netdev - Re: [PATCH net-next 09/10] netfilter: get ipv6 pktlen properly in length

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Date:   Thu, 19 Jan 2023 04:13:32 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Xin Long <lucien.xin@...il.com>
Cc:     David Ahern <dsahern@...il.com>,
        network dev <netdev@...r.kernel.org>, davem@...emloft.net,
        kuba@...nel.org, Paolo Abeni <pabeni@...hat.com>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Pravin B Shelar <pshelar@....org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Florian Westphal <fw@...len.de>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Ilya Maximets <i.maximets@....org>,
        Aaron Conole <aconole@...hat.com>,
        Roopa Prabhu <roopa@...dia.com>,
        Nikolay Aleksandrov <razor@...ckwall.org>,
        Mahesh Bandewar <maheshb@...gle.com>,
        Paul Moore <paul@...l-moore.com>,
        Guillaume Nault <gnault@...hat.com>
Subject: Re: [PATCH net-next 09/10] netfilter: get ipv6 pktlen properly in length_mt6

On Thu, Jan 19, 2023 at 2:19 AM Xin Long <lucien.xin@...il.com> wrote:

> I think that IPv6 BIG TCP has a similar problem, below is the tcpdump in
> my env (RHEL-8), and it breaks too:
>
> 19:43:59.964272 IP6 2001:db8:1::1 > 2001:db8:2::1: [|HBH]
> 19:43:59.964282 IP6 2001:db8:1::1 > 2001:db8:2::1: [|HBH]
> 19:43:59.964292 IP6 2001:db8:1::1 > 2001:db8:2::1: [|HBH]
> 19:43:59.964300 IP6 2001:db8:1::1 > 2001:db8:2::1: [|HBH]
> 19:43:59.964308 IP6 2001:db8:1::1 > 2001:db8:2::1: [|HBH]
>

Please make sure to use a not too old tcpdump.

> it doesn't show what we want from the TCP header either.
>
> For the latest tcpdump on upstream, it can display headers well for
> IPv6 BIG TCP. But we can't expect all systems to use the tcpdump
> that supports HBH parsing.

User error. If an admin wants to diagnose TCP potential issues, it should use
a correct version.

>
> For IPv4 BIG TCP, it's just a CFLAGS change to support it in "tcpdump,"
> and 'tshark' even supports it by default.

Not with privacy _requirements_, where only the headers are captured.

I am keeping a NACK, until you make sure you do not break this
important feature.

>
> I think we should NOT go with "adjust tot_len" or "truncate packets" way,
> and it makes more sense to make it supported in "tcpdump" by default,
> just like in "tshark". I believe commit [1] was added for some problems
> they've met, we should enable it for both.
>
> [1] https://github.com/the-tcpdump-group/tcpdump/commit/c8623960f050cb81c12b31107070021f27f14b18
>
> Thanks.