Uptime Kuma reports DOWN, but the service can be accessed

Tip

In case you did not know: docker has more than one network type with only some of them allowing access to the local network and some not even allowing access to remote networks

If your Uptime Kuma reports DOWN of your service, knowing if it is a bug of Uptime Kuma / a docker network misconfiguration or a firewall is a good start to fixing the issue.

To debug this, go into your container's bash via

docker exec -it uptime-kuma bash

Install curl

apt update && apt --yes install curl

Then you can debug this issue with commands like ping, curl, ... Examples:

curl https://google.com
ping google.com

IPv6

If you are running Uptime Kuma on top of Docker and the service can only be access via IPv6. Please follow the Docker's official wiki to enable IPv6 support.
IPv6 are not supported out of the box on Docker.

For docker compose, this is as simple as adding a network with IPv6 enabled to your compose file. Upating the template from the project would look like this:

  services:
    uptime-kuma:
      image: louislam/uptime-kuma:2
      volumes:
        - ./data:/app/data
      ports:
        # <Host Port>:<Container Port>
        - 3001:3001
      restart: unless-stopped
+     networks:
+       - uptime-kuma-net
+ networks:
+   uptime-kuma-net:
+     enable_ipv6: true

The network is created at the bottom, with enable_ipv6: true and then added to the service.