17.9 DNS

17.9.6.1 make-localhost

Be sure to:

    # cd /etc/namedb
    # sh make-localhost

to properly create the local reverse DNS zone file in /etc/namedb/localhost.rev.

17.9.6.2 /etc/namedb/named.conf

    // $FreeBSD$
    //
    // Refer to the named(8) manual page for details.  If you are ever going
    // to setup a primary server, make sure you've understood the hairy
    // details of how DNS is working.  Even with simple mistakes, you can
    // break connectivity for affected parties, or cause huge amount of
    // useless Internet traffic.
    
    options {
            directory "/etc/namedb";
    
    // 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 name servers 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 8.1 uses an unprivileged
             * port by default.
             */
            // query-source address * port 53;
    
            /*
             * If running in a sandbox, you may have to specify a different
             * location for the dumpfile.
             */
            // dump-file "s/named_dump.db";
    };
    
    // Note: the following will be supported in a future release.
    /*
    host { any; } {
            topology {
                    127.0.0.0/8;
            };
    };
    */
    
    // Setting up secondaries is way easier and the rough picture for this
    // is explained below.
    //
    // If you enable a local name server, don't forget to enter 127.0.0.1
    // into your /etc/resolv.conf so this server will be queried first.
    // 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 "localhost.rev";
    };
    
    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.IP6.INT" {
            type master;
            file "localhost.rev";
    };
    
    // NB: Do not use the IP addresses below, they are faked, and only
    // serve demonstration/documentation purposes!
    //
    // Example secondary config entries.  It can be convenient to become
    // a secondary at least for the zone where 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 the first bytes of the respective IP address, in reverse
    // order, with ".IN-ADDR.ARPA" appended.)
    //
    // Before starting to setup a primary zone, better make sure you fully
    // understand how DNS and BIND works, however.  There are sometimes
    // unobvious pitfalls.  Setting up a secondary is comparably simpler.
    //
    // NB: Don't blindly enable the examples below. :-)  Use actual names
    // and addresses instead.
    //
    // NOTE!!! FreeBSD runs bind in a sandbox (see named_flags in rc.conf).
    // The directory containing the secondary zones must be write accessible
    // to bind.  The following sequence is suggested:
    //
    //      mkdir /etc/namedb/s
    //      chown bind:bind /etc/namedb/s
    //      chmod 750 /etc/namedb/s

For more information on running BIND in a sandbox, see Running named in a sandbox.

    /*
    zone "domain.com" {
            type slave;
            file "s/domain.com.bak";
            masters {
                    192.168.1.1;
            };
    };
    
    zone "0.168.192.in-addr.arpa" {
            type slave;
            file "s/0.168.192.in-addr.arpa.bak";
            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 "example.org";
    };

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

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

17.9.6.3 Zone Files

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

    $TTL 3600
    
    example.org. IN SOA ns1.example.org. admin.example.org. (
                            5               ; Serial
                            10800           ; Refresh
                            3600            ; Retry
                            604800          ; Expire
                            86400 )         ; Minimum TTL
    
    ; DNS Servers
    @       IN NS           ns1.example.org.
    @       IN NS           ns2.example.org.
    
    ; Machine Names
    localhost       IN A    127.0.0.1
    ns1             IN A    3.2.1.2
    ns2             IN A    3.2.1.3
    mail            IN A    3.2.1.10
    @               IN A    3.2.1.30
    
    ; Aliases
    www             IN CNAME        @
    
    ; MX Record
    @               IN MX   10      mail.example.org.

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. (
                            5               ; 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. The @ as seen here could have been example.org. The @ translates to the origin.

    localhost       IN A    127.0.0.1
    ns1             IN A    3.2.1.2
    ns2             IN A    3.2.1.3
    mail            IN A    3.2.1.10
    @               IN A    3.2.1.30

The A record indicates machine names. As seen above, ns1.example.org would resolve to 3.2.1.2. Again, the origin symbol, @, is used here, thus meaning example.org would resolve to 3.2.1.30.

    www             IN CNAME        @

The canonical name record is usually used for giving aliases to a machine. In the example, www is aliased to the machine addressed to the origin, or example.org (3.2.1.30). 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 3, 2, 1. A mail server attempting to deliver to example.org would first try the highest priority MX, 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
    
    1.2.3.in-addr.arpa. IN SOA ns1.example.org. admin.example.org. (
                            5               ; Serial
                            10800           ; Refresh
                            3600            ; Retry
                            604800          ; Expire
                            3600 )          ; Minimum
    
    @       IN NS   ns1.example.org.
    @       IN NS   ns2.example.org.
    
    2       IN PTR  ns1.example.org.
    3       IN PTR  ns2.example.org.
    10      IN PTR  mail.example.org.
    30      IN PTR  example.org.

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