From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Miller Subject: Re: [PATCH] tcp: Restore RFC5961-compliant behavior for SYN packets Date: Fri, 21 Nov 2014 15:34:38 -0500 (EST) Message-ID: <20141121.153438.43820255874630231.davem@davemloft.net> References: <1416526940.8629.62.camel@edumazet-glaptop2.roam.corp.google.com> <20141121014727.GA1536781@mail.thefacebook.com> <1416536527.8629.74.camel@edumazet-glaptop2.roam.corp.google.com> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: calvinowens@fb.com, kuznet@ms2.inr.ac.ru, jmorris@namei.org, edumazet@google.com, kernel-team@fb.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org To: eric.dumazet@gmail.com Return-path: In-Reply-To: <1416536527.8629.74.camel@edumazet-glaptop2.roam.corp.google.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org From: Eric Dumazet Date: Thu, 20 Nov 2014 18:22:07 -0800 > On Thu, 2014-11-20 at 17:47 -0800, Calvin Owens wrote: > >> That's actually not what led to finding this, but it's a good point. :) >> >> What if the challenge-ACK counter were decremented in tcp_validate_incoming() >> when a valid RST packet is seen? That would allow legitimate remote >> hosts to reestablish connections without being ratelimited, and still >> prevent a malicious host from guessing sequence numbers. >> >> There would need to be a way to tell if a challenge ACK had in fact been >> sent and only decrement in that case, since otherwise a local attacker >> could establish and immediately reset lots of connections to keep the >> counter below the ratelimit threshold and guess sequence numbers. >> >> Simply adding a flag to struct tcp_sock would work: just set the flag >> whenever a challenge ACK is sent, and clear it and decrement the counter >> only if it is set when a valid RST packet is seen. > > Seems tricky, a Challenge ACK do not necessarily gives an RST. > > Anyway this certainly can wait, as we already have a sysctl to > eventually work around the issue. > > Acked-by: Eric Dumazet Applied, thanks everyone.