A memo when I took the time to investigate the subject
First, as a common pattern, execute the following to reboot
net.ipv6.conf.all.disable_ipv6=1 net.ipv6.conf.default.disable_ipv6=1 net.ipv6.conf.lo.disable_ipv6 = 1 net.ipv6.conf.enp0s3.disable_ipv6 = 1
If you check with
ip a after restarting, it remains persistently.
$sudo sysctl --all | grep disable_ipv6 net.ipv6.conf.all.disable_ipv6 = 1 net.ipv6.conf.default.disable_ipv6 = 1 net.ipv6.conf.enp0s3.disable_ipv6 = 0 net.ipv6.conf.lo.disable_ipv6 = 1
It doesn't seem to work because it is managed by Network manager. Then execute the following magic and reboot (enp0s3 should be read depending on the environment)
$sudo nmcli connection modify enp0s3 ipv6.method ignore
Now when you look at
ip a, it's disabled.
Even if you look at netstat in this state, apache etc. is displayed as LISTEN in IPv6. More on this below. https://qiita.com/fetaro/items/d5164ba8271114d8a0c8