so_keepalive option for outbound sockets

[Maxim Dounin]

Hello!

On Thu, Jul 21, 2016 at 07:02:48PM -0700, Aditya Umrani wrote:

> Hello,
> 
> I've configured the so_keepalive option for a server (this is also my
> default server). I do have other server configs listening to the same
> port.
> ========
> listen 9822 default_server so_keepalive=on;
> listen 29822 ssl http2 default_server so_keepalive=on;
> ========
> 
> My sysctl settings are set to :
> ========
> net.ipv4.tcp_keepalive_time = 120
> net.ipv4.tcp_keepalive_probes = 10
> net.ipv4.tcp_keepalive_intvl = 120
> ========
> 
> However, I see that no tcp keepalive packets are being sent on
> outbound connections (from nginx to the upstream). I also checked the
> output of 'netstat -an --timers' I see that no outbound sockets have
> the 'keepalive' flag. All of them are 'off'. If it matters, the server
> config which actually servers this request is not the default one, but
> one of the other configs.
> I took a quick look at the code and the 'SO_KEEPALIVE' options only
> shows up on functions which deal with listening sockets. Does this
> mean that nginx does not honor this option for outbound connections?

Yes.  The so_keepalive parameter is configured on listening 
sockets and applies to connections accepted though these sockets.

Using SO_KEEPALIVE on sockets to backends are not generally 
needed, as these connections are usually local and 
well-controlled.  (And if it's not the case, many operation 
systems have an option to request TCP keepalive for all 
connection, like net.inet.tcp.always_keepalive on FreeBSD.  
Though looks like Linux isn't able to do so.)

-- 
Maxim Dounin
http://nginx.org/