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);
	}