netdev - Re: net.ipv4.tcp_max_syn_backlog implementation

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Date:   Mon, 28 Aug 2017 21:17:23 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Harsha Chenji <cjkernel@...il.com>
Cc:     netdev@...r.kernel.org
Subject: Re: net.ipv4.tcp_max_syn_backlog implementation

On Mon, 2017-08-28 at 23:47 -0400, Harsha Chenji wrote:
> 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?

See 2nd argument in listen() system call, ie backlog 

man listen

Without a synflood, just look at "ss -t state listening" 

The backlog is the 2nd column (Send)