nginx docker container: 502 bad gateway response

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The Problem

Localhost is a bit tricky when it comes to containers. Within a docker container, localhost points to the container itself.
This means, with an upstream like this:

upstream foo{
  server 127.0.0.1:8080;
}

or

upstream foo{
  server 0.0.0.0:8080;
}

you are telling nginx to pass your request to the local host.
But in the context of a docker-container, localhost (and the corresponding ip addresses) are pointing to the container itself:

enter image description here

by addressing 127.0.0.1 you will never reach your host machine, if your container is not on the host network.

Solutions

Host Networking

You can choose to run nginx on the same network as your host:

docker run --name nginx -d -v /root/nginx/conf:/etc/nginx/conf.d --net=host nginx

Note that you do not need to expose any ports in this case.

This works though you lose the benefit of docker networking. If you have multiple containers that should communicate through the docker network, this approach can be a problem. If you just want to deploy nginx with docker and do not want to use any advanced docker network features, this approach is fine.

Access the hosts remote IP Address

Another approach is to reconfigure your nginx upstream directive to directly connect to your host machine by adding its remote IP address:

upstream foo{
  //insert your hosts ip here
  server 192.168.99.100:8080;
}

The container will now go through the network stack and resolve your host correctly:

enter image description here

You can also use your DNS name if you have one. Make sure docker knows about your DNS server.

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