@evgeni Fixed up if you can review again

I just saw voxpupuli/facterdb#340 and noticed you can also read the sysctl. So technically you can enable it even if it's in /proc/cmdline, but I still think it's a good check because it means that after a reboot it'll also work. Or at least, more likely.

No, you really can't, ipv6.disable=1 completely removes IPv6 support from the kernel:

[root@centos9-stream ~]# cat /proc/cmdline 
BOOT_IMAGE=(hd0,msdos1)/boot/vmlinuz-5.14.0-354.el9.x86_64 root=UUID=a91e762e-6199-45c4-8989-1bba94306968 ro no_timer_check console=tty0 console=ttyS0,115200n8 net.ifnames=0 biosdevname=0 elevator=noop ipv6.disable=1
[root@centos9-stream ~]# sysctl -a |grep ipv6
[root@centos9-stream ~]# sysctl -w net.ipv6.conf.all.disable_ipv6=0
sysctl: cannot stat /proc/sys/net/ipv6/conf/all/disable_ipv6: No such file or directory
[root@centos9-stream ~]# ls -alh /proc/sys/net/ipv6/
ls: cannot access '/proc/sys/net/ipv6/': No such file or directory

Ah, so you can enable/disable it at runtime if the kernel was booted with IPv6 support, but if it's in /proc/cmdline then it surely isn't there.

That makes me wonder if we need both checks.

That makes me wonder if we need both checks.

I don't think so. ipv6.disable breaks things like bind to [::]. But disabling IPv6 on all interfaces (that's what the all in net.ipv6.conf.all is) means you have no ::1 etc, but I think a bind of [::] should still work, you'd just not get any v6 binds by that, so it's the same as 0.0.0.0.