bind configuration help

[Błażej Ślusarek]

Hello,
I'd like to ask for help in setting up my DNS server. When I start the
server, everything is fine, but only for some time. After the "some
time" passes, my external domain name cannot be resolved from anywhere
on the Internet. When I restart the Named, everything is back to
normal after few seconds, again for the "some time". Here are some
fragments of my DNS configuration:

***
options {
    directory "/var/bind";
    forward first;
    forwarders {
        some.ip;
    };
    allow-query { any; };
    allow-recursion { any; };
    listen-on-v6 { none; };
    listen-on { 127.0.0.1; internal.ip; external.ip; };

zone "my.domain.name" IN {
    type master;
    file "pri/costam.zone";
    allow-update { none; };
    //allow-transfer { slaves; };
    allow-transfer { any; };
    notify yes;
};
***
I've got no clue what could be the cause of this behavior. The server
should provide service to internal and external networks and allow
zone transfers. I'd also like to ask for correct iptables
configuration for the above dns settings. I'm quite not sure that if I
have the "forwarders" option, I have to enable port 53 in FORWARD
chain, or maybe just INPUT and OUTPUT is enough. Also, what rules are
necessary for the zone transfer to work?