[Zope] passive ftp?

howdy. i'm running zope on linux server that also runs iptables. i'm having trouble getting iptables to accept passive ftp connections to the zope ftp server. i'm assuming that this is because the iptables ftp module doesn't know that port 8021 is ftp as well. i suppose i could set the zope ftp port to 21. has anyone found any other ways around this problem? thanks!