Comment 3 for bug 1556419

​https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/commit/?id=a9006892643a8f4e885b692de0708bcb35a7d530
netfilter: nf_ct_helper: allow to disable automatic helper assignment

This patch allows you to disable automatic conntrack helper lookup based on TCP/UDP ports, eg. echo 0 > /proc/sys/net/netfilter/nf_conntrack_helper [ Note: flows that already got a helper will keep using it even if automatic helper assignment has been disabled ] Once this behaviour has been disabled, you have to explicitly use the iptables CT target to attach helper to flows. There are good reasons to stop supporting automatic helper assignment, for further information, please read: ​http://www.netfilter.org/news.html#2012-04-03 This patch also adds one message to inform that automatic helper assignment is deprecated and it will be removed soon (this is spotted only once, with the first flow that gets a helper attached to make it as less annoying as possible).

​https://home.regit.org/netfilter-en/secure-use-of-helpers/