[Zope] firewall and passive ftp

Hi, I have a problem using Zope-ftp with our firewall. With passive ftp one cannot know wich ports are being used for communication. Port 8021 is open and works. Ftp on the local machine works as well (passive and active). Opening all high ports on the firewall is not an option. Is it possible to specify (for example in zope.conf) which ports are allowed for passive ftp (like PassivePortRange 30000 40000 for other ftp servers), so that we can reconfigure the firewall for these ports? If not, are there any other ways of solving this? Thanks in advance for all help! Michael