On RHEL-6 systems listens to only IPv4 by default. Switching listen=NO
and listen_ipv6=YES causes the daemon listens to only IPv6.

But on RHEL-7 listens to IPv6 as well as IPv4 by default. Be default listening
on the IPv6 'any' address will accept connections from both (IPv6 and IPv4)
clients. If you want listen on specific adresses then you must run two copies
of vsftpd with two configuration files. For more details see man 5 vsftpd.conf.
