Here is the database layout
It is fairly simple, with 3 tables

Table domains:
create table domains ( 
  domain_id int auto_increment, 
  domain varchar(100) NOT NULL, 
  owner_id int, 
  key( domain_id, domain))

domain_id: is an auto incremented integer used to identify 
           the domain. It is used as a key to the "records" table.

domain:    is the  domain name

owner_id: it is a key to the owner table. 

Table records:
create table records ( 
  domain_id int NOT NULL, 
  host varchar(100) not null, 
  type char(1), val varchar(100) )

domain_id: key from the domains table

host: fully qualified hostname

type: dns record type, 
      N = name server
      M = mail exchange server
      H = IN PTR for bind or Host for djbdns
      A = A record
      C = CNAME

val: the value associated with the type
     N = no value
     M = distance 
     H = IP number
     A = IP number
     C = host name

An index on records is created after the table loads.
This significantly increases dnsloadzone times

Index record_idx
 create index records_idx on records ( domain_id, host ) 

Table owners:
create table owners ( 
  owner_id int auto_increment, 
  username char(30) NOT NULL, 
  password char(30),
  key (owner_id, username ) )

onwer_id: id of the owner, used in the domains table too
username: username for login
password: password for login

Sample information "normal" zone:
mysql> select * from domains where domain_id = 53000;
+-----------+-----------+-------------+
| domain_id |  owner_id | domain      |
+-----------+-----------+-------------+
|     53000 |     52999 | beo-tech.it |
+-----------+-----------+-------------+
1 row in set (0.00 sec)

mysql> select * from records where domain_id = 53000;
+-----------+-----------------------+------+------------------+
| domain_id | host                  | type | val              |
+-----------+-----------------------+------+------------------+
|     53000 | beo-tech.it           | A    | 62.149.128.18    |
|     53000 | dns.technorail.com    | N    |                  |
|     53000 | dns2.technorail.com   | N    |                  |
|     53000 | ftp                   | C    | www.beo-tech.it  |
|     53000 | localhost.beo-tech.it | A    | 127.0.0.1        |
|     53000 | mail.beo-tech.it      | A    | 62.149.128.18    |
|     53000 | mx.beo-tech.it        | M    | 10               |
|     53000 | mx.beo-tech.it        | A    | 62.149.128.17    |
|     53000 | pop3                  | C    | mail.beo-tech.it |
|     53000 | smtp.beo-tech.it      | A    | 62.149.128.21    |
|     53000 | www.beo-tech.it       | A    | 62.149.130.44    |
+-----------+-----------------------+------+------------------+
11 rows in set (0.07 sec)


Sample reverse:
mysql> select * from domains where domain_id = 1;
+-----------+-----------+-------------------------+
| domain_id |  owner_id | domain                  |
+-----------+-----------+-------------------------+
|         2 |         0 | 128.149.62.in-addr.arpa |
+-----------+-----------+-------------------------+
1 row in set (0.00 sec)

mysql> select * from records where domain_id = 2;
+-----------+-------------------------------+------+---------------+
| domain_id | host                          | type | val           |
+-----------+-------------------------------+------+---------------+
|         2 | dns.technorail.com            | N    |               |
|         2 | dns.technorail.com            | H    | 62.149.128.2  |
|         2 | dns2.technorail.com           | N    |               |
|         2 | gw-adsl-atm.aruba.it          | H    | 62.149.128.20 |
|         2 | gw-dialup-atm.aruba.it        | H    | 62.149.128.25 |
|         2 | gw-interbusiness-atm.aruba.it | H    | 62.149.128.1  |
|         2 | mail.technorail.com           | M    | 10            |
|         2 | maildomini.aruba.it           | H    | 62.149.128.16 |
|         2 | mailmaxsql.aruba.it           | H    | 62.149.128.10 |
|         2 | mxdomini.aruba.it             | H    | 62.149.128.17 |
|         2 | ourwebs.aruba.it              | H    | 62.149.128.9  |
|         2 | pop3domini.aruba.it           | H    | 62.149.128.18 |
|         2 | radius.technorail.com         | H    | 62.149.128.23 |
|         2 | radiusadsl.aruba.it           | H    | 62.149.128.13 |
|         2 | radtac.technorail.com         | H    | 62.149.128.12 |
|         2 | scambiobanner.aruba.it        | H    | 62.149.128.40 |
|         2 | smtp.technorail.com           | H    | 62.149.128.19 |
|         2 | smtpdomini.aruba.it           | H    | 62.149.128.21 |
|         2 | sql.aruba.it                  | H    | 62.149.128.11 |
|         2 | sqldomini.aruba.it            | H    | 62.149.128.14 |
|         2 | web.aruba.it                  | H    | 62.149.128.15 |
+-----------+-------------------------------+------+---------------+
21 rows in set (0.00 sec)