There are 2 ways to do this :
1. Disable IPv6 in kernel module (requires reboot)
2. Disable IPv6 using sysctl settings (no reboot required)
To verify if IPv6 is enabled or not, execute :
# ifconfig -a | grep inet6
1 Disable IPv6 in kernel module (requires reboot)
1.1 Edit /etc/default/grub and add ipv6.disable=1 in line GRUB_CMDLINE_LINUX, e.g.:
# cat /etc/default/grub
GRUB_TIMEOUT=5
GRUB_DEFAULT=saved
GRUB_DISABLE_SUBMENU=true
GRUB_TERMINAL_OUTPUT=”console”
GRUB_CMDLINE_LINUX=”ipv6.disable=1 crashkernel=auto rhgb quiet”
GRUB_DISABLE_RECOVERY=”true”
1.2 Regenerate a GRUB configuration file and overwrite existing one:
# grub2-mkconfig -o /boot/grub2/grub.cfg
1.3 Restart system and verify no line “inet6” in “ifconfig” command output.
# shutdown -r now
# ifconfig -a | grep inet6
2 Disable IPv6 using sysctl settings (no reboot required)
2.1 Append below lines in /etc/sysctl.conf:
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
NOTE : To disable IPv6 on a single interface add below lines to /etc/sysctl.conf :
net.ipv6.conf.[interface].disable_ipv6 = 1 ### put interface name here [interface]
net.ipv6.conf.default.disable_ipv6 = 1
2.2 To make the settings affective, execute :
# sysctl -p
2.3 Add the AddressFamily line to sshd_config :
# vi /etc/ssh/sshd_config
….
AddressFamily inet
….
Restart sshd for changes to get get effect :
# systemctl restart sshd
NOTE : make sure the file /etc/ssh/sshd_config contains the line AddressFamily inet to avoid breaking SSH Xforwarding if you are using the sysctl method