netdev - Re: Why not use all the syn queues? in the function "tcp_conn

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Date:   Tue, 4 Sep 2018 00:23:32 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Ttttabcd <ttttabcd@...tonmail.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Why not use all the syn queues? in the function
 "tcp_conn_request", I have some questions.



On 09/03/2018 10:31 PM, Ttttabcd wrote:
> Hello everyone,recently I am looking at the source code for handling TCP three-way handshake(Linux Kernel version 4.18.5).
> 
> I found some strange places in the source code for handling syn messages.
> 
> in the function "tcp_conn_request"
> 
> This code will be executed when we don't enable the syn cookies.
> 
> 		if (!net->ipv4.sysctl_tcp_syncookies &&
> 		    (net->ipv4.sysctl_max_syn_backlog - inet_csk_reqsk_queue_len(sk) <
> 		     (net->ipv4.sysctl_max_syn_backlog >> 2)) &&
> 		    !tcp_peer_is_proven(req, dst)) {
> 			/* Without syncookies last quarter of
> 			 * backlog is filled with destinations,
> 			 * proven to be alive.
> 			 * It means that we continue to communicate
> 			 * to destinations, already remembered
> 			 * to the moment of synflood.
> 			 */
> 			pr_drop_req(req, ntohs(tcp_hdr(skb)->source),
> 				    rsk_ops->family);
> 			goto drop_and_release;
> 		}
> 
> But why don't we use all the syn queues?


Isn't it explained in the comment ?

Anyway, I am not sure anyone disables syn cookies.