If this is a serious concern, the Gopher proxy should be located on the outside of the firewall, so that it is not hampered by the packet filter's restrictions. If we accept that clients should always perform active opens, it might be worthwhile enhancing the FTP protocol to eliminate the extra exchange entirely. At least two independent implementations of the modified clients exist. Source code to one is freely available. To our knowledge, APSV has not been implemented.
Some people feel that packet filters are dangerous, since they are very hard to configure properly.
But they are quite popular. Another common complaint is that permitting arbitrary outgoing calls is dangerous, since it allows free export of sensitive data through a firewall. Some firewalls impose artificial bandwidth limits to discourage this. Using PASV does enhances the security of gateway machines, since they no longer need to create ports that an outsider might connect to before the real FTP client. Concerns have been expressed that this use of PASV just trades one problem for another.