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

[Eric Dumazet <edumazet@google.com>]

On Wed, Oct 30, 2019 at 8:36 PM Willy Tarreau <w@1wt.eu> wrote:
>
> 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 ?
>

There is no relationship like that.

The only place somaxconn is use is in __sys_listen() to cap the
user-provided backlog.

somaxconn = sock_net(sock->sk)->core.sysctl_somaxconn;
if ((unsigned int)backlog > somaxconn)
       backlog = somaxconn;

There is a second place in fastopen_queue_tune() but this is not
relevant for this discussion.