Правильный способ отключения IPv6 в RedHat Linux 6 / CentOS 6
/etc/sysctl.conf : net.ipv6.conf.all.disable_ipv6 = 1
/etc/sysconfig/network : NETWORKING_IPV6=no
/etc/sysconfig/network-scripts/ifcfg-eth0 : IPV6INIT=”no”
disable ip6tables – chkconfig –level 345 ip6tables off
reboot
(http://blog.acsystem.sk/linux/rhel-6-centos-6-disabling-ipv6-in-system)