nginx reverse proxy to other nginx 502 bad gateway - docker

Question

I would like to create two services, both of them having their own nginx. I would like to use third nginx as a reverse proxy to these two but I got
502 Bad Gateway
when I request
http://127.0.0.1:8080/
http://127.0.0.1:8080/one
http://127.0.0.1:8080/two
accessing:
http://127.0.0.1:8081
http://127.0.0.1:8082
works ok
I have this docker-compose.yml
version: "3.3"
services:
nginx-one:
image: nginx:1.17.8
ports:
- "8081:80"
networks:
- frontend
- backend
volumes:
- ./nginx-one/html:/usr/share/nginx/html
nginx-two:
image: nginx:1.17.8
ports:
- "8082:80"
networks:
- frontend
- backend
volumes:
- ./nginx-two/html:/usr/share/nginx/html
nginx-reverse-proxy:
image: nginx:1.17.8
ports:
- "8080:80"
networks:
- frontend
- backend
volumes:
- ./nginx-reverse-proxy/html:/usr/share/nginx/html
- ./nginx-reverse-proxy/conf.d:/etc/nginx/conf.d
debian-network:
image: cslev/debian_networking
stdin_open: true # docker run -i
tty: true # docker run -t
networks:
- frontend
- backend
networks:
frontend:
internal: false
backend:
internal: true
and dir structure
.
├── docker-compose.yml
├── nginx-one
│   └── html
│   └── index.html
├── nginx-reverse-proxy
│   ├── conf.d
│   │   └── default.conf
│   └── html
│   └── index.html
├── nginx-two
   └── html
   └── index.html
nginx.conf content
user nginx;
worker_processes 1;
error_log /var/log/nginx/error.log warn;
pid /var/run/nginx.pid;
events {
worker_connections 1024;
}
http {
include /etc/nginx/mime.types;
default_type application/octet-stream;
log_format main '$remote_addr - $remote_user [$time_local] "$request" '
'$status $body_bytes_sent "$http_referer" '
'"$http_user_agent" "$http_x_forwarded_for"';
access_log /var/log/nginx/access.log main;
sendfile on;
#tcp_nopush on;
keepalive_timeout 65;
#gzip on;
include /etc/nginx/conf.d/*.conf;
}
and
conf.d/default.conf
server {
listen 80;
location /one {
proxy_pass http://127.0.0.1:8081/;
}
location /two {
proxy_pass http://127.0.0.1:8082/;
}
}
When I comment this line of docker-compose.yml
# ./nginx-reverse-proxy/conf.d:/etc/nginx/conf.d
so conf.d/default.conf is not used, and I request from the host's browser:
http://127.0.0.1:8080/
it gives a proper response from the nginx-reverse-proxy itself but obviously
http://127.0.0.1:8080/one
http://127.0.0.1:8080/two
don't provide any response from
http://127.0.0.1:8081
http://127.0.0.1:8082
but 404 instead.
docker ps output:
IMAGE COMMAND CREATED STATUS PORTS NAMES
cslev/debian_networking "bash" 25 minutes ago Up 3 minutes nginxproblem_debian-network_1
nginx:1.17.8 "nginx -g 'daemon of…" 47 minutes ago Up 3 minutes 0.0.0.0:8080->80/tcp nginxproblem_nginx-reverse-proxy_1
nginx:1.17.8 "nginx -g 'daemon of…" 14 hours ago Up 3 minutes 0.0.0.0:8082->80/tcp nginxproblem_nginx-two_1
nginx:1.17.8 "nginx -g 'daemon of…" 14 hours ago Up 3 minutes 0.0.0.0:8081->80/tcp nginxproblem_nginx-one_1
runnnig this script:
#!/bin/sh
docker exec nginxproblem_debian-network_1 curl -sS 127.0.0.1:8080
docker exec nginxproblem_debian-network_1 curl -sS nginxproblem_nginx-reverse-proxy_1:8080
docker exec nginxproblem_debian-network_1 curl -sS 127.0.0.1:8080/one
docker exec nginxproblem_debian-network_1 curl -sS nginxproblem_nginx-reverse-proxy_1:8080/one
docker exec nginxproblem_debian-network_1 curl -sS 127.0.0.1:8080/two
docker exec nginxproblem_debian-network_1 curl -sS nginxproblem_nginx-reverse-proxy_1:8080/two
docker exec nginxproblem_debian-network_1 curl -sS 127.0.0.1:8081
docker exec nginxproblem_debian-network_1 curl -sS nginxproblem_nginx-one_1:8080
docker exec nginxproblem_debian-network_1 curl -sS 127.0.0.1:8082
docker exec nginxproblem_debian-network_1 curl -sS nginxproblem_nginx-two_1:8082
gives:
curl: (7) Failed to connect to 127.0.0.1 port 8080: Connection refused
curl: (7) Failed to connect to nginxproblem_nginx-reverse-proxy_1 port 8080: Connection refused
curl: (7) Failed to connect to 127.0.0.1 port 8080: Connection refused
curl: (7) Failed to connect to nginxproblem_nginx-reverse-proxy_1 port 8080: Connection refused
curl: (7) Failed to connect to 127.0.0.1 port 8080: Connection refused
curl: (7) Failed to connect to nginxproblem_nginx-reverse-proxy_1 port 8080: Connection refused
curl: (7) Failed to connect to 127.0.0.1 port 8081: Connection refused
curl: (7) Failed to connect to nginxproblem_nginx-one_1 port 8080: Connection refused
curl: (7) Failed to connect to 127.0.0.1 port 8082: Connection refused
curl: (7) Failed to connect to nginxproblem_nginx-two_1 port 8082: Connection refused
when - ./nginx-reverse-proxy/conf.d:/etc/nginx/conf.d is not commented,
and the same output when - ./nginx-reverse-proxy/conf.d:/etc/nginx/conf.d is commented in contrary to accessing ip addresses from the browser.

Answer 1

Unless you have host networking enabled, 127.0.0.1 is pointing to the container itself. You can refer to the other two containers from inside a container in the same network by the service name, e.g. nginx-one or nginx-two.
You're also mapping the container port 80 to port 8080/8081/8082 on the host machine. This however does nothing for communication between containers in the same network. Check the docs:
By default, when you create or run a container using docker create or docker run, it does not publish any of its ports to the outside world. To make a port available to services outside of Docker, or to Docker containers which are not connected to the container’s network, use the --publish or -p flag. This creates a firewall rule which maps a container port to a port on the Docker host to the outside world.
So, try changing http://127.0.0.1:8081/; to http://nginx-one/; and it should work.