Toralf Förster <toralf.foers...@gmx.de> wrote:
> On 03/10/2018 10:16 PM, Florian Westphal wrote:
> > Toralf Förster <toralf.foers...@gmx.de> wrote:
> >> At my server (stable hardened Gentoo with vanilla 4.15.7) I do have this 
> >> rule:
> >>
> >> /sbin/iptables -A OUTPUT -p tcp --destination-port 443 --syn --match 
> >> connlimit --connlimit-above 3000 --connlimit-mask 0 --connlimit-daddr 
> >> --match limit --limit 1/second --limit-burst 1 -j LOG --log-prefix "443 
> >> hammered "
> >>
> >> Sometimes (usually after 2-4 days uptime) however this rule fires too 
> >> often - even over hours.
> >>
> >> When I restart the iptable script then the rule stopps to fire. This 
> >> happened few times in a row over the last weeks. So I'm convinced that 
> >> there wasn't an external event related to this behaviour.
> > 
> > You could check via conntrack -L.
> 
> HHm, the output of "conntrack -L" let me wonder if the rule gave the output I 
> wanted to achieve.
> I'd like to get a log entry, if there are more than 3,000 new SYN connections 
> within the last second, not, if there are 3,000 SYN at all.

connlimit keeps state of total connetions.
So if there are 3000 outgoing connections to port 443, and there is one
more, then connlimit will start to match.

AFAIU from your description you should replace connlimit with

-m hashlimit --hashlimit-above 3003/sec --hashlimit-name synhash

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