netdev - net.ipv4.tcp_max_syn_backlog implementation

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Message-ID: <CAMB9Wx+q9fidTnh3Tyias8KKdbRcKpoXM-ntSqjE_oW36F1W_A@mail.gmail.com>
Date:   Mon, 28 Aug 2017 23:47:41 -0400
From:   Harsha Chenji <cjkernel@...il.com>
To:     netdev@...r.kernel.org
Subject: net.ipv4.tcp_max_syn_backlog implementation

So I have ubuntu 12.04 x32 in a VM with syncookies turned off. I tried
to do a syn flood (with netwox) on 3 different processes. Each of them
returns a different value with netstat -na | grep -c RECV :

nc -l 5555 returns 16 (netcat-traditional)
apache2 port 80 returns 256
vsftpd on 21 returns 64.
net.ipv4.tcp_max_syn_backlog is 512.

Why do these different processes on different ports have different
queue lengths for incomplete connections? Where exactly in the kernel
is this decided?