FTP Protocol extensions permit the use of a special "EPSV" command.
NATs may be able to put the connection on a "fast path" through the
translator using EPSV, as the EPRT command will never be used
and therefore, translation of the data portion of the segments
will never be needed.
EPSV is often required to interoperate with FTP servers on IPv6
networks. On the other hand, it may break some IPv4 servers.
By default, EPSV may try EPSV with any FTP server. To fine tune
that decision, you may restrict EPSV to certain clients or servers
using ACLs:
ftp_epsv allow|deny al1 acl2 ...
WARNING: Disabling EPSV may cause problems with external NAT and IPv6.
Only fast ACLs are supported.
Requires ftp_passive to be ON (default) for any effect.