In Red Hat Enterprise Linux 5, vsfptd listens to only IPv4 by default. Switching listen=NO
and listen_ipv6=YES causes the daemon to listen to IPv6 only.

But in Red Hat Enterprise Linux 7, vsfptd listens to IPv6 as well as IPv4 by default. By default, listening
on IPv6 "any" address will accept connections from both IPv6 and IPv4
clients. If you want to listen on specific adresses, you must run two copies
of vsftpd with two configuration files. For more details see man(5) vsftpd.conf.
