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https://askubuntu.com/questions/41543/h ... -interface

With at the bottom the comment "I wanted to stress that there is no way to re-enable without a reboot"
Setting the lines in the sysctl.conf to 0 does not work in any case.

MDeen wrote: ↑Tue Aug 21, 2018 7:44 pm https://askubuntu.com/questions/41543/h ... -interface

With at the bottom the comment "I wanted to stress that there is no way to re-enable without a reboot"
Setting the lines in the sysctl.conf to 0 does not work in any case.

That comment refers to that change in /etc/default/grub, but that change does not have an effect on a running system. Sure, if you reboot with that option set, you won't have IPv6. But you haven't rebooted yet.

Note: Before you try these, make sure you have some sort of a console access to your Ubuntu (not ssh) because there is a good chance that networking does not come up as expected.

I set up a test virtual machine running Ubuntu 18.04.1 with IPv6 connectivity (manual). I then disabled its IPv6 by running sysctl net.ipv6.conf.all.disable_ipv6=1, and sure enough, my IPv6 address disappeared. I then re-enabled IPv6 functionality in kernel with sysctl net.ipv6.conf.all.disable_ipv6=0. However, when I disabled IPv6 it made my IPv6 address disappear from ifconfig output, and re-enabling IPv6 did not bring it back. For that, I needed to ip link set ens3 down && ip link set ens3 up (where ens3 is the name of my network interface, yours is probably different) and voilà, my IPv6 address shows up in ifconfig again. Perhaps you need to do that same "ip link set.." trick.

Well, that borked the network settings up good. I had no connectivity after that I had to manually configure my network after that.
I've got IPv6 disabled again, maybe I'll try again later.