Disable IPv6 on Ubuntu 20.04

Question

Simple way that worked for a very long time was adding in /etc/sysctl.conf

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

Afterwards sysctl -p for activation of the setting.
It works for a while, but suddenly the IPv6 is enabled.
We are facing two things:

1. IPv6 is randomly enabled
2. After restart of the server, IPv6 is enabled again.

Checking the proc:

$ cat /proc/sys/net/ipv6/conf/eth0/disable_ipv6
1

I want this disabled, there is a postfix server and gmail is objecting while IPv6 is active.

Solution

To permanently disable ipv6, you can update your grub file. You'll need administrative rights.

Edit /etc/default/grub with your preferred text editor. Make sure ipv6.disable=1 is added to the GRUB_CMDLINE_LINUX lines as follows:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash ipv6.disable=1"
GRUB_CMDLINE_LINUX="ipv6.disable=1"

Then

sudo update-grub

And you should be good after a reboot.

Comments

[Admin]

However, seems to be ok lately (few months already), I didn't figure out what was wrong. Maybe some ubuntu update fixed that.

[krubo]

Worked great for a few months, but now on 5.11.0-44 it still uses IPv6 even with setting ipv6.disable=1 :/

[Jakke]

It is no issue with 5.11.0.44, I'm running the same and still disabled. Double check if nothing has changed your grub config... linuxconfig.org/… Also you can check if ipv6 is really disabled in different ways golinuxcloud.com/linux-check-ipv6-enabled