How to disable IPv6 on CentOS 7

To verify if IPv6 is enabled or not, execute :

# ifconfig -a | grep inet6
inet6 fe80::211:aff:fe6a:9de4 prefixlen 64 scopeid 0x20
inet6 ::1 prefixlen 128 scopeid 0x10[host]

1. Disable IPv6 in kernel module (requires reboot)
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"

2. Regenerate a GRUB configuration file and overwrite existing one:

# grub2-mkconfig -o /boot/grub2/grub.cfg

3. Restart system and verify no line “inet6” in “ip addr show” command output.

# shutdown -r now

# ip addr show | grep net6

2. Disable IPv6 using sysctl settings (no reboot required)

1. Append below lines in /etc/sysctl.conf:

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1

2. To make the settings affective, execute :

# sysctl -p

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

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

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