On Wed, Oct 18, 2023 at 8:19 AM Martin KaFai Lau <martin.lau@xxxxxxxxx> wrote:
>
> On 10/17/23 9:48 AM, Kuniyuki Iwashima wrote:
> > From: Martin KaFai Lau <martin.lau@xxxxxxxxx>
> > Date: Mon, 16 Oct 2023 22:53:15 -0700
> >> On 10/13/23 3:04 PM, Kuniyuki Iwashima wrote:
> >>> Under SYN Flood, the TCP stack generates SYN Cookie to remain stateless
> >>> After 3WHS, the proxy restores SYN and forwards it and ACK to the backend
> >>> server. Our kernel module works at Netfilter input/output hooks and first
> >>> feeds SYN to the TCP stack to initiate 3WHS. When the module is triggered
> >>> for SYN+ACK, it looks up the corresponding request socket and overwrites
> >>> tcp_rsk(req)->snt_isn with the proxy's cookie. Then, the module can
> >>> complete 3WHS with the original ACK as is.
> >>
> >> Does the current kernel module also use the timestamp bits differently?
> >> (something like patch 8 and patch 10 trying to do)
> >
> > Our SYN Proxy uses TS as is. The proxy nodes generate a random number
> > if TS is in SYN.
> >
> > But I thought someone would suggest making TS available so that we can
> > mock the default behaviour at least, and it would be more acceptable.
> >
> > The selftest uses TS just to strengthen security by validating 32-bits
> > hash. Dropping a part of hash makes collision easier to happen, but
> > 24-bits were sufficient for us to reduce SYN flood to the managable
> > level at the backend.
>
> While enabling bpf to customize the syncookie (and timestamp), I want to explore
> where can this also be done other than at the tcp layer.
>
> Have you thought about directly sending the SYNACK back at a lower layer like
> tc/xdp after receiving the SYN? There are already bpf_tcp_{gen,check}_syncookie
> helper that allows to do this for the performance reason to absorb synflood. It
> will be natural to extend it to handle the customized syncookie also.
>
> I think it should already be doable to send a SYNACK back with customized
> syncookie (and timestamp) at tc/xdp today.
>
> When ack is received, the prog@tc/xdp can verify the cookie. It will probably
> need some new kfuncs to create the ireq and queue the child socket. The bpf prog
> can change the ireq->{snd_wscale, sack_ok...} if needed. The details of the
> kfuncs need some more thoughts. I think most of the bpf-side infra is ready,
> e.g. acquire/release/ref-tracking...etc.
>
I think I mostly agree with this.
I am rebasing a patch adding usec resolution to TCP TS,
that we used for about 10 years at Google, because it is time to upstream it.
I am worried about more changes/conflicts caused by Kuniyuki patch set...
