netdev - Re: [PATCH net] netfilter: socket: Lookup orig tuple for IPv6 SNAT

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Message-ID: <20250318201323.GB840@breakpoint.cc>
Date: Tue, 18 Mar 2025 21:13:23 +0100
From: Florian Westphal <fw@...len.de>
To: Maxim Mikityanskiy <maxtram95@...il.com>
Cc: Pablo Neira Ayuso <pablo@...filter.org>,
	Jozsef Kadlecsik <kadlec@...filter.org>,
	"David S. Miller" <davem@...emloft.net>,
	David Ahern <dsahern@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>, Patrick McHardy <kaber@...sh.net>,
	KOVACS Krisztian <hidden@...abit.hu>,
	Balazs Scheidler <bazsi@...abit.hu>,
	netfilter-devel@...r.kernel.org, coreteam@...filter.org,
	netdev@...r.kernel.org, Maxim Mikityanskiy <maxim@...valent.com>
Subject: Re: [PATCH net] netfilter: socket: Lookup orig tuple for IPv6 SNAT

Maxim Mikityanskiy <maxtram95@...il.com> wrote:
> nf_sk_lookup_slow_v4 does the conntrack lookup for IPv4 packets to
> restore the original 5-tuple in case of SNAT, to be able to find the
> right socket (if any). Then socket_match() can correctly check whether
> the socket was transparent.
> 
> However, the IPv6 counterpart (nf_sk_lookup_slow_v6) lacks this
> conntrack lookup, making xt_socket fail to match on the socket when the
> packet was SNATed. Add the same logic to nf_sk_lookup_slow_v6.
> 
> IPv6 SNAT is used in Kubernetes clusters for pod-to-world packets, as
> pods' addresses are in the fd00::/8 ULA subnet and need to be replaced
> with the node's external address. Cilium leverages Envoy to enforce L7
> policies, and Envoy uses transparent sockets. Cilium inserts an iptables
> prerouting rule that matches on `-m socket --transparent` and redirects
> the packets to localhost, but it fails to match SNATed IPv6 packets due
> to that missing conntrack lookup.
> 
> Closes: https://github.com/cilium/cilium/issues/37932
> Fixes: b64c9256a9b7 ("tproxy: added IPv6 support to the socket match")

Note that this commit predates IPv6 NAT support in netfilter.
No need to send a v2, just saying.

Reviewed-by: Florian Westphal <fw@...len.de>