Nginx hang and do not respond with large number of network connection in FIN_WAIT state

[gnusys]

The TCP state graph for the situation is:

https://i.imgur.com/USECPtc.png

You can see at 16:55 the FIN_WAIT1 ,CLOSE_WAIT and ESTABLISHED takes a steep
climb, At this point Nginx hangs as the server has a script that checks stub
status and this doesn't finish. The server itself and all other services
like sshd works pretty fine ,except Nginx 

At 17:06 you can see the connection states drop steeply too. this is the
point where Nginx is stopped and Apache takes over as the webserver 
The ESTABLISHED and CLOSE_WAIT drop fully and the FIN_WAIT1 reduces slowly
and finally attain normalcy at 17:15

Apache continued working fine with the same amount of external traffic and
same firewall setup

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,282613,282650#msg-282650