Re: [PATCH nf v2] netfilter: conntrack: connection timeout after re-register

Florian Westphal
2020-10-09T20:08:46Z

Jozsef Kadlecsik <kadlec@netfilter.org> wrote:
> > The "delay unregister" remark was wrt. the "all rules were deleted"
> > case, i.e. add a "grace period" rather than acting right away when
> > conntrack use count did hit 0.
>
> Now I understand it, thanks really. The hooks are removed, so conntrack
> cannot "see" the packets and the entries become stale.

Yes.

> What is the rationale behind "remove the conntrack hooks when there are no
> rule left referring to conntrack"? Performance optimization? But then the
> content of the whole conntrack table could be deleted too... ;-)

Yes, this isn't the case at the moment -- only hooks are removed,
entries will eventually time out.

> > Conntrack entries are not removed, only the base hooks get unregistered.
> > This is a problem for tcp window tracking.
> >
> > When re-register occurs, kernel is supposed to switch the existing
> > entries to "loose" mode so window tracking won't flag packets as
> > invalid, but apparently this isn't enough to handle keepalive case.
>
> "loose" (nf_ct_tcp_loose) mode doesn't disable window tracking, it
> enables/disables picking up already established connections.
>
> nf_ct_tcp_be_liberal would disable TCP window checking (but not tracking)
> for non RST packets.

You are right, mixup on my part.

> But both seems to be modified only via the proc entries.

Yes, we iterate table on re-register and modify the existing entries.