Debugging gateway timeouts in a Docker homelab

I recently resolved a recurring gateway timeout issue in my Docker-based homelab. Here is how I identified and fixed the routing problem.

My homelab relies on Docker Compose and Traefik for reverse proxy duties. I chose this setup for its simplicity, which makes mobile debugging easier. Each application operates in a dedicated Docker Compose file, and Traefik handles the routing and SSL certificates.

The system initially worked well, but I began experiencing occasional gateway timeouts in Synapse. Adding Immich to the server increased the frequency of these timeouts. When I bypassed Traefik and accessed the services directly, they functioned normally. This indicated the routing layer caused the timeouts.

The affected containers were connected to two Docker networks:

• A backend network for inter-container communication within each service
• A frontend network shared by all HTTP-serving containers for Traefik routing

When containers attach to multiple networks, Traefik randomly selects one for traffic routing. The initial tests succeeded because Traefik temporarily routed through the correct network. Subsequent requests failed when it switched to the internal backend network.

To fix this, you must explicitly define the routing network. Traefik provides a setting to specify the network via a container label:

- "traefik.docker.network=mynetwork"

Traefik will now consistently use the specified network, resolving the timeout errors. Explicitly defining the network ensures reliable routing for multi-network containers.