netdev - Re: [PATCH net] net: increase SOMAXCONN to 4096

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Date:   Thu, 31 Oct 2019 04:36:32 +0100
From:   Willy Tarreau <w@....eu>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>, Yue Cao <ycao009@....edu>
Subject: Re: [PATCH net] net: increase SOMAXCONN to 4096

On Wed, Oct 30, 2019 at 09:36:20AM -0700, Eric Dumazet wrote:
> SOMAXCONN is /proc/sys/net/core/somaxconn default value.
> 
> It has been defined as 128 more than 20 years ago.
> 
> Since it caps the listen() backlog values, the very small value has
> caused numerous problems over the years, and many people had
> to raise it on their hosts after beeing hit by problems.
> 
> Google has been using 1024 for at least 15 years, and we increased
> this to 4096 after TCP listener rework has been completed, more than
> 4 years ago. We got no complain of this change breaking any
> legacy application.
> 
> Many applications indeed setup a TCP listener with listen(fd, -1);
> meaning they let the system select the backlog.
> 
> Raising SOMAXCONN lowers chance of the port being unavailable under
> even small SYNFLOOD attack, and reduces possibilities of side channel
> vulnerabilities.

Just a quick question, I remember that when somaxconn is greater than
tcp_max_syn_backlog, SYN cookies are never emitted, but I think it
recently changed and there's no such constraint anymore. Do you
confirm it's no more needed, or should we also increase this latter
one accordingly ?

Willy