Please consider backporting the following commit to 4.14:
commit 094009531612246d9e13f9e0c3ae2205d7f63a0a Author: Nicolas Dichtel nicolas.dichtel@6wind.com Date: Tue Nov 14 14:21:32 2017 +0100
ipv6: set all.accept_dad to 0 by default
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i...
This fixes a regression introduced in 4.14 that has since been fixed in mainline.
Thanks, Dan