master/empty.db"; };

// TEST-NET for Documentation (RFC 3330)
zone "2.0.192.in-addr.arpa" { type master; file "master/empty.db"; };

// Router Benchmark Testing (RFC 3330)
zone "18.198.in-addr.arpa"  { type master; file "master/empty.db"; };
zone "19.198.in-addr.arpa"  { type master; file "master/empty.db"; };

// IANA Reserved - Old Class E Space
zone "240.in-addr.arpa"     { type master; file "master/empty.db"; };
zone "241.in-addr.arpa"     { type master; file "master/empty.db"; };
zone "242.in-addr.arpa"     { type master; file "master/empty.db"; };
zone "243.in-addr.arpa"     { type master; file "master/empty.db"; };
zone "244.in-addr.arpa"     { type master; file "master/empty.db"; };
zone "245.in-addr.arpa"     { type master; file "master/empty.db"; };
zone "246.in-addr.arpa"     { type master; file "master/empty.db"; };
zone "247.in-addr.arpa"     { type master; file "master/empty.db"; };
zone "248.in-addr.arpa"     { type master; file "master/empty.db"; };
zone "249.in-addr.arpa"     { type master; file "master/empty.db"; };
zone "250.in-addr.arpa"     { type master; file "master/empty.db"; };
zone "251.in-addr.arpa"     { type master; file "master/empty.db"; };
zone "252.in-addr.arpa"     { type master; file "master/empty.db"; };
zone "253.in-addr.arpa"     { type master; file "master/empty.db"; };
zone "254.in-addr.arpa"     { type master; file "master/empty.db"; };

// IPv6 Unassigned Addresses (RFC 4291)
zone "1.ip6.arpa"       { type master; file "master/empty.db"; };
zone "3.ip6.arpa"       { type master; file "master/empty.db"; };
zone "4.ip6.arpa"       { type master; file "master/empty.db"; };
zone "5.ip6.arpa"       { type master; file "master/empty.db"; };
zone "6.ip6.arpa"       { type master; file "master/empty.db"; };
zone "7.ip6.arpa"       { type master; file "master/empty.db"; };
zone "8.ip6.arpa"       { type master; file "master/empty.db"; };
zone "9.ip6.arpa"       { type master; file "master/empty.db"; };
zone "a.ip6.arpa"       { type master; file "master/empty.db"; };
zone "b.ip6.arpa"       { type master; file "master/empty.db"; };
zone "c.ip6.arpa"       { type master; file "master/empty.db"; };
zone "d.ip6.arpa"       { type master; file "master/empty.db"; };
zone "e.ip6.arpa"       { type master; file "master/empty.db"; };
zone "0.f.ip6.arpa"     { type master; file "master/empty.db"; };
zone "1.f.ip6.arpa"     { type master; file "master/empty.db"; };
zone "2.f.ip6.arpa"     { type master; file "master/empty.db"; };
zone "3.f.ip6.arpa"     { type master; file "master/empty.db"; };
zone "4.f.ip6.arpa"     { type master; file "master/empty.db"; };
zone "5.f.ip6.arpa"     { type master; file "master/empty.db"; };
zone "6.f.ip6.arpa"     { type master; file "master/empty.db"; };
zone "7.f.ip6.arpa"     { type master; file "master/empty.db"; };
zone "8.f.ip6.arpa"     { type master; file "master/empty.db"; };
zone "9.f.ip6.arpa"     { type master; file "master/empty.db"; };
zone "a.f.ip6.arpa"     { type master; file "master/empty.db"; };
zone "b.f.ip6.arpa"     { type master; file "master/empty.db"; };
zone "0.e.f.ip6.arpa"       { type master; file "master/empty.db"; };
zone "1.e.f.ip6.arpa"       { type master; file "master/empty.db"; };
zone "2.e.f.ip6.arpa"       { type master; file "master/empty.db"; };
zone "3.e.f.ip6.arpa"       { type master; file "master/empty.db"; };
zone "4.e.f.ip6.arpa"       { type master; file "master/empty.db"; };
zone "5.e.f.ip6.arpa"       { type master; file "master/empty.db"; };
zone "6.e.f.ip6.arpa"       { type master; file "master/empty.db"; };
zone "7.e.f.ip6.arpa"       { type master; file "master/empty.db"; };

// IPv6 ULA (RFC 4193)
zone "c.f.ip6.arpa"     { type master; file "master/empty.db"; };
zone "d.f.ip6.arpa"     { type master; file "master/empty.db"; };

// IPv6 Link Local (RFC 4291)
zone "8.e.f.ip6.arpa"       { type master; file "master/empty.db"; };
zone "9.e.f.ip6.arpa"       { type master; file "master/empty.db"; };
zone "a.e.f.ip6.arpa"       { type master; file "master/empty.db"; };
zone "b.e.f.ip6.arpa"       { type master; file "master/empty.db"; };

// IPv6 Deprecated Site-Local Addresses (RFC 3879)
zone "c.e.f.ip6.arpa"       { type master; file "master/empty.db"; };
zone "d.e.f.ip6.arpa"       { type master; file "master/empty.db"; };
zone "e.e.f.ip6.arpa"       { type master; file "master/empty.db"; };
zone "f.e.f.ip6.arpa"       { type master; file "master/empty.db"; };

// IP6.INT is Deprecated (RFC 4159)
zone "ip6.int"          { type master; file "master/empty.db"; };

// 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
// master name server.
//
// Do not forget to include the reverse lookup zone!
// This is named after the first bytes of the IP address, in reverse
// order, with ".IN-ADDR.ARPA" appended, or ".IP6.ARPA" for IPv6.
//
// Before starting to set up a master zone, make sure you fully
// understand how DNS and BIND work.  There are sometimes
// non-obvious pitfalls.  Setting up a slave zone is usually simpler.
//
// NB: Don't blindly enable the examples below. :-)  Use actual names
// and addresses instead.

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

/* Example of a slave reverse zone
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.

29.6.6.2 Zone Files

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

$TTL 3600        ; 1 hour default TTL
example.org.    IN      SOA      ns1.example.org. admin.example.org. (
                                2006051501      ; Serial
                                10800           ; Refresh
                                3600            ; Retry
                                604800          ; Expire
                                300             ; Negative Reponse 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   example.org.

Note that every hostname ending in a “.” is an exact hostname, whereas everything without a trailing “.” is relative to the origin. For example, ns1 is translated into ns1.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
                        300 )           ; Negative Reponse TTL

       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 whose name happens to be the same as the domain name example.org (192.168.1.1). CNAMEs can never be used together with another kind of record for the same hostname.

               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
                        300 )           ; Negative Reponse TTL

        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.

It is worth noting that all names on the right side of a PTR record need to be fully qualified (i.e., end in a “.”).