Docker stack conditional environment variable - docker

Question

I have a docker stack file that is deployed across my swarm, which have many nodes.
For a specific reason one of the nodes (lets call it Node A) has a connection to the outside (internet), and the others dont, so when deploying a container on the other nodes, I need to set the HTTP_PROXY environment variable.
Question is: how do I set this ONLY on the nodes with a specific label (and not on the A node)
docker-compose.yml
version: '3.6'
services:
app:
image: my_image
ports:
- "8033:8000"
environment:
- HTTP_PROXY=proxy.server.com:3128
- HTTPS_PROXY=proxy.server.com:3128
deploy:
replicas: 10
placement:
constraints: [node.labels.app_server == app_server]
volumes:
- /etc/timezone:/etc/timezone:ro
- /etc/localtime:/etc/localtime:ro
The only alternative so far would be to have the stack deployed with different variables, and place a constraint on deployment. But I am trying to avoid it.

Answer 1

How about setting those environment variables in the necessary hosts and passing them through to your container like so:
version: '3'
services:
app:
image: ubuntu
environment:
- HTTP_PROXY
- HTTPS_PROXY
They will only be set in the container if they are also set in the host environment. See documentation here.
Example/More Info:
# When HTTP_PROXY is set in the host environment, value is passed through to the container.
$ HTTP_PROXY=test docker-compose run app env | grep -i proxy
Creating some-nodes-only_app_run ... done
HTTP_PROXY=test
# When HTTP_PROXY is not set in the host environment, nothing is set in container.
$ docker-compose run app env | grep -i proxy
Creating some-nodes-only_app_run ... done
You could also write an entrypoint script to set the proxy when needed. I would recommend checking the connectivity of the container and then falling back to a proxy if necessary, but if you want to do it based on the hostname you could use something like this:
entrypoint.sh
#!/bin/bash
PROXY_HOSTNAME=some-host
if [ -f /etc/host_hostname ]; then
HOST_HOSTNAME=$(cat /etc/host_hostname)
if [ "$HOST_HOSTNAME" = "$PROXY_HOSTNAME" ]; then
echo "Setting fallback http proxy"
export HTTP_PROXY=${FALLBACK_HTTP_PROXY}
fi
fi
exec $#
Dockerfile
# test:latest
FROM ubuntu
COPY entrypoint.sh /entrypoint.sh
ENTRYPOINT ["./entrypoint.sh"]
docker-compose.yml
version: '3'
services:
app:
image: test:latest
environment:
- FALLBACK_HTTP_PROXY=test
volumes:
- /etc/hostname:/etc/host_hostname:ro
Example run:
$ docker-compose run app env | grep -i http_proxy
Creating some-nodes-only_app_run ... done
Setting fallback http proxy
FALLBACK_HTTP_PROXY=test
HTTP_PROXY=test