linux-kernel - Re: [External] Re: [PATCH] tcp: fix silent loss when syncookie is trigered

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Message-ID: <CAPXF5UVG+c0STZORvdaz6Mk8fwxE7DTBtTp=uF51xMrFL0R02w@mail.gmail.com>
Date:   Sat, 17 Apr 2021 06:45:35 +0800
From:   赵亚 <zhaoya.gaius@...edance.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     David Miller <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Florian Westphal <fw@...len.de>
Subject: Re: [External] Re: [PATCH] tcp: fix silent loss when syncookie is trigered

On Fri, Apr 16, 2021 at 7:52 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Fri, Apr 16, 2021 at 12:52 PM zhaoya <zhaoya.gaius@...edance.com> wrote:
> >
> > When syncookie is triggered, since $MSSID is spliced into cookie and
> > the legal index of msstab  is 0,1,2,3, this gives client 3 bytes
> > of freedom, resulting in at most 3 bytes of silent loss.
> >
> > C ------------seq=12345-------------> S
> > C <------seq=cookie/ack=12346-------- S S generated the cookie
> >                                         [RFC4987 Appendix A]
> > C ---seq=123456/ack=cookie+1-->X      S The first byte was loss.
> > C -----seq=123457/ack=cookie+1------> S The second byte was received and
> >                                         cookie-check was still okay and
> >                                         handshake was finished.
> > C <--------seq=.../ack=12348--------- S acknowledge the second byte.
>
>
> I think this has been discussed in the past :
> https://kognitio.com/blog/syn-cookies-ate-my-dog-breaking-tcp-on-linux/
>
> If I remember well, this can not be fixed "easily"
>
> I suspect you are trading one minor issue with another (which is
> considered more practical these days)
> Have you tried what happens if the server receives an out-of-order
> packet after the SYN & SYN-ACK ?
> The answer is : RST packet is sent, killing the session.
>
> That is the reason why sseq is not part of the hash key.

Yes, I've tested this scenario. More sessions do get reset.

If a client got an RST, it knew the session failed, which was clear. However,
if the client send a character and it was acknowledged, but the server did not
receive it, this could cause confusion.
>
> In practice, secure connexions are using a setup phase where more than
> 3 bytes are sent in the first packet.
> We recommend using secure protocols over TCP. (prefer HTTPS over HTTP,
> SSL over plaintext)

Yes, i agree with you. But the basis of practice is principle.
Syncookie breaks the
semantics of TCP.
>
> Your change would severely impair servers under DDOS ability to really
> establish flows.

Would you tell me more details.
>
> Now, if your patch is protected by a sysctl so that admins can choose
> the preferred behavior, then why not...

The sysctl in the POC is just for triggering problems easily.

So the question is, when syncookie is triggered, which is more important,
the practice or the principle?