On Thu, Feb 17, 2022 at 02:01:49PM +0900, Christian Balzer via Exim-users wrote
> If found it excruciatingly hard to correlate tcpdump and nf_conntrack
> flows,

 These data can be related via timestamps, they may be enabled for
 conntrack output:

   conntrack -o timestamp,ktimestamp -E ...

 Note that timestamping for kernel module should be enabled via option
 net.netfilter.nf_conntrack_timestamp (read man conntrack for details).

> but those ICMP6 destination unreachable packets are the result of
> the local iptables rejecting a connection to port 43922 (the originating
> outbound SMTP session from here), something it allowed for the first 2
> seconds just fine.
> 
> The:
> ---
> -A INPUT -p icmpv6 -j ACCEPT
> -A INPUT -i bond+ -m state --state ESTABLISHED,RELATED -j ACCEPT
> ---

 No rejection rules here. Look for your iptables rules to find sources
 of rejection, then insert logging rules to debug.
-- 
 Eugene Berdnikov

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