Hello,

We are using nginx as a loadbalancer in front of docker containers. Most of the time this works without problems. But sometimes (~0.1% of the requests) the requests are sent to the server group name instead of one of the members of the servergroup.

upstream gateway {
server 127.0.0.1:6000 weight=1000000;
server 1.2.3.4:6000;
}

server {
listen <ip>:443;
ssl on;
server_name <public dns>;

<ssl config>

location / {
proxy_pass http://gateway;
}
}

Most of the time the requests are logged as expected:

03/Feb/2016:04:00:25 +0100 "/api/v1/login" 200 52 "Jersey/2.7 (HttpUrlConnection 1.8.0_51)" "time=0.192" "<public dns>" "upstream=127.0.0.1:6000"
or
03/Feb/2016:04:00:25 +0100 "/api/v1/login" 200 52 "Jersey/2.7 (HttpUrlConnection 1.8.0_51)" "time=0.192" "<public dns>" "upstream=1.2.3.4:6000"

But randomly this happens:

03/Feb/2016:04:00:25 +0100 "/api/v1/login" 502 52 "Jersey/2.7 (HttpUrlConnection 1.8.0_51)" "time=0.192" "<public dns>" "upstream=gateway"

As you can see, it uses the server group name as the upstream target (which fails of course since this is not a valid host).

What could be the cause of this behaviour?

All the best,
Armin