There are many ways to disable IPv6 in Linux, so you'll have to check for them all.
First, your kernel version does not quite look like Debian 10 standard AMD64 kernel. If you are using a custom kernel, IPv6 may be disabled at kernel compilation time, in which case you'll need to recompile the kernel (or switch to another kernel) to get it enabled.
grep CONFIG_IPV6 /boot/config-$(uname -r) and see what it says. If the response includes a line:
# CONFIG_IPV6 is not set
then your current kernel had its IPv6 support disabled at compilation time.
If the response includes a line:
then the IPv6 support is compiled as a kernel module.
If there is instead a line:
then IPv6 support is compiled in to the main kernel. In this case, one way to disable IPv6 would be to add boot option
ipv6.disable=1 to the kernel command line (in
/etc/default/grub) but you said you've already checked for that.
IPv6 compiled as a module
modprobe -c |grep "options ipv6" as root. If the response includes a line:
options ipv6 disable=1
then IPv6 is disabled by kernel module configuration, and you should find that line in one of the files in
To re-enable IPv6 in this case, find that line, comment it out, and run
update-initramfs -u as root to make sure IPv6 does not get disabled at early boot, as this disabling strategy may require a reboot to re-enable.
(In theory, unloading and re-loading the
ipv6 after commenting out the disable option should suffice, but because of dependencies between modules, it might be hard to actually do without rebooting.)
Methods applicable for both modular and compiled-in IPv6
You should also check the sysctl settings:
grep "disable_ipv6" /etc/sysctl.conf /etc/sysctl.d/*.conf
If the output includes lines like this, then IPv6 has been disabled through sysctl settings:
net.ipv6.conf.<something>.disable_ipv6 = 1
<something> might be the word
default, or a name of a specific network interface. To re-enable, comment out the
disable_ipv6 lines and run
sysctl -p as root, then reboot (or just reconfigure your network interfaces).