TCP connection problems (FIN_WAIT2/LAST_ACK) when upstream server is keepalived

[dwass]

Have a strange problem. We have a configuration (this is only a part of
the configuration) that has problems if the upstream server is a
load-balanced server using ipvs/keepalived.

Here is the relevant part of the configuration:

upstream fb_server {
        server foobar:8080;
}
server {
  listen 80;
  server_name f.b.com;
  location /fb/ {
     proxy_pass http://fb_server;
  }
}

The hostname "foobar" resolves to an IP address that is load-balanced
over 2 physical servers ("foo" and "bar") using ipvs/keepalived.

What we see is that on the 2 upstream servers "foo" and "bar" there are
a lot of connections in FIN_WAIT2 state and on the machine where nginx
is running we see an increasing number of connections in LAST_ACK state.
This would imply that nginx is not managing the upstream connection
shutdown properly.

In order to prove that the problem is related to the load-balancing, if
we change the upstream server to either "foo" or "bar" the problem goes
away.

Now I realize that we can also use nginx to do the load-balancing and
not use ipvs/keepalived, but I'd like to know why this doesn't work
properly in this configuration. It is also probably a useful thing for
the nginx authors and users to know as well.

If anyone has any idea why this is behaving like this I'd love to hear
from you.

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,120319,120319#msg-120319