Cannot Connect Container on Overlay Network: [emerg] invalid host - docker

Question

I am trying to reverse proxy a request through an nginx container in a swarm to a standalone container which shares the same overlay network.
tldr; I receive the following error:
2018/03/15 19:00:35 [emerg] 1#1: invalid host in upstream "http://nginx" in /etc/nginx/nginx.conf:96
nginx: [emerg] invalid host in upstream "http://nginx" in /etc/nginx/nginx.conf:96
The standalone container contains an app which has another nginx frontend:
version: "3"
services:
nginx:
restart: always
container_name: my.nginx
build: ./nginx
networks:
- default
- my-overlay-network
depends_on:
- another-service
... other services
networks:
my-overlay-network:
external: true
I start this app with docker-compose up -d.
My swarm contains the reverse proxy:
version: "3"
services:
reverseproxy:
build: ./reverseproxy
image: reverse_proxy
networks:
- my-overlay-network
ports:
- "80:80"
- "443:443"
volumes:
- /etc/letsencrypt:/etc/letsencrypt
deploy:
replicas: 10
restart_policy:
condition: on-failure
networks:
my-overlay-network:
external: true
If I startup the nginx swarm without specifying a proxy_pass for the standalone app, I can successfully ping the other host like so:
ping http://nginx/
I can confirm the other host receives this request based on the nginx logs.
However, if I specify the docker standalone app in the reverse proxy:
upstream standalone {
server http://nginx/;
}
and
server {
listen 443 ssl http2;
listen [::]:443 ssl http2;
... other stuff ...
location / {
proxy_pass http://standalone/;
}
}
I get the following errors:
2018/03/15 19:00:35 [emerg] 1#1: invalid host in upstream "http://nginx" in /etc/nginx/nginx.conf:96
nginx: [emerg] invalid host in upstream "http://nginx" in /etc/nginx/nginx.conf:96

Answer 1

Try to add resolver before upstream:
resolver 127.0.0.11;
upstream standalone {
server http://nginx/;
}
127.0.0.11 is an address of embedded docker DNS server

Answer 2

The issue is that the syntax for upstream is incorrect: it takes a host and port, so it should be:
upstream standalone {
server nginx;
}