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.