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I've been wrestling with sysctl to get it to keep iPv6 disabled and find it works fine if I set it up like so:
$ cat /etc/sysctl.d/10-ipv6-off.conf
net.ipv6.conf.all.disable_ipv6=1
net.ipv6.conf.default.disable_ipv6=1
net.ipv6.conf.lo.disable_ipv6=1
net.ipv6.conf.eth0.disable_ipv6=1
And then reload sysctl:
$ sysctl --system
But after rebooting I find that IPv6 gets re-enabled. How can I permanently disable IPv6?
So what ended up working in this situation was to simply tell the kernel to disable IPv6 during boot. To accomplish this I merely added this string ipv6.disable=1 to the cmdline.txt file in `/boot.
$ cat /boot/firmware/cmdline.txt
net.ifnames=0 dwc_otg.lpm_enable=0 console=serial0,115200 console=tty1 root=LABEL=writable rootfstype=ext4 elevator=deadline rootwait fixrtc ipv6.disable=1
After doing so you can confirm that the kernel no longer provides access to IPv6 by looking in this directory:
$ ls /proc/sys/net/
core ipv4 netfilter unix
When IPv6 is enabled, the sub-directory, ipv6 shows up in this location.
You can also check to make sure none of your NIC interfaces have inet6 on them as well:
$ ip a l | grep inet6
$