Disable IPv6 addresses on Ubuntu

IPv6 may be undesirable in certain situations, for systems that can run without IPv6 and only operate on IPv4, the IPv6 protocol stack can be disabled.

On Debian and Ubuntu the IPv6 stack can be disabled as follows

sudo vi /etc/sysctl.d/99-sysctl.conf

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

If only one certain interface should run without IPv6, e.g. ens160.

net.ipv6.conf.ens160.disable_ipv6 = 1

Save the sysctl.conf file and enable the setting.

sudo sysctl -p

If you want to deactivate IPv6 during operation, the following command applies.

echo 1 > /proc/sys/net/ipv6/conf/all/disable_ipv6
echo 1 > /proc/sys/net/ipv6/conf/default/disable_ipv6

Alternatively you can run this command in the terminal shell.

sysctl -w net.ipv6.conf.all.disable_ipv6=1
sysctl -w net.ipv6.conf.default.disable_ipv6=1

IPv6 is now disabled, a reboot is not necessary.

If the removal of IPv6 for running services is a disaster, e.g. SSH server, the configuration can be adjusted.

vi /etc/ssh/sshd_config

#AddressFamily any
AddressFamily inet
#or remove hash(#)
#ListenAddress 0.0.0.0
ListenAddress 0.0.0.0

Then restart the SSH daemon.

systemctl restart sshd

Postfix may also require to customize the ipv4 loopback interface.

vi /etc/postfix/main.cf

#inet_interfaces = localhost
inet_interfaces = 127.0.0.1

Checking for IPv6 addresses.

ifconfig | grep inet6