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Date: Sun, 28 Oct 2012 21:02:48 +0100
From: Eric Dumazet <eric.dumazet@...il.com>
To: Julian Anastasov <ja@....bg>
Cc: David Miller <davem@...emloft.net>,
Vijay Subramanian <subramanian.vijay@...il.com>,
netdev@...r.kernel.org, ncardwell@...gle.com,
Venkat Venkatsubra <venkat.x.venkatsubra@...cle.com>,
Elliott Hughes <enh@...gle.com>,
Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH net-next] tcp: better retrans tracking for defer-accept
On Sun, 2012-10-28 at 18:51 +0200, Julian Anastasov wrote:
> In fact, my concern was for a case where client can
> flood us with same SYN. My idea was if 5 SYN-ACKs were
> sent in first second, request_sock to expire even when
> num_timeout is changing from 0 to 1. I.e. request_sock
> to expire based on SYN-ACK count, not on fixed time.
>
> But I'm not sure what is better here,
> to expire request_sock immediately when SYN-ACK reaches
> limit or to keep it 63 secs so that we can reduce our
> SYN-ACK rate under such SYN attacks. And not only
> under attack.
>
> Here is what happens if we add DROP rule for
> SYN-ACKs. We can see that every SYN retransmission is
> followed by 2 SYN-ACKs, here is example with loopback:
>
> Initial SYN and SYN-ACK:
> 12:21:45.773023 IP 127.0.0.1.38450 > 127.0.0.1.22: Flags [S], seq 2096477888, win 32792, options [mss 16396,sackOK,TS val 7978589 ecr 0,nop,wscale 6], length 0
> 12:21:45.773051 IP 127.0.0.1.22 > 127.0.0.1.38450: Flags [S.], seq 1774312921, ack 2096477889, win 32768, options [mss 16396,sackOK,TS val 7978589 ecr 7978589,nop,wscale 6], length 0
>
> SYN retr 1:
> 12:21:46.775816 IP 127.0.0.1.38450 > 127.0.0.1.22: Flags [S], seq 2096477888, win 32792, options [mss 16396,sackOK,TS val 7979592 ecr 0,nop,wscale 6], length 0
> immediate SYN-ACK from tcp_check_req:
> 12:21:46.775843 IP 127.0.0.1.22 > 127.0.0.1.38450: Flags [S.], seq 1774312921, ack 2096477889, win 32768, options [mss 16396,sackOK,TS val 7979592 ecr 7978589,nop,wscale 6], length 0
> SYN-ACK from inet_csk_reqsk_queue_prune timer:
> 12:21:46.975807 IP 127.0.0.1.22 > 127.0.0.1.38450: Flags [S.], seq 1774312921, ack 2096477889, win 32768, options [mss 16396,sackOK,TS val 7979792 ecr 7978589,nop,wscale 6], length 0
>
> same for retr 2..5:
> 12:21:48.779809 IP 127.0.0.1.38450 > 127.0.0.1.22: Flags [S], seq 2096477888, win 32792, options [mss 16396,sackOK,TS val 7981596 ecr 0,nop,wscale 6], length 0
> 12:21:48.779837 IP 127.0.0.1.22 > 127.0.0.1.38450: Flags [S.], seq 1774312921, ack 2096477889, win 32768, options [mss 16396,sackOK,TS val 7981596 ecr 7978589,nop,wscale 6], length 0
> 12:21:48.975789 IP 127.0.0.1.22 > 127.0.0.1.38450: Flags [S.], seq 1774312921, ack 2096477889, win 32768, options [mss 16396,sackOK,TS val 7981792 ecr 7978589,nop,wscale 6], length 0
>
> This is a waste of bandwidth too. It is true that
> client can use different TCP_TIMEOUT_INIT value and this timing
> may look different if both sides use different value.
> The most silly change I can think of is to add something
> like this in syn_ack_recalc (not tested at all):
>
> /* Avoid double SYN-ACK if client is resending SYN faster:
> * (num_timeout - num_retrans) >= 0
> */
> *resend = !((req->num_timeout - req->num_retrans) & 0x40);
>
> if (!rskq_defer_accept) {
> *expire = req->num_timeout >= thresh;
> return;
> }
> *expire = req->num_timeout >= thresh &&
> (!inet_rsk(req)->acked || req->num_timeout >= max_retries);
> /*
> * Do not resend while waiting for data after ACK,
> * start to resend on end of deferring period to give
> * last chance for data or ACK to create established socket.
> */
> if (inet_rsk(req)->acked)
> *resend = req->num_timeout >= rskq_defer_accept - 1;
>
> If we add some checks in tcp_check_req we can also
> restrict the immediate SYN-ACKs up to tcp_synack_retries.
>
> The idea is:
>
> - expire request_sock as before, based on num_timeout with
> the idea to catch many SYN retransmissions and to reduce
> SYN-ACK rate from 2*SYN_rate to 1*SYN_rate, up to
> tcp_synack_retries SYN-ACKs
>
> - num_retrans accounts sent SYN-ACKs, they can be sent in
> response to SYN retr or from timer. If num_retrans increases
> faster than num_timeout it means client uses lower
> TCP_TIMEOUT_INIT value and sending SYN-ACKs from
> tcp_check_req is enough because we apply tcp_synack_retries
> once as a SYN-ACK limit and second time as expiration
> period.
>
> - If we get 10 SYNs in 1 second, we will send 5 SYN-ACKs
> immediately (will be restricted in tcp_check_req), from
> second +1 to +31 we will not send SYN-ACKs if
> tcp_synack_retries is reached, we will wait for ACK and
> for more SYNs to drop, silently. Finally, at +63 we expire
> the request_sock. inet_csk_reqsk_queue_prune still
> can reduce the expiration period (thresh value) under load.
>
> Of course, this is material for separate patch,
> if idea is liked at all.
>
> Regards
On a SYNFLOOD attack, we end up sending one SYNACK per SYN message
anyway ?
If we want to address a non SYNFLOOD attack, why not resetting
req->expire when we send a SYNACK to a retransmitted SYN ?
tcp_check_req()
...
if (!inet_rtx_syn_ack(sk, req)) {
req->expire = jiffies +
min(TCP_TIMEOUT_INIT << req->num_timeout,
TCP_RTO_MAX);
}
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