How to enable IPv6 on CentOS / RHEL 6

IPv6 is by default enabled on CentOS/RHEL 6 systems. But in case, if it was disabled for some reason, you can follow the short howto below to re-enable it. There are basically 2 ways to disable IPv6 on CentOS/RHEL 6.

1. Disable IPv6 in kernel module (requires reboot)
2. Disable IPv6 using sysctl settings (no reboot required)

Method 1 : Using /etc/modprobe.d/ipv6.conf (requires reboot)

1. Confirm if ipv6 is used. IPV6 is not used if no lines with inet6 shows up.

# ifconfig |grep inet6
#

2. Modify the configuration file /etc/modprobe.d/*.conf or /etc/modprobe.conf which prevents kernel from loading ipv6 module

# grep ipv6 /etc/modprobe.conf /etc/modprobe.d/*.conf
/etc/modprobe.d/ipv6.conf:1:options ipv6 disable=1        ### we need to remove this line

After removing (or hashing it out) the above line, verify:

# cat /etc/modprobe.d/ipv6.conf
# options ipv6 disable=1

3. Enable ip6tables service if it is needed

# chkconfig ip6tables on

4. Reboot

# shutdown -r now

5. Verify if you can now see the inet6 line in the ifconfig command output.

# ifconfig |grep inet6
          inet6 addr: 2405:200:a60:fd00:250:56ff:fe96:f0e/64 Scope:Global
          inet6 addr: fe80::250:56ff:fe96:f0e/64 Scope:Link
          inet6 addr: ::1/128 Scope:Host

Method 2 : Using /etc/sysctl.conf (online)

1. Append below line to the file /etc/sysctl.conf.

# vi /etc/sysctl.conf
# IPv6 support in the kernel, set to 0 by default
net.ipv6.conf.all.disable_ipv6 = 0
net.ipv6.conf.default.disable_ipv6 = 0

2. To apply the changes for the current sysctl parameters, execute :

# sysctl -p

3. Use, ‘sysctl -a’ to verify the parameter values.

# sysctl -a | grep net.ipv6.conf
net.ipv6.conf.all.disable_ipv6 = 0
net.ipv6.conf.default.disable_ipv6 = 0