How to check or disable IPv6

Check for IPv6 support in the current running kernel

To check, whether your current running kernel supports IPv6, take a look into your /proc-file-system. Following entry must exists:

$ cat /proc/net/if_inet6
Output:

00000000000000000000000000000001 01 80 10 80       lo
fe8000000000000002179afffe0af644 03 40 20 80      ra0
fe800000000000000219d1fffe2abaa8 02 40 20 80     eth0

IPv6 protocol stack for Linux via ipv6 module - Turn on IPv6 Support

ipv6 module has IPv6 protocol stack for Linux. If above cat command fails, it is quite likely, that the IPv6 module is not loaded. Just login as root and type the following command, enter:
# modprobe ipv6
Now test again, enter:
$ lsmod | grep ipv6Output:

ipv6                  411425  18

Try following simple shell conditional code to display human readable output, enter:
$ [ -f /proc/net/if_inet6 ] && echo 'IPv6 ready system!' || echo 'No IPv6 support found! Compile the kernel!!'
Output:

IPv6 ready system!

Disabling IPv6 on more recent systems works the same way as described in the Debian wiki:

Disable ipv6 in kernel : echo net.ipv6.conf.all.disable_ipv6=1 > /etc/sysctl.d/disableipv6.conf will disable ipv6 at next reboot.

 sysctl --system

Keep in mind that, whatever problem you are having, this should be an absolute last resort after you have exhausted all other options for fixing the problem. IPv6 is not optional and you will have to turn it back on sooner or later.

To check if IPv6 is disabled through sysctl :

sysctl -a|grep disable_ipv6