netdev - Re: [PATCH net] ipv6/netfilter: Drop Packet Too Big with invalid payload

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Message-ID: <20210624090553.GB24271@breakpoint.cc>
Date:   Thu, 24 Jun 2021 11:05:53 +0200
From:   Florian Westphal <fw@...len.de>
To:     Georg Kohmann <geokohma@...co.com>
Cc:     netdev@...r.kernel.org, coreteam@...filter.org,
        netfilter-devel@...r.kernel.org, kuba@...nel.org,
        davem@...emloft.net, fw@...len.de, kadlec@...filter.org,
        pablo@...filter.org
Subject: Re: [PATCH net] ipv6/netfilter: Drop Packet Too Big with invalid
 payload

Georg Kohmann <geokohma@...co.com> wrote:
> PMTU is updated even though received ICMPv6 PTB do not match any
> transmitted traffic. This breaks TAHI IPv6 Core Conformance Test
> Revision 5.0.1, v6LC.4.1.12 Validate Packet Too Big[1].
> 
> Referring to RFC8201 IPv6 Path MTU Discovery, section 4: "Nodes should
> appropriately validate the payload of ICMPv6 PTB messages to ensure
> these are received in response to transmitted traffic (i.e., a reported
> error condition that corresponds to an IPv6 packet actually sent by the
> application) per [ICMPv6]."
> 
> nf_conntrack_inet_error() return -NF_ACCEPT if the inner header of
> ICMPv6 error packet is not related to an existing connection. Drop PTB
> packet when this occur. This will prevent ipv6 from handling the packet
> and update the PMTU.

This is intentional. We try to not auto-drop packets in conntrack.

Packet is marked as invalid, users can add nft/iptables rules to discard
such packets if they want to do so.