$TTL 604800
@ IN    SOA     hostname.domain.com hostmaster.domain.com (
                                2002080100      ;serial YYMMDD##
                                86400   ;refresh every 1 day
                                3600    ;retry every hour
                                3600000 ;expire after 42 days
                                604800 )  ;minimum 1 semaine

        IN      NS      localhost.
1       IN      PTR     machine1.
2       IN      PTR     machine2.
3       IN      PTR     machine3.