[RFC nf-next 0/4] netfilter: conntrack: allow insertion of clashing entries

[Florian Westphal <fw@strlen.de>]

This series allows conntrack to insert a duplicate conntrack entry
if the reply direction doesn't result in a clash with a different
original connection.

Background:

kubernetes creates load-balancing rules for DNS using
-m statistics, e.g.:
-p udp --dport 53 -m statistics --mode random ... -j DNAT --to-destination x
-p udp --dport 53 -m statistics --mode random ... -j DNAT --to-destination y

When the resolver sends an A and AAAA request back-to-back from
different threads on the same socket, this has a high chance of a connection
tracking clash at insertion time.

This in turn results in a drop of the clashing udp packet which then
results in a 5 second DNS timeout.

The clash cannot be resolved with the current logic because the
two conntracks entries have different NAT transformations, the first one
from s:highport to x.53, the second from s:highport to y.53.

One solution is to change rules to use a consistent mapping, e.g.
using -m cluster or nftables 'jhash' expression.  This would cause
the A and AAAA requests coming from same socket to match the same
rule and thus share the same NAT information.

This change adds a second clash resolution/drop avoidance step:
A clashing entry will be added anyway provided the reply direction
is unique.

Because this results in duplicate conntrack entries for the original
direction, this comes with strings attached:
1. The clashed conntrack entry will only be around for 3 seconds
2. The clashed entry will still fail to be inserted if hash
   chain grew too large.

This entire series isn't nice but so far I did not find a better
solution.

Comments welcome.